Light and Optics

Unit 8: Light and OpticsChapter 23: The Physical Nature

of Light23.1 Electromagnetic Spectrum

23.2 Interference, Diffraction, and Polarization

23.3 The Dual Nature of Light

23.1 Investigation: The Electromagnetic

SpectrumKey Question:

What is the electromagnetic spectrum?

Objectives: Research one type of wave that is part of the

electromagnetic spectrum. Prepare a poster containing the information they

discover. Make an oral presentation to share their findings with

the class.

Electromagnetic waves If you could shake the

magnet up and down 450 trillion times per second, you would make waves of red light with a frequency of about 450 THz.

Light and radio waves are waves of electromagnetism.

Electromagnetic waves

If you switch electricity on and off repeatedly, the oscillating electricity makes an electromagnetic wave.

This is exactly how radio towers make radio waves.

The electromagnetic spectrum

Light, like sound and heat, is a form of energy.

The visible light we see is part of the electromagnetic spectrum.

Electromagnetic spectrumThe entire range of electromagnetic waves,

including all possible frequencies, is called the electromagnetic spectrum.

This spectrum includes both visible light and invisible waves:— radio wave— microwaves— infrared light— ultraviolet light— X-rays— gamma rays

Wavelength and Frequency of Light

Because the wavelength of light is so small, scientists measure it in nanometers.

One nanometer (nm) is one billionth of a meter (0.000000001 m).

Frequencies and wavelengths of lightSince color is related to energy, there is a direct relationship between color (energy) and frequency and an inverse relationship between color (energy) and wavelength.

Electromagnetic wave speedAll electromagnetic waves travel at the same

speed in a vacuum, the speed of light—3 × 108 m/s.

When moving through a material, the frequency of light stays the same.

Index of refraction

The index of refraction (n) for a material is the ratio of the speed of light in a vacuum to the speed of light in that material.

Low-energy electromagnetic wavesWe classify the energy of electromagnetic waves by comparing it to the energy it takes to remove an electron from an atom.

Low energy waves, like visible light, do not have enough energy to break most chemical bonds:— Radio waves are the lowest-frequency waves.— Microwaves range in length from 1 mm to 30 cm.— Infrared waves include wavelengths from 1 mm

to about 700 nm.

High-energy electromagnetic wavesUltraviolet light has enough energy to remove electrons and to break chemical bonds.

X-rays are high-frequency waves that are used extensively in medical and manufacturing applications.

Gamma rays are generated in nuclear reactions, and can strip the innermost electrons out of an atom.

Unit 8: Light and OpticsChapter 23: The Physical Nature

of Light23.1 Electromagnetic Spectrum

23.2 Interference, Diffraction, and Polarization

23.3 The Dual Nature of Light

23.2 Investigation: The Wave Nature of Light: Polarization

Key Question:

What are some ways light behaves like a wave?

Objectives:

Use a string to demonstrate the behavior of a light wave.

Explain the interaction of polarizers, using the wave theory of light.

Diffraction and shadowsLike sound and water waves,

light shows interference, diffraction and polarization.

Diffraction occurs when a wave passes through an opening not too much wider than the wavelength of the wave.

Observing diffraction with light is evidence that light is a wave.

Diffraction and shadowsYou can see diffraction

in a shadow cast by a sharp edge with light from a laser.

The edge of the shadow has ripples in it.

The ripples are caused by diffraction.

Young’s double slit experiment In 1807, Thomas Young proved light was a

wave when he showed that two beams of light could interfere with each other.

Light is a waveThe bright bands in an

interference pattern are where the light waves from both slits are in phase at the screen (constructive interference).

The dark bands appear where the light waves reach the screen out of phase (destructive interference).

Diffraction gratingsA diffraction grating actually a series of thin

parallel grooves on a piece of glass or plastic.

When light goes through a diffraction grating, each groove scatters the light so the grating acts like many parallel slits.

Spectrometers

A spectrometer is a calibrated diffraction grating used to create a spectrum.

The spectrometer has a scale that allows you to read different wavelengths of light directly from the pattern of light made by the grating.

Polarization

The orientation of light is called its polarization.

Only transverse waves can have polarization.

Polarizers

A polarizer is a material that allows light of only one polarization to pass through it.

Light with a single polarization is called polarized light.

Applications of polarization

Polarized sunglasses reduce glare because they selectively absorb light with horizontal polarization while letting other light through.

Applications of polarization Images on a LCD (liquid crystal display)are

made using polarized light.

Each liquid crystal window can be electronically controlled to act like a polarizer, or not.

Unit 8: Light and OpticsChapter 23: The Physical Nature

of Light23.1 Electromagnetic Spectrum

23.2 Interference, Diffraction, and Polarization

23.3 The Dual Nature of Light

23.3 Investigation: The Particle Nature of Light: Phosphorescence

Key Question: How does light fit into the atomic theory of matter?

Objectives:

Explore the quantum theory of light.

Experiment with a photoluminescent material

Energy, color and light

The lowest-energy photons we can see are the ones that appear red to our eyes.

White light is a mixture of photons with a range of energy.

Energy and intensity of lightThe intensity of light is a

combination of both the number of photons and the energy per photon.

To make a red light with an intensity of 100 W/m2 takes a lot more photons than it does to make the same intensity with blue light.

Energy and intensity of light If glow-in-the-dark

plastic is exposed to light, it stores some energy and releases the energy later by giving off light.

The process of releasing stored light energy is called photoluminescence.

Glow-in-the-dark plastic demonstrates that a single atom only absorbs asingle photon at a time.

Light and atoms

Almost all atoms absorb and emit light.

For most atoms, the absorption and emission of light happens in less than one-millionth of a second.

How 3-D Movies Work

Cinematographers, ophthalmologists, optical engineers, and computer graphic designers all play a role in the development of modern 3-D movie technology.

To create the illusion of three-dimensions on a flat screen, each eye must receive its own separate image of the movie, from a slightly different perspective, mimicking the way your eyes take in a real three-dimensional scene.