Light and Optics Nature of Light

Light and Optics

Nature of Light

Light

In order to do justice to a light and optics unit, we have to first look at the different sources of light!

LightLight is a form of

energy that you can see. This is the first basic principle of light. The sun is the most impressive source of light in our solar system. It is considered a natural form of light

Light and Radiation

As light leaves the sun, it radiates out. This movement of energy is called radiation. Any type of light that travels by radiation is called radiant energyThis includes visible light, x-rays, radio

waves, microwaves, ect…

Light Light can be transformed into other forms of

energy. Thermal energy – Pavement in the summer feels hot

while under the sun Electrical energy – Solar cells transform light energy to

electricity Chemical energy – Photosynthesis in plants creates

chemical bonds to form sugar molecules

Natural Light Sources

Energy created by the sun and other stars by nuclear fusion

Flames give off light when the gas particles around it becomes them thermally energized that they begin releasing the energy as light energy.

Natural Light Sources

Artificial Light Sources

Incandescent lights Includes all light bulbsElectrical currents flow through the bulb and

into the resistor in the middle. The resistor builds up thermal energy to a point where it begins releasing that energy as visible light

Electrical Energy Thermal Energy Visible Light

Fluorescent SourcesElectric current heats up mercury inside of

the tube, which starts giving off ultraviolet light.

That ultraviolet light is absorbed by phosphor and releases it as visible light

Ultraviolet Phosphor Visible Light

Artificial Light Sources

Phosphorescent SourcesLight energy is absorbed by certain types of

particles and stored. It is slowly released over time

Most glow in the dark things utilize phosphorescence

Artificial Light Sources

Chemiluminescent LightsLight energy released from chemical

reactionsGlow sticks utilize the combination of a

couple chemicals (you need to ‘break’ them)

Artificial Light Sources

Bioluminescence LightsUses chemical reactions

to produce light that is primarily used to attract prey

Artificial Light Sources

The Ray Model of Light

So, we’ve stated that light is a form of energy

But it is also matter!Light bounces off of objects, such as

when something blocks a projector, the light is bounced off of the object, and doesn’t reach the screen.

This suggests that light travels in straight directions

The Ray Model of Light

You can use the ray model to predict where shadows will form, or how large they will be.

The closer to the light source an object is, the more rays it blocks

The Ray Model of Light

Different Media

A ‘medium’ is a type of substance (like air, water, oil, Pyrex, ect…

If the substance is clear and light can travel in a straight path through it, it is called Transparent

If the type of substance scatters the light, but doesn’t entirely block it, it is considered Translucent.

Different Media

Finally, if a material completely blocks the light, and prevents any of it from passing through, it is called opaque.

Different Media

Light and Optics

Reflection

Reflection

In your own words, write a definition for ‘Reflection’ Think about looking in a mirror.

Reflection is the process in which light strikes a surface and bounces back off that surface. The words you are reading, is possible because the

light is reflecting from the screen to your eyes!

Reflection

So, how is how we see an image in a mirror or an image we see on a screen similar?

Types of Surfaces

Rough SurfaceThis type of surface is one where light tends

to reflect off into many different directions

Smooth SurfaceThis type of surface is one where light from

the same source tends to bounce in the same direction.

Types of Surfaces

Types of Rays

Incident RayThe ray that comes from the light source

and strikes the surfaceReflected Ray

The ray that bounces off the surface

Predicting Angles of Reflection

Draw the ‘normal’ line.The perpendicular (straight away from the

surface) line splits the angle in halfMatch your incident ray with the reflected

ray

Law of Reflection

As we’ve looked at before, the angle of reflection is the same as the angle of incidence. There is no exception to this, thus it is considered a law. The Law of Reflection states that: “the angle of reflection equals the

angle of incidence”

Forming an Image

So, how do we see images in a mirror? Light rays from a light source strike an object on all surfaces which reflects the light in all directions

When that light reaches our eye, our brain interprets it as an image, hence, vision!

Forming an Image

Forming an Image

Smooth surfaces allow a very accurate image to be formed, because all of the light rays reflect in the same direction, so if our eye happens to be in the way, we see a near exact image of the object!

Even though the light has been reflected, our eyes ‘assume’ the light has come from a straight line. So while looking into the mirror, we see an image ‘inside’ of the mirror.

Rough Surfaces

Smooth surfaces reflect light uniformly (the light goes in the same direction)

Objects that don’t reflect an image back (like a mirror) are considered ‘rough’.

Rough Surfaces

Since the normal lines all point in different directions, the reflected rays go in different directions.

Appears as though the reflected rays were scattered randomly.Can’t form an image.

Rough Surfaces

However, some of that light still can reach our eyes. So we can still see the object!

Curved Mirrors

Mirrors that bulge out are called convex mirrors.

Mirrors that cave in are called concave mirrors.

Images in convex mirrors are always smaller.

Convex and Concave Mirrors

Images in concave mirrors are always bigger.

RefractionTopic 3

Refraction

Process in which light is bent when it travels from one medium to another.

Light bends because it changes speed when it moves through materials that have different densities.

Light travels slower in materials that are more dense because there are more particles.

The bending of light makes the object’s image appear to be in a different position than it really is.

Around a Bend of Light The Law of Refraction: When light travels from one medium to a more dense

medium, the light will be bent toward the normal,

When it exits the denser medium into a less dense medium, it will bend away from the normal.

The new direction of light is called the

angle of refraction.

When the angle of incidence increases the angle of refraction also increases.

Image

Refraction can also occur when light travels through air at different temperatures.

Warm air is less dense than cold air.

The refraction of light through air is called a mirage.

Mirage

The pools of water you see on a hot summer day are often caused by this effect.

The air closer to the ground is hotter than the air about it.

As you approach these pools, they disappear because they were never there.

The pools of water were actually images of the sky refracted by warm air near the ground.

Mirage

What happens when light strikes a surface?

Type of behaviour

What happens to light striking a surface

Nature of surface

What else?

Absorption Energy transformation

Rough, dark, opaque

Some light is reflected

Reflection Bounces Off Smooth, shiny Some light is absorbed

Refraction Travels through new direction

Different transparent medium

Some light is reflected

Topic 4: Lenses & Vision

A lens is a curved piece of transparent material (glass/plastic).  When light rays pass through it, the light is refracted, causing the rays to bend.

A double concave lens is thinner and flatter in the middle than the edges.  Light passing through the thicker more curved areas of the lens will bend more than light passing through the thinner areas, causing the light to spread out or diverge.

A double convex lens is thicker in the middle than around the edges.  This causes the light to come together at a focal point, or converge.

Lenses and MirrorsLenses are useful optical devices.  Eyeglasses, have been made from lenses since the thirteenth century.  A convex lens refracts the light rays from an object so they can be focused. 

Different size lenses can converge the light rays at different distances, enabling corrections to be made to focal points.

However, light from the left portion of the object is directed to the right and the light from the top is directed to the bottom.  This inverts the image.  Overhead projectors and film projectors do this

Eye SpyThe lens in the human eye is a convex lens, which focuses the light rays entering your eye to a point on your retina (a light sensitive area at the back of the eye).  The image you see is formed on the retina. 

Eyeball: The eyeball is about 2.5 cm in diameter and approximately a sphere. It contains all of the parts that make up the eye.

Some people however have eyes that are too long or too short. If their eye is too long, the image forms in front of the retina - this is a condition called Myopic, or near-sightedness (they have trouble seeing distant objects). 

If their eye is too short, the image forms behind the retina, making objects that are close to them difficult to see.  This condition is called far-sightedness.

Knowledge of how light behaves when it travels through lenses helps eye specialists correct vision problems.

Hyperopia (Farsightedness)

Knowledge of how light behaves when it travels through lenses helps eye specialists correct vision problems.

Myopia (Nearsightedness)

Knowledge of how light behaves when it travels through lenses helps eye specialists correct vision problems.

Astigmatism

Uhh?

http://www.youtube.com/watch?v=TDldZrAeZQ8

Comparing the Eye and the CameraThere are many similarities between the human eye and the camera.

Putting It in FocusIn a camera, if an object moves closer to the film, the lens must move away to keep the image in focus. 

In the human eye, the lens cannot move, so the ciliary muscles change the shape of the lens (by making the lens bulge in the middle if the image comes closer to you and stretching if the object is further away).  This is done so that the eyeball isn't stretched.  The process of changing the shape of the lens is called accommodation. 

As people become older, the lens stiffens and loses its' ability to change shape (doesn't bulge) and many people need to wear (convex lens) reading glasses, so that the images can be focused.

