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Notes: Light

and Optics

Light goes straight

• Light travels in a straight line unless

it interacts with a medium.

• The material through which a wave

travels is called a medium.

• Light can be reflected, refracted

(bent), scattered, or absorbed.

Reflection

• Reflection: when an object or wave

bounces back off a surface through

which it cannot pass.

Law of Reflection

• The law of reflection states that the

angle of incidence equals the angle

of reflection.

Video

Two Types of Reflection

Refraction

• Refraction: the bending of light

waves due to change in speed.

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Refraction

• Refraction: when light rays enter a

medium at an angle, the change in

speed causes the rays to bend, or

change direction.

• Refraction is what causes white light

to break into colors like in a rainbow.

Prisms

Rainbows Light & Matter

• Transparent objects let light through.

• Translucent objects scatter light.

• Opaque objects reflect or absorb light.

Color of Light

• Color of Objects

– White light is the presence of ALL the

colors of the visible spectrum.

– Black objects absorb ALL the colors and

no light is reflected back.

© 2000 Microsoft Clip Gallery

16.2 How we see other

colors

• The additive primary colors

are red, green, and blue.

• We don’t see everything

white because the strength

of the signal matters.

• All the different shades of

color we can see are made

by changing the proportions

of red, green, and blue.

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16.2 How we see the

color of things

• Colored fabrics and paints

get color from a subtractive

process.

• Chemicals, known as

pigments, in the dyes and

paints absorb some colors

and allow the color you

actually see to be reflected.

• Magenta, yellow, and cyan

are the three subtractive

primary colors.

16.2 Color and Vision

• When all the colors of the rainbow are

combined, we do not see any particular color.

• We see light without any color.

• We call this combination of all the colors of light

"white light".

16.2 Color and Vision

Key Question:

How do we see

color?

*Students read Section 16.2

AFTER Investigation 16.2

16.2 How we see the

color of things

When we see an object, the

light that reaches our

eyes can come from two

different processes:

1. The light can be emitted

directly from the object,

like a light bulb or glow

stick.

2. The light can come from

somewhere else, like the

sun, and we see the

objects by reflected light.

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16.2 Why are

plants green?

• Plants absorb energy

from light and convert

it to chemical energy

in the form of sugar

(food for the plant).

• Chlorophyll is an

important molecule

that absorbs blue and

red light.

How the Eye Works Retina

Correcting Vision LASIK Surgery

The process:

• After your eye has been numbed with "eye drop" anesthesia, an instrument known as an eyelid speculum will be positioned to hold your eyelids open. You will remain awake and comfortable throughout the procedure.

• A small suction ring will be placed around the cornea and serves as a platform for the microkeratome.

• The microkeratome separates the surface layers of the cornea, and the corneal flap is folded back.

• You will be asked to look at a target light while the Excimer laser reshapes the corneal tissue. A clicking sound can be heard as each microscopic layer of tissue is vaporized. This process will last from seconds to minutes, depending on the amount of correction necessary.

• The corneal flap is then placed back into its original position and allowed to dry for a few minutes.

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How the human eye

sees color

• The retina in the back of

the eye contains

photoreceptors.

• These receptors release

chemical signals.

• Chemical signals travel

to the brain along the

optic nerve.

optic nerve

Photoreceptors in the Eye

• Cones respond to

three colors: red,

green and blue.

• Rods detect

intensity of light:

black, white,

shades of gray.

Brain interprets Fooling your brain

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http://www.michaelbach.de/ot/

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Fooling your brain

Mirrors

• Mirror: a plane sheet of glass or

similar material with a smooth, shiny

coating on one side.

Types of mirrors

• Plane mirror: a flat mirror that usually

shows a “correct” virtual image.

• A virtual image is an upright image

whose left and right sides are

reversed.

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Types of mirrors

• Concave mirrors: a mirror with a

surface that curves inward, thinner in

the middle.

• Concave mirrors can

form virtual images or

a real image depending

on the bend of the mirror.

Types of mirrors

• Concave mirrors rely on the focal

point in order to produce images.

• Focal point: the point in which light

rays meet to form a picture.

Types of mirrors

• Convex mirrors: a mirror with a surface that curves outward, thicker in the middle.

• Convex mirrors form virtual images that are always smaller than the originals.

• Convex mirrors allow you to see a larger area.

Lenses

• Lens: a curved piece of glass or

other transparent material that

refracts light.

• A lens forms an image by refracting

light rays that pass through it.

