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Material after quiz and still on

everyone’s Unit 11 test.

Material from here on out though

is NOT on the final exam for seniors but IS on the final for non-

seniors

When light travels from a fast material like air into a slow material

like glass, Snell’s Law always works.

When light travels out of glass

or water and back into air, something unusual happens as

the angle of incidence gets

larger.

Calculating the Critical Angle (CA)

• The CA is the angle of incidence that provides an angle of refraction of 90o.

• Substituting into Snell’s Law reveals that the CA can be calculated by taking the inverse sine of the ratio of the indices of refraction.

• The ratio of nr/ni is a value less than one. It has to be. This ratio must be less than one. Try it!

Θcrit = sin-1 (nr/ni)

• I won’t be providing this equation. It can still be determined from Snell’s law bc angle of refraction is 90o, and you’ll have both n’s.

What is the critical angle for water?

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• The critical angle is the largest angle of

incidence when refraction still occurs

BUT

• If the angle of incidence exceeds the

critical angle, refraction no longer occurs.

Instead the light is internally reflected!

Total Internal Reflection

• Occurs when the reflection of all the

incident light bounces off the boundary

– Light is in the more dense medium and approaching a less dense medium

– Angle of incidence is greater than the so-

called critical angle

• Will happen for light traveling from water to

air, but not air to water

Fish Tank TIR Fish View of the World

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Sparkling Diamond TIR and Fiber Optics

Fiber Optics

Interesting Refraction

Phenomena

https://tinyurl.com/hoffspanglens3

Prisms, Rainbows, Mirages

TIR and Prisms

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Dispersion• The separation of visible light into its

different colors

• Each color has a distinct wave frequency

and will bend varying amounts upon passage through a prism

• For instance for some types of glass, the n value for frequencies of violet light is

1.53; and the n value for frequencies of

red light is 1.51.

• ROYGBIV!

Dispersion

Each color of light has a slightly different index of refraction, so each color bends a different amount.

Rainbows• To view a rainbow your back must be to the sun as you

look at approximated 40o above the ground with suspended droplets of water, even a mist.

• Each droplet then acts as a little prism

Rainbows

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Double Rainbow Double Rainbow

• https://youtu.be/OQSNhk5ICTI

Structures of the Human Eye

https://tinyurl.com/hoffspanglens4

• Blind spot cards

• Look at your blood flowing through your eye!

• Why don’t you normally see this?

Human Eye Diagram (p500)

Refraction Structures

Cornea (3/4) - fixed

Lens (1/4) - adjustable

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Common Disorders

• Cataract – cloudy cornea, fixed with surgery.

• Glaucoma – high pressure in eye fluid, fixed with medicine or surgery.

• Macular Degeneration – leading cause of blindness over 50. Thinning and degeneration of the central part of the retina.

• The lens is composed of transparent, flexible tissue and is located directly behind the iris and the pupil. It is the second part of your eye, after the cornea,

that helps to focus light and images on your retina.

• Because the lens is flexible and elastic, it can change its curved shape to focus on objects and people that are either nearby or at a distance.

• The ciliary muscles, which are part of the ciliary body, are attached to the lens and contract or release to change the lens shape and curvature.

• The lens becomes more rounded to focus on near objects (left pic) and more elongated (or stretched) to focus on objects that are far away (right pic)

• Over time, the lens loses some of its elasticity and therefore loses some of its ability to focus on near objects. This is called presbyopia and explains why

people need reading glasses as they become older.

Presbyopia

• Presbyopia makes it difficult to focus on

close-up words or images. Most people are between 40 and 50 when they begin to

realize that the letters of the telephone

book are "too small" or that it's necessary to hold the newspaper further away in

order to see clearly. At the same time, the

ability to focus on objects that are far away remains intact.

Near sightedness• You can see near things: With myopia, you can see

close-up objects clearly, but distant objects are blurred. Myopia occurs when the eyeball is too long for light rays to focus correctly on your retina. The more myopic you are, the blurrier your vision will be at a distance and objects will have to be closer to you in order to see them clearly.

Far Sightedness

• With hyperopia, you can see distant

objects clearly – far things, but close-up objects are blurred. Hyperopia occurs

when the eyeball is too short for light rays

to focus correctly on your retina

Astigmatism

• If you have astigmatism, it means that the surface of your eye (the cornea) is not smoothly curved; instead, the surface is irregular. Normally, the surface of your cornea is rounded, much like a basketball; with astigmatism, however, your cornea is shaped more a like a football (American football, that is). This doesn't allow light rays to focus clearly on a single point on your retina. Astigmatism is usually accompanied by myopia or hyperopia and is usually correctable with eyeglasses.

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Corrective Lenses (what type of

lens fixes what?)

Nearsightedness Farsightedness Astigmatism

Good Lens and Cheap Lens

Polarization of Light Are Light Waves Longitudinal or Transverse?

EM waves are caused by vibrating

charged particles

(often electrons).

Light waves are transverse.The charged particles vibrate in a

direction perpendicular to the direction of travel of the light wave.

Unpolarized Light -A collection of waves from charges vibrating

in many directions.

The sun, a lamp in the classroom, a candle

all emit unpolarized light.

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Polarized light

• It is possible to polarize unpolarized light

Three Ways to Polarize Light

1. Polarizing Filters – selective absorption.

Polaroid filters

• Unpolarized light passes through the filter

and emerges with HALF the intensity and vibrations in a single plane

• Only vibrations parallel to the filter pass through

• Is it possible to polarize ALL of the light?

Polarized Light Are the glasses really polarizing?

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2. Calcite – double refraction

• https://www.youtube.com/watch?v=4byrfj5

ZJ9E

3. Reflection from Non-metal Surface

Vertical Glare from Vertical Surfaces

Adding a Filter to a Camera

Light reflected off water is

partially polarized in a direction parallel to the water’s surface.

Which direction is this filter

polarized?

Which way should sun glasses be made?

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Which way should sun glasses be made?

“Glare” is greatly reduced Non-reflective coating

• Put a coating which is the average between glass and air on your lens, so the amount of

reflection is about equal for both medium-changes. Destructive interference can occur completely for one wavelength, and partially for

other nearby wavelengths. Often, coatings use 550 nm (right in the middle of visible spectrum). Red and violet are not usually affected, which is

why people with glasses seem to have a certain glare associated with those colors.

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In Summary: Polarized Light• Polarized light lies along the same plane as that of the

vibrating electron that produced it.

– A vertically vibