23
Electromagnetic Radiation Jean Brainard, Ph.D. Say Thanks to the Authors Click http://www.ck12.org/saythanks (No sign in required)

Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

  • Upload
    others

  • View
    16

  • Download
    0

Embed Size (px)

Citation preview

Page 1: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

Electromagnetic Radiation

Jean Brainard, Ph.D.

Say Thanks to the AuthorsClick http://www.ck12.org/saythanks

(No sign in required)

Page 2: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

To access a customizable version of this book, as well as otherinteractive content, visit www.ck12.org

CK-12 Foundation is a non-profit organization with a mission toreduce the cost of textbook materials for the K-12 market bothin the U.S. and worldwide. Using an open-content, web-basedcollaborative model termed the FlexBook®, CK-12 intends topioneer the generation and distribution of high-quality educationalcontent that will serve both as core text as well as provide anadaptive environment for learning, powered through the FlexBookPlatform®.

Copyright © 2014 CK-12 Foundation, www.ck12.org

The names “CK-12” and “CK12” and associated logos and theterms “FlexBook®” and “FlexBook Platform®” (collectively“CK-12 Marks”) are trademarks and service marks of CK-12Foundation and are protected by federal, state, and internationallaws.

Any form of reproduction of this book in any format or medium,in whole or in sections must include the referral attribution linkhttp://www.ck12.org/saythanks (placed in a visible location) inaddition to the following terms.

Except as otherwise noted, all CK-12 Content (including CK-12Curriculum Material) is made available to Users in accordancewith the Creative Commons Attribution-Non-Commercial 3.0Unported (CC BY-NC 3.0) License (http://creativecommons.org/licenses/by-nc/3.0/), as amended and updated by Creative Com-mons from time to time (the “CC License”), which is incorporatedherein by this reference.

Complete terms can be found at http://www.ck12.org/terms.

Printed: September 7, 2014

AUTHORJean Brainard, Ph.D.

EDITORBradley Hughes, Ph.D.

Page 3: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

www.ck12.org Chapter 1. Electromagnetic Radiation

CHAPTER 1 Electromagnetic RadiationCHAPTER OUTLINE

1.1 Electromagnetic Waves

1.2 Properties of Electromagnetic Waves

1.3 The Electromagnetic Spectrum

1.4 References

Why is this picture of a cat so colorful? No cat looks like this to the human eye. The picture was taken with a specialcamera that senses infrared light. This is a form of energy given off by warm objects. Areas that appear yellow arethe warmest, and areas that appear purple are the coolest. The picture shows that the cat’s eyes are the warmest partof its head. Why can’t people see images like this without a camera? The answer has to do with the wavelengths ofinfrared light. Its wavelengths are too long for the human eye to detect. In fact, the human eye can detect light onlyin a very narrow range of wavelengths, called visible light. You’ll learn more about infrared light, visible light, andother forms of electromagnetic radiation in this chapter.Flickr:yellowcloud. www. f lickr.com/photos/yellowcloud/6375882963/. CC BY 2.0.

1

Page 4: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

1.1. Electromagnetic Waves www.ck12.org

1.1 Electromagnetic Waves

Lesson Objectives

• Describe electromagnetic waves.• Explain how electromagnetic waves begin.• State how electromagnetic waves travel.• Summarize the wave-particle theory of light.• Identify sources of electromagnetic waves.

Lesson Vocabulary

• electromagnetic radiation• electromagnetic wave• photon

Introduction

Both infrared light and visible light are examples of electromagnetic radiation. Electromagnetic radiation is thetransfer of energy by waves traveling through matter or across empty space. The waves that transfer this energy arecalled electromagnetic waves. In this lesson, you’ll learn how electromagnetic waves differ from mechanical wavessuch as ocean waves and sound waves. For an excellent video introduction to electromagnetic waves, go to thisURL: http://www.youtube.com/watch?v=cfXzwh3KadE (5:20).

MEDIAClick image to the left for use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/5046

What Are Electromagnetic Waves?

An electromagnetic wave is a wave that consists of vibrating electric and magnetic fields. A familiar example willhelp you understand the fields that make up an electromagnetic wave. Think about a common bar magnet. It exertsmagnetic force in an area surrounding it, called the magnetic field. You can see the magnetic field of a bar magnet inFigure 1.1. Because of this force field, a magnet can exert force on objects without touching them. They just haveto be in its magnetic field. An electric field is similar to a magnetic field (see Figure 1.1). An electric field is an area

2

Page 5: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

www.ck12.org Chapter 1. Electromagnetic Radiation

of electrical force surrounding a charged particle. Like a magnetic field, an electric field can exert force on objectsover a distance without actually touching them.

FIGURE 1.1Magnetic and electric fields are invisible areas of force surrounding magnets and charged particles. The fieldlines in the diagrams represent the direction and location of the force.

How Electromagnetic Waves Begin

An electromagnetic wave begins when an electrically charged particle vibrates. This is illustrated in Figure 1.2.When a charged particle vibrates, it causes the electric field surrounding it to vibrate as well. A vibrating electricfield, in turn, creates a vibrating magnetic field (you can learn how this happens in the chapter "Electromagnetism").The two types of vibrating fields combine to create an electromagnetic wave. You can see an animation of anelectromagnetic wave at this URL: http://www.youtube.com/watch?v=Qju7QnbrOhM (1:31).

MEDIAClick image to the left for use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/5047

3

Page 6: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

1.1. Electromagnetic Waves www.ck12.org

FIGURE 1.2An electromagnetic wave starts with avibrating charged particle.

How Electromagnetic Waves Travel

As you can see in Figure 1.2, the electric and magnetic fields that make up an electromagnetic wave occur are atright angles to each other. Both fields are also at right angles to the direction that the wave travels. Therefore, anelectromagnetic wave is a transverse wave.

No Medium Required

Unlike a mechanical transverse wave, which requires a medium, an electromagnetic transverse wave can travelthrough space without a medium. Waves traveling through a medium lose some energy to the medium. However,when an electromagnetic wave travels through space, no energy is lost, so the wave doesn’t get weaker as it travels.However, the energy is "diluted" as it spreads out over an ever-larger area as it travels away from the source. This issimilar to the way a sound wave spreads out and becomes less intense farther from the sound source.

Wave Interactions

Electromagnetic waves can travel through matter as well as across space. When they strike matter, they interact withit in the same ways that mechanical waves interact with matter. They may reflect (bounce back), refract (bend whentraveling through different materials), or diffract (bend around objects). They may also be converted to other formsof energy. Microwaves are a familiar example. They are a type of electromagnetic wave that you can read aboutlater on in this chapter, in the lesson "The Electromagnetic Spectrum." When microwaves strike food in a microwaveoven, they are converted to thermal energy, which heats the food.

Wave or Particle?

Electromagnetic radiation behaves like waves of energy most of the time, but sometimes it behaves like particles.As evidence accumulated for this dual nature of electromagnetic radiation, the famous physicist Albert Einstein

4

Page 7: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

www.ck12.org Chapter 1. Electromagnetic Radiation

developed a new theory about electromagnetic radiation, called the wave-particle theory. This theory explains howelectromagnetic radiation can behave as both a wave and a particle. In brief, when an electron returns to a lowerenergy level, it is thought to give off a tiny "packet" of energy called a photon (see Figure 1.3). The amount ofenergy in a photon may vary. It depends on the frequency of electromagnetic radiation. The higher the frequency is,the more energy a photon has.

FIGURE 1.3A photon of light energy is given off whenan electron returns to a lower energylevel.

Sources of Electromagnetic Radiation

The most important source of electromagnetic radiation on Earth is the sun. Electromagnetic waves travel from thesun to Earth across space and provide virtually all the energy that supports life on our planet. Many other sources ofelectromagnetic waves that people use depend on technology. Radio waves, microwaves, and X rays are examples.We use these electromagnetic waves for communications, cooking, medicine, and many other purposes. You’ll learnabout all these types of electromagnetic waves in this chapter’s lesson on "The Electromagnetic Spectrum."