The shortest distance at which an object is in focus is called the near point of the eye.  The longest distance is called the far point of the eye.  On average, an adult has a near point of about 25 cm, whereas babies have a near point of only 7 cm.  The far point is infinite (because you can see the stars).

Bringing In The LightIn order to adjust the amount of light that enters the eye and the camera, a special device opens and closes to let just the right amount of light in.  In the camera, the diaphragm controls the aperture (opening) of the lens and the shutter limits the passage of light.

In the eye, the device (or part of the eye) that controls the amount of light entering is called the iris (the coloured part of the eye), which changes the size of the pupil - in much the same way as the diaphragm controls the aperture (opening) of the camera lens.  

The parts of a camera are housed in a rigid light-proof box, whereas layers of tissue hold the different parts of the eye together.  The eyeball contains fluids, called humours, which prevent the eyeball from collapsing and refract the light that enters the eye.

Topic 5: Extending Human

Vision

Tools have been developed, to extend our vision, enabling us to see tiny micro-organisms, far-off distances and the vast reaches of outer space.

Telescopes Telescopes help us to see distant objects more clearly.

In a refracting telescope, light from a distant object is collected and focused by a convex lens called the objective lens.  A second lens, called the eyepiece lens, works as a magnifying glass to enlarge the image.

A reflecting telescope uses a concave mirror to collect rays of light from a distant object.  This mirror is called the primary, or objective mirror, which forms a real image magnified by the eyepiece lens.

The lens in a refracting telescope and the mirror in a reflecting telescope collect as much light as possible from distant objects.  These collectors then focus the light into an image.  The further away the image is from the lens, or the mirror, the greater the magnification.  For the greatest magnification the telescope needs to have as large a distance as possible between the object being viewed and its image.

BinocularsBinoculars are actually two reflecting telescopes mounted side by side.  In binoculars, the telescopes are shortened by placing prisms inside, which serve as plane mirrors.  In this way, the light entering the binoculars can be reflected back and forth inside a short tube.

Microscopes, Telescopes and Scientific KnowledgeA magnifying glass is a very simple microscope, which typically magnifies about 10 times.  In 1676, a Dutch scientist, Anton Van Leeuwenhock used a simple convex lens to view bacteria (magnified about 280 times).  Compound microscopes (as you learned in Unit 1) have an objective lens that forms a real image of the object, which is then magnified by an eyepiece lens.  Usually more than one objective and eyepiece lens are used to increase the magnification and improve the sharpness of the image.

New DiscoveriesScientists have learned many new things as a result of the development of microscopes and telescopes.  Living tissue is composed of living cells, in which functions and reproduction can be viewed, as well as activity in relation to cancerous growth and destruction by viruses.  Scientists can also now study the genetic make-up of cells.  Similarly, the improvements in the telescope has opened up the universe for viewing and study.  Telescopes and microscopes have their limitations, which reveal the nature of light.

The Source of ColoursTopic 6

Prism: Rainbow Effect

In the 17th century, Sir Isaac Netwon conducted a famous experiment.

He placed a prism so that a thin beam of white light could pass through it.

When white light travelled through the prism, Newton saw bands of colour emerge.

Each colour was refracted at a different angle.

Conclusion

Newton concluded that the prism was not the source of colour but that the colours existed in the white light.

Using a reverse prism the colours changed back into white light.

Therefore colour is a property of visible light.

The Spectrum

Spectrum: when white light is refracted into different colours.

ROYGBIV = solar spectrum.Recall: When light strikes an object, the

light may be reflected, absorbed or transmitted.

Why do we see a rose as red?

Seeing Red

The colour you see when light strikes an opaque object depends on which colours are reflected and which ones are absorbed.

White objects reflect all colours.Black objects absorb all colours.

Additive Primary Colours

Additive primary colours: RedGreenBlue Additive because adding all three

together in the proper amounts will make white light.

Red + Green + Blue = White

Secondary Colours

The light of two additive primary colours will produce a secondary colour.

Secondary colours:Red + green = yellow

Green + blue = cyanBlue + red = magenta.

We See Colours

The retina contains 2 cell types that respond to light.

1. Rods = detect presence of light2. Cones = detect colour ( green, red, blue)

Colour blindness is where cone cells are unable to detect certain colours

Topic 7: The Wave Model of Light

Remember that light travels in straight lines.  Sir Isaac Newton tried to explain why.  He proposed that light beams are made of streams of extremely tiny, fast-moving particles.  These tiny particles of light, he suggested, could only travel in straight lines, not around objects.