Convex Lens

• Convex lens: thicker in the center than at the edges that focuses rays of light (magnify).

• Convex lenses magnify

things or make them

look bigger.

• Examples are

microscopes,

magnifying glasses,

and telescopes.

Convex Lens

• Used in things like microscopes,

magnifying lenses, and in some

telescopes.

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• Concave lenses: thinner in the center and

thicker at the edges (minimize).

• Concave lenses form

virtual images that are

usually smaller than

the original.

• Used in eyeglasses

to correct for nearsightedness.

Concave Lens Concave lens

• Mainly used to correct vision in eye

glasses and contact lenses.

16.2 How does a color

TV work?

• Televisions give off light.

• To make color with a TV, you can use red, green, and

blue (RGB) directly.

• The screen is made of tiny

red, green, and blue dots.

• The dots are called pixels

and each pixel gives off its

own light.

• TV sets can mix the three

colors to get millions of

different colors.

Uses

• Lasers- produce very bright beams of

light.

– The atoms all give out their light together

Uses of Laser (1)

• In medicine

– to break up gallstones and kidney stones,

– to weld broken tissue (e.g. detached retina)

– to destroy cancerous and precancerous cells;

at the same time, the heat seal off capillaries,

– to remove plaque clogging human arteries.

Uses of Laser (2)

• In industry

– to drill tiny holes in

hard materials,

– for welding and

machining,

– for lining up

equipment precisely,

especially in

inaccessible places.

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Uses of laser (3)

• In everyday life

– to be used as bar-code

readers,

– to be used in compact disc

players,

– to produce short pulses of

light used in digital

communications,

– to produce holograms.

CDs

• CDs use laser light

– and reflection-

– The surface has tiny pits and smooth bits in a spiral pattern

– the pits scatter light from a laser

– the smooth bits reflect the light

– so a coded message is read.

spiral

Holography

• Holography is the production of holograms by the use of laser.

• A hologram is a 3D image recorded in a special photographic plate.

• The image appears to float in space and to move when the viewer moves.

Holograms

How holograms work Laser show

• Physics is in everything

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Total Internal Reflection

• Involves both reflection and refraction

• When light bends (refracts) enough it is

reflected rather than transmitted.

Total Internal Reflection

• Fiber Optics

– Transmit signals as

flashes of light.

– Since light has a higher

frequency than radio

waves, much more

information can be

carried on a light beam

than on a radio waves.

16.1 Light carries

information

• The fiber-optic networks you read

about are pipelines for information

carried by light.

16.1 Light carries

information

• In some cities, a

fiber-optic cable

comes directly

into homes and

apartments

carrying

telephone,

television, and

Internet signals.

• The process of making light with heat

is called incandescence.

• Incandescent bulbs generate light

when electricity passes through a thin

piece of metal wire called a filament.

16.1 Electric Light

• The filament heats up

and gives off light.

• The other common kind of electric light is the

fluorescent bulb.

16.1 Electric Light

• Fluorescent bulbs

convert electricity

directly to light without

generating a lot of

heat.

• Fluorescent bulbs use

high-voltage

electricity to energize

atoms of gas that fill

the bulb.

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16.1 Light intensity

• The intensity of light from a small source follows

an inverse square law because its intensity

diminishes as the square of the distance.

16.1 The speed of light

• The speed at which light travels through

air is approximately 300 million meters

per second.

• Light travels almost a million times

faster than sound.

16.1 The speed of light

• The speed of light is so important in

physics that it is given its own symbol,

a lower case c.

• The best accepted experimental

measurement for the speed of light in

air is 299,792,500 m/sec.

• For most purposes, we do not need to

be this accurate and may use a value

for c of 3 × 108 m/sec.

16.1 Reflection and

refraction

• Another example of

refraction of light is the

twinkling of a star in the

night sky

• As starlight travels from

space into the Earth’s

atmosphere, the rays are

refracted.

• Since the atmosphere is

constantly changing, the

amount of refraction also

changes.

16.3 Photons and Atoms

• Key Question:

How does light fit

into the atomic

theory of matter?

*Students read Section 16.3

AFTER Investigation 16.3

16.2 Photons and

intensity

• Intensity measures power per unit area.

• There are two ways to make light of high intensity.

– One way is to have high- energy photons.

– A second way is to have a lot of photons even if they

are low-energy.

The number and

energy of photons

determine the

intensity of the light.

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Application: Color Printing