Lesson Summary

• An electromagnetic wave consists of vibrating electric and magnetic fields.• An electromagnetic wave begins when an electrically charged particle vibrates.• Electromagnetic waves are transverse waves that can travel across space without a medium. When the waves

strike matter, they may reflect, refract, or diffract, or they may be converted to other forms of energy.• Electromagnetic radiation behaves like particles as well as waves. This prompted Albert Einstein to develop

his wave-particle theory.• The most important source of electromagnetic waves on Earth is the sun, which provides virtually all the

energy that supports life on Earth. Other sources of electromagnetic radiation depend on technology and areused for communications, cooking, and other purposes.

5

Page 8: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

1.1. Electromagnetic Waves www.ck12.org

Lesson Review Questions

Recall

1. Define electromagnetic radiation.2. What is an electromagnetic wave?3. How do electromagnetic waves interact with matter?4. What is a photon?5. Identify sources of electromagnetic waves.

Apply Concepts

6. Create a diagram to represent an electromagnetic wave. Explain your diagram to another student who has noprior knowledge of electromagnetic waves.

Think Critically

7. Explain how an electromagnetic wave begins.8. Compare and contrast mechanical transverse waves and electromagnetic transverse waves.

Points to Consider

In this lesson, you learned that electromagnetic waves are transverse waves. Like other transverse waves, electro-magnetic waves have certain properties.

• Based on your knowledge of other transverse waves, such as waves in a rope, what is the wavelength of anelectromagnetic wave? How is it measured?

• How do you think the wavelengths of electromagnetic waves are related to their frequencies? (Hint: How isthe speed of waves calculated?)

6

Page 9: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

www.ck12.org Chapter 1. Electromagnetic Radiation

1.2 Properties of Electromagnetic Waves

Lesson Objectives

• Describe the speed of electromagnetic waves.• Relate wavelength and frequency of electromagnetic waves.

Lesson Vocabulary

• speed of light

Introduction

Some electromagnetic waves are harmless. The light we use to see is a good example. Other electromagnetic wavesare very harmful. They can penetrate virtually anything and destroy living cells. Why do electromagnetic wavesvary in these ways? It depends on their properties. Like other waves, electromagnetic waves have properties ofspeed, wavelength, and frequency.

Speed of Electromagnetic Waves

All electromagnetic waves travel at the same speed through empty space. That speed, called the speed of light, is300 million meters per second (3.0 × 108 m/s). Nothing else in the universe is known to travel this fast. If you couldmove that fast, you would be able to travel around Earth 7.5 times in just 1 second! The sun is about 150 millionkilometers (93 million miles) from Earth, but it takes electromagnetic radiation only 8 minutes to reach Earth fromthe sun. Electromagnetic waves travel more slowly through a medium, and their speed may vary from one mediumto another. For example, light travels more slowly through water than it does through air (see Figure 1.4). You canlearn more about the speed of light at this URL: http://videos.howstuffworks.com/discovery/29407-assignment-discovery-speed-of-light-video.htm .

FIGURE 1.4Light slows down when it enters water from the air. This causes the waveto refract, or bend.

7

Page 10: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

1.2. Properties of Electromagnetic Waves www.ck12.org

Wavelength and Frequency of Electromagnetic Waves

Although all electromagnetic waves travel at the same speed, they may differ in their wavelength and frequency.

Defining Wavelength and Frequency

Wavelength and frequency are defined in the same way for electromagnetic waves as they are for mechanical waves.Both properties are illustrated in Figure 1.5.

• Wavelength is the distance between corresponding points of adjacent waves. Wavelengths of electromagneticwaves range from many kilometers to a tiny fraction of a millimeter.

• Frequency is the number of waves that pass a fixed point in a given amount of time. Frequencies of electro-magnetic waves range from thousands to trillions of waves per second. Higher frequency waves have greaterenergy.

FIGURE 1.5Wavelength and frequency of electromagnetic waves.

Speed, Wavelength, and Frequency

The speed of a wave is a product of its wavelength and frequency. Because all electromagnetic waves travel at thesame speed through space, a wave with a shorter wavelength must have a higher frequency, and vice versa. Thisrelationship is represented by the equation:

Speed = Wavelength×Frequency

The equation for wave speed can be rewritten as:

Frequency =Speed

Wavelengthor Wavelength =

SpeedFrequency

8

Page 11: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

www.ck12.org Chapter 1. Electromagnetic Radiation

Therefore, if either wavelength or frequency is known, the missing value can be calculated. Consider an electromag-netic wave that has a wavelength of 3 meters. Its speed, like the speed of all electromagnetic waves, is 3.0 × 108

meters per second. Its frequency can be found by substituting these values into the frequency equation:

Frequency =3.0×108 m/s

3.0 m= 1.0×108 waves/s, or 1.0×108 hertz (Hz)

You Try It!

Problem: What is the wavelength of an electromagnetic wave that has a frequency of 3.0 × 108 hertz?

For more practice calculating the frequency and wavelength of electromagnetic waves, go to these URLs:

• http://www.youtube.com/watch?v=GwZvtfZRNKk• http://www.youtube.com/watch?v=wjPk108Ua8k

Lesson Summary

• All electromagnetic waves travel at the same speed through space, called the speed of light, which equals 3.0× 108 meters per second. Electromagnetic waves travel more slowly through a medium.

• Electromagnetic waves differ in their wavelengths and frequencies. The higher the frequency of an electro-magnetic wave, the greater its energy. The speed of an electromagnetic wave is the product of its wavelengthand frequency, so a wave with a shorter wavelength has a higher frequency, and vice versa.

Lesson Review Questions

Recall

1. What is the speed of light?2. What is the wavelength of an electromagnetic wave?3. Describe the range of frequencies of electromagnetic waves.

Apply Concepts

4. If an electromagnetic wave has a wavelength of 1 meter, what is its frequency?

Think Critically

5. Explain why light waves bend when they pass from air to water at an angle.6. Explain the relationship between frequency and wavelength of electromagnetic waves.

Points to Consider

In this lesson, you learned that electromagnetic waves vary in their wavelength and frequency. The complete rangeof wavelengths and frequencies of electromagnetic waves is outlined in the next lesson, "The Electromagnetic

9

Page 12: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

1.2. Properties of Electromagnetic Waves www.ck12.org

Spectrum."

• What do you think are the longest-wavelength electromagnetic wave?• What might be the electromagnetic waves with the highest frequencies?

10

Page 13: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

www.ck12.org Chapter 1. Electromagnetic Radiation

1.3 The Electromagnetic Spectrum

Lesson Objectives

• Define the electromagnetic spectrum.• Describe radio waves and their uses.• Identify three forms of light.• Describe X rays and gamma rays.

Lesson Vocabulary

• electromagnetic spectrum• gamma ray• infrared light• microwave• radar• radio wave• ultraviolet light• visible light• X ray

Introduction

Imagine playing beach volleyball, like the young men in Figure 1.6. They may not realize it, but they are beingbombarded by electromagnetic radiation as play in the sunlight. The only kinds of radiation they can detect arevisible light, which allows them to see, and infrared light, which they feel as warmth on their skin. What other kindsof electromagnetic radiation are they being exposed to in sunlight? In this lesson, you’ll find out.

What Is The Electromagnetic Spectrum?

Electromagnetic radiation occurs in waves of different wavelengths and frequencies. Infrared light and visiblelight make up just a small part of the full range of electromagnetic radiation, which is called the electromagneticspectrum. The electromagnetic spectrum is summarized in the diagram in Figure 1.7.

• On the far left of the diagram are radio waves, which include microwaves. They have the longest wavelengthsand lowest frequencies of all electromagnetic waves. They also have the least amount of energy.

• On the far right are X rays and gamma rays. The have the shortest wavelengths and highest frequencies of allelectromagnetic waves. They also have the greatest amount of energy.

11

Page 14: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

1.3. The Electromagnetic Spectrum www.ck12.org

FIGURE 1.6Electromagnetic radiation from the sunreaches Earth across space. It strikeseverything on Earth’s surface, includingthese volleyball players.

• Between these two extremes, wavelength, frequency, and energy change continuously from one side of thespectrum to the other. Waves in this middle section of the electromagnetic spectrum are commonly calledlight.

As you will read below, the properties of electromagnetic waves influence how the different waves behave and howthey can be used.

FIGURE 1.7How do the wavelength and frequency of waves change across the electromagnetic spectrum?