Looking at WavelengthLight is not made up of tiny particles that travel in straight lines as Newton suggests. When light passes through a small opening, it spreads out around each side of the opening. To explain this, Dutch scientist Christiaan Huygens (1629-1695) suggested that light travels in a wave, not as a stream of fast moving particles.

Wavelength TerminologyThe high parts of the wave are called crests. The low parts of the wave are called troughs.

The distance from crest to crest is called wavelength (the distance from one complete crest and one complete trough).

The height of the crest or the depth of the trough from rest position is called the amplitude.

The Frequency is the rate at which the crest and the trough move up and down.

The number of cycles in a period of time - which is usually measured in hertz, or cycles per second.

The Wave Model of LightThe wave model of light pictures light traveling as a wave. It doesn't explain everything about how light behaves but it helps us visualize it. Thinking about light traveling in waves helps to explain unpredictable behaviour, like when light curves around an opening.

When light passes through a small opening, the waves spread out. If the wavelength is long, the waves spread out very little, whereas shorter wavelengths spread out more.

Light Waves In ActionSunsets can be explained using the wave model of light. As light waves from the sun travel through Earth's atmosphere, they strike particles of different sizes, including dust and other elements.

The longer wavelengths of the reds and oranges tend to pass around these particles, whereas, the shorter wavelengths of blue and violet, strike the particles and reflect and scatter.

At sunset, the light we see passes through about 700 kms of the Earth's atmosphere. There are many more particles in the atmosphere at this time of the day, due to the activity going on during the day - so many more blue and violet waves are reflected away. Red and orange are the vibrant colours we see at sunset.

Laser LightIn 1966, Theodore H. Maiman, a physicist at Hughes Aircraft Company in California became the first person to use a process called ... laser light.

light amplification by the stimulated emmission of radiationor

Incandescent lights give off many different colours and therefore have many different frequencies and wavelengths. The waves are jumbled and crests from one wavelength might overlap the trough of another, making the waves work against each other. This type of light is incoherent.

Laser light is quite different. It gives off a single wavelength (frequency) of coherent light.

Lasers have many useful applications:•Scanners (bar codes in retail shops are scanned to give the price) •Digitized data are read by a laser on a compact disk (CD) •Lasers are use by law enforcement officers to detect the speed of vehicles.

•Laser light can be released in pulses or in a continuous beam. In either form, it is so powerful, that it can make precise cuts through metal and can also be used in surgery, as a scalpel - or, to instantly seal broken blood vessels, because it produces such intense heat.

•Eye surgeons use lasers to correct vision defects (shaving off areas of the cornea - to correct problems caused by irregularities in the shape of the eyeball) •They can also 'spot weld' a detached retina •One day dentists may use lasers to vaporize cavities, instead of drilling into them.

Intro to Electromagnetic Spectrum

The sun is the most abundant source of direct, natural light on Earth.

There are also forms of energy that are invisible! The tiny band of visible light that we see is only part

of the entire spectrum of light energy we recieve. The electromagnetic spectrum is composed of light,

electricl and magnetic waves that vibrate to radiate the earth!

Colours

Different colours on the electromagnetic spectrum have different wavelengths that are measured in nanometers.

They also have different frequencies that are measured in hertz.

Radiation in the Environment

Radiation is a natural part of our environment.

Humans have always lived on Earth in the presence of radiation.

Natural radiation reaches Earth from outer space and continuously radiates from the rocks, soils and water.

Background radiation is that which is naturally and inevitable present in our environment.

Levels can vary greatly. People in granite areas or on mineralized

sands receive more terrestrial radiation. While people living at high altitudes

receive more cosmic radiation. A lot of our natural exposure is due to

radon, a gas which seeps from the earth’s crust and present in air.

Radiation and Life

Radiation – energy travelling through space.

Sunshine is a form of radiation (light, heat, suntans)

We control our exposure through sunglasses, shade, air conditioners, hats, clothes and sunscreen.

There would be no life on earth without sunlight but we have recognised that too much of it on our persons is not a good thing.

Sunshine consists of radiation in a range of wavelengths from long-wave infra-red to shorter wavelength ultraviolet.