12

Page 15: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

www.ck12.org Chapter 1. Electromagnetic Radiation

Radio Waves

Radio waves are the broad range of electromagnetic waves with the longest wavelengths and lowest frequencies.In Figure 1.7, you can see that the wavelength of radio waves may be longer than a soccer field. With their lowfrequencies, radio waves have the least energy of electromagnetic waves, but they still are extremely useful. Theyare used for radio and television broadcasts, microwave ovens, cell phone transmissions, and radar. You can learnmore about radio waves, including how they were discovered, at this URL: http://www.youtube.com/watch?v=al7sFP4C2TY (3:58).

MEDIAClick image to the left for use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/5048

AM and FM Radio

In radio broadcasts, sounds are encoded in radio waves that are sent out through the atmosphere from a radio tower.A receiver detects the radio waves and changes them back to sounds. You’ve probably listened to both AM and FMradio stations. How sounds are encoded in radio waves differs between AM and FM broadcasts.

• AM stands for amplitude modulation. In AM broadcasts, sound signals are encoded by changing the amplitudeof radio waves. AM broadcasts use longer–wavelength radio waves than FM broadcasts. Because of theirlonger wavelengths, AM radio waves reflect off a layer of the upper atmosphere called the ionosphere. Youcan see how this happens in Figure 1.8. This allows AM radio waves to reach radio receivers that are very faraway from the radio tower.

• FM stands for frequency modulation. In FM broadcasts, sound signals are encoded by changing the frequencyof radio waves. Frequency modulation allows FM waves to encode more information than does amplitudemodulation, so FM broadcasts usually sound clearer than AM broadcasts. However, because of their shorterwavelength, FM waves do not reflect off the ionosphere. Instead, they pass right through it and out into space(see Figure 1.8). As a result, FM waves cannot reach very distant receivers.

FIGURE 1.8AM radio waves reflect off the ionosphereand travel back to Earth. Radio wavesused for FM radio and television passthrough the ionosphere and do not reflectback.

13

Page 16: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

1.3. The Electromagnetic Spectrum www.ck12.org

Television

Television broadcasts also use radio waves. Sounds are encoded with frequency modulation, and pictures areencoded with amplitude modulation. The encoded radio waves are broadcast from a TV tower like the one inFigure 1.9. When the waves are received by television sets, they are decoded and changed back to sounds andpictures.

FIGURE 1.9This television tower broadcasts signalsusing radio waves.

Microwaves

The shortest wavelength, highest frequency radio waves are called microwaves (see Figure 1.7). Microwaves havemore energy than other radio waves. That’s why they are useful for heating food in microwave ovens. Microwaveshave other important uses as well, including cell phone transmissions and radar, which is a device for determiningthe presence and location of an object by measuring the time for the echo of a radio wave to return from it and thedirection from which it returns. These uses are described in Figure 1.10. You can learn more about microwaves andtheir uses in the video at this URL: http://www.youtube.com/watch?v=YgQQb1BVnu8 (3:23).

MEDIAClick image to the left for use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/5049

Light

Mid-wavelength electromagnetic waves are commonly called light. This range of electromagnetic waves has shorterwavelengths and higher frequencies than radio waves, but not as short and high as X rays and gamma rays. Lightincludes visible light, infrared light, and ultraviolet light. If you look back at Figure 1.7, you can see where thesedifferent types of light waves fall in the electromagnetic spectrum.

Visible Light

The only light that people can see is called visible light. It refers to a very narrow range of wavelengths in theelectromagnetic spectrum that falls between infrared light and ultraviolet light. Within the visible range, we see light

14

Page 17: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

www.ck12.org Chapter 1. Electromagnetic Radiation

FIGURE 1.10Microwaves are used for cell phones and radar.

of different wavelengths as different colors of light, from red light, which has the longest wavelength, to violet light,which has the shortest wavelength. You can see the spectrum of colors of visible light in Figure 1.11. When all ofthe wavelengths are combined, as they are in sunlight, visible light appears white. You can learn more about visiblelight in the chapter "Visible Light" and at the URL below.

15

Page 18: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

1.3. The Electromagnetic Spectrum www.ck12.org

http://www.youtube.com/watch?v=PMtC34pzKGc (4:50)

MEDIAClick image to the left for use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/5050

FIGURE 1.11Red light (right) has the longest wave-length, and violet light (left) has the short-est wavelength.