Infrared Radiation

Red light has a wavelength of 700 nm, but it could stretch out to 100 nm.

It would become heat radiation, or INFRARED radiation.

It is invisible but you can sense it with your skin.

Anything that is warmer than its surrounds emit infrared rays!

Practical Applications

Motion sensorsBuglar AlarmsHeat Lamps

Herschel and infrared

Directed sunlight through a glass prism to create a spectrum and then measured the temperature of each color. Herschel used three thermometers with blackened bulbs and, for each color of the spectrum, placed one bulb in a visible color while the other two were placed beyond the spectrum as control samples. As he measured the individual temperatures of the violet, blue, green, yellow, orange, and red light, he noticed that all of the colors had temperatures higher than the controls.

He found that the temperatures of the colors increased from the violet to the red part of the spectrum. After noticing this pattern Herschel decided to measure the temperature just beyond the red portion of the spectrum in a region where no sunlight was visible. To his surprise, he found that this region had the highest temperature of all.

Radio Waves

If you could stretch the infrared wave out it would go into radio waves.

Radio waves have longer wavelength and a lower frequency than visible light.

Different types of radio waves have different uses.

Microwaves

Shortest wavelength and highest frequency of the radio waves.

The are reflected by metalThey pass through glass, paper, plasitc,

and similar material.They are absorbed by food.

Applications

Detect speeding carsSend telephone, satellite and

television communicationsTreat muscle soreness.Dry and cure plywoodCure rubber and resinsRaise bread and doughnutsCook potato chips

Electromagnetic Spectrum

Cosmic Rays - background radiation - particles of enormous energy given off  by starsGamma Radiation - deadly high energy given out by the sun and other starsX-rays - high energy used in x-ray equipmentUltraviolet Rays - invisible sunlight energy waves that cause the skin to tanVisible Light - the basic colours of light emitted by the sun and visible to the eye - colours of the spectrum - ROYGBIVInfrared Rays - rays of heat energy - felt by our nervous systemsRadio Waves - microwaves. radio energy, TVs

Ultraviolet Radiation

About 200 nmThis radiation is very energetic.

It causes tanning but it can also do irrepairable damage to us.

UV rays can damage the cornea of the eye: fogging which can lead to a slow loss of vision.

OZONE

Due to the thinning of the ozone, more UV rays are reaching us.

The damage to the ozone layer is speeded up by the use of aeorsol sprays and Freon gas, which breaks up ozone particles.

Though some ultraviolet waves from the Sun penetrate Earth's atmosphere, most of them are blocked from entering by various gases like Ozone. Some days, more ultraviolet waves get through our atmosphere. Scientists have developed a UV index to help people protect themselves from these harmful ultraviolet waves.

The Far UV Camera/Spectrograph deployed and left on the Moon by the crew of Apollo 16 took this picture. The part of the Earth facing the Sun reflects much UV light. Even more interesting is the side facing away from the Sun. Here, bands of UV emission are also apparent. These bands are the result of aurora caused by charged particles given off by the Sun. They spiral towards the Earth along Earth's magnetic field lines.

X-RAYS

Waves pass through tissue (skin and muscle) and are absorbed by bones.

This radiation always stays in the bone and builds up over time.

Therefore people who work as technicians taking the x-rays must protect themselves, by leaving the room where the xray is taken and also must protect the patient’s other areas of the body with lead vests to prevent over-exposure.

Credit: NASA/CXC/SAO This is the same supernova remnant but this image shows only X-ray emission

                                                                             This image is special - it shows a supernova remnant - the remnant of a star that exploded in a nearby galaxy known as the Small Magellanic Cloud. The false-colors show what this supernova remnant looks like in X-rays (in blue), visible light (green) and radio (red).

Gamma Rays

SHORTEST WAVELENGTHHIGHEST FREQUENCYResult from nuclear reactions and can kill

cells.Useful to kill cancer cells.Radiation Therapy – using gamma rays to

kill cancerous growth.

Where do they come from?These waves are generated by

radioactive atoms and in nuclear explosions.

Things like supernova explosions (the way massive stars die), neutron stars and pulsars, and black holes are all sources of celestial gamma-rays.

Music to my ears!!!

http://www.youtube.com/watch?v=bjOGNVH3D4Y

http://imagine.gsfc.nasa.gov/docs/science/quiz_l1/emspectrum_quiz.html