Infrared Light

Light with the longest wavelengths is called infrared light. The term infrared means "below red." Infrared light isthe range of light waves that have longer wavelengths than red light in the visible spectrum. You can’t see infraredlight waves, but you can feel them as heat on your skin. The sun gives off infrared light as do fires and livingthings. The picture of a cat that opened this chapter was made with a camera that detects infrared light waves andchanges their energy to colored light in the visible range. Night vision goggles, which are used by law enforcementand the military, also detect infrared light waves. The goggles convert the invisible waves to visible images. For adeeper understanding of infrared light, watch the video at this URL: http://www.youtube.com/watch?v=2–0q0XlQJ0 (6:46).

MEDIAClick image to the left for use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/5051

Ultraviolet Light

Light with wavelengths shorter than visible light is called ultraviolet light. The term ultraviolet means "aboveviolet." Ultraviolet light is the range of light waves that have shorter wavelengths than violet light in the visiblespectrum. Humans can’t see ultraviolet light, but it is very useful nonetheless. It has higher-frequency waves thanvisible light, so it has more energy. It can be used to kill bacteria in food and to sterilize laboratory equipment (seeFigure 1.12). The human skin also makes vitamin D when it is exposed to ultraviolet light. Vitamin D is needed forstrong bones and teeth. You can learn more about ultraviolet light and its discovery at this URL: http://www.youtube.com/watch?v=QW5zeVy8aE0 (3:40).

16

Page 19: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

www.ck12.org Chapter 1. Electromagnetic Radiation

MEDIAClick image to the left for use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/1587

FIGURE 1.12This sterilizer for laboratory equipmentuses ultraviolet light to kill bacteria.

Too much exposure to ultraviolet light can cause sunburn and skin cancer. You can protect your skin from ultravioletlight by wearing clothing that covers your skin and by applying sunscreen to any exposed areas. The SPF, or sun-protection factor, of sunscreen gives a rough idea of how long it protects the skin from sunburn (see Figure 1.13).A sunscreen with a higher SPF protects the skin longer. You should use sunscreen with an SPF of at least 15 evenon cloudy days, because ultraviolet light can travel through clouds. Sunscreen should be applied liberally and often.You can learn more about the effects of ultraviolet light on the skin at this URL: http://www.youtube.com/watch?v=np-BBJyl-go (5:59).

MEDIAClick image to the left for use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/5052

X Rays and Gamma Rays

The shortest-wavelength, highest-frequency electromagnetic waves are X rays and gamma rays. These rays haveso much energy that they can pass through many materials. This makes them potentially very harmful, but it alsomakes them useful for certain purposes.

17

Page 20: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

1.3. The Electromagnetic Spectrum www.ck12.org

FIGURE 1.13If your skin normally burns in 10 minutes of sun exposure, using sunscreenwith an SPF of 30 means that, ideally, your skin will burn only after 30times 10 minutes, or 300 minutes, of sun exposure. How long doessunscreen with an SPF of 50 protect skin from sunburn?

X Rays

X rays are high-energy electromagnetic waves. They have enough energy to pass through soft tissues such as skinbut not enough to pass through bones and teeth, which are very dense. The bright areas on the X ray film in Figure1.14 show where X rays were absorbed by the teeth. X rays are used not only for dental and medical purposes butalso to screen luggage at airports (see Figure 1.14). Too much X ray exposure may cause cancer. If you’ve haddental X rays, you may have noticed that a heavy apron was placed over your body to protect it from stray X rays.The apron is made of lead, which X rays cannot pass through. You can learn about the discovery of X rays as wellas other uses of X rays at this URL: http://www.guardian.co.uk/science/blog/2010/oct/26/x-ray-visions-disease-forgeries .

FIGURE 1.14Two common uses of X rays are illustratedhere.

Gamma Rays

Gamma rays are the most energetic of all electromagnetic waves. They can pass through most materials, includingbones and teeth. Nonetheless, even these waves are useful. For example, they can be used to treat cancer. A medicaldevice sends gamma rays the site of the cancer, and the rays destroy the cancerous cells. If you want to learn moreabout gamma rays, watch the video at the URL below.

http://www.youtube.com/watch?v=okyynBaSOtA (2:45)

MEDIAClick image to the left for use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/5015

18

Page 21: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

www.ck12.org Chapter 1. Electromagnetic Radiation

KQED: Seeing Cells in 3-D

Scientists in Berkeley have developed a powerful new microscope which uses X rays to scan a whole cell and in amanner of minutes, generate a 3D view of the cell and its genetic material. This groundbreaking tool is helping toadvance research into the development of biofuels, the treatment of malaria and it may even help to more rapidlydiagnose cancer. For more information on X ray microscopes, see http://science.kqed.org/quest/video/x-ray-microscope-seeing-cells-in-3-d/ .

MEDIAClick image to the left for use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/129635

Lesson Summary

• The electromagnetic spectrum is the full range of wavelengths and frequencies of electromagnetic radiation.Wavelength, frequency, and energy change continuously across the electromagnetic spectrum.

• Radio waves are the broad range of electromagnetic waves with the longest wavelengths and lowest frequen-cies. They are used for radio and television broadcasts, microwave ovens, cell phone transmissions, and radar.

• Mid-wavelength electromagnetic waves are called light. Light consists of visible, infrared, and ultravioletlight. Humans can see only visible light. Infrared light has longer wavelengths than visible light and isperceived as warmth. Ultraviolet light has shorter wavelengths than visible light and has enough energy to killbacteria. It can also harm the skin.

• X rays and gamma rays are the electromagnetic waves with the shortest wavelengths and highest frequencies.X rays are used in medicine and dentistry and to screen luggage at airports. Gamma rays are used to kill cancercells.

Lesson Review Questions

Recall

1. What is the electromagnetic spectrum?2. Describe how wave frequency changes across the electromagnetic spectrum, from radio waves to gamma rays.3. List three uses of radio waves.4. How are X rays and gamma rays used in medicine?

Apply Concepts

5. Create a public service video warming people of the dangers of ultraviolet light. Include tips for protectingthe skin from ultraviolet light.

19

Page 22: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

1.3. The Electromagnetic Spectrum www.ck12.org

Think Critically

6. Explain two ways that sounds can be encoded in electromagnetic waves.7. Explain how radar works.8. Compare and contrast infrared, visible, and ultraviolet light.

Points to Consider

This chapter introduces visible light. The chapter "Visible Light" discusses visible light in greater detail.

• In this lesson, you read that visible light consists of light of different colors. Do you know how visible lightcan be separated into its different colors? (Hint: How does a rainbow form?)

• In the next chapter, Visible Light, you’ll read that visible light interacts with matter in certain characteristicways. Based on your own experiences with visible light, how does it interact with matter? (Hint: Whathappens to visible light when it strikes a wall, window, or mirror?)

20

Page 23: Electromagnetic Radiation - wsfcs.k12.nc.us...•electromagnetic radiation •electromagnetic wave •photon Introduction Both infrared light and visible light are examples of electromagnetic

www.ck12.org Chapter 1. Electromagnetic Radiation

1.4 References

1. Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.02. Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.03. Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.04. Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.05. Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.06. Alicia Bruce. http://www.flickr.com/photos/aliciaabruce/6883325921/ . CC BY 2.07. Courtesy of NASA. http://commons.wikimedia.org/wiki/File:EM_Spectrum3-new.jpg . Public Domain8. Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.09. KaMa Photography. http://www.flickr.com/photos/katharina_90/3858883369/ . CC BY 2.0

10. Christopher Auyeung and Laura Guerin. CK-12 Foundation . CC BY-NC 3.011. User:Phrood/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:Spectrum-sRGB.svg . Public

Domain12. User:Newbie/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:UV-ontsmetting_laminaire-vloe

ikast.JPG . Public Domain13. Tom Newby. http://www.flickr.com/photos/noodle93/5392321454/ . CC BY 2.014. Dental X ray: Paulo Ordoveza (Flickr:brownpau); Airport X ray: Gwydion M. Williams. Dental X ray:

http://www.flickr.com/photos/brownpau/4879544448/; Airport X ray: http://www.flickr.com/photos/45909111@N00/3766521804/ . CC BY 2.0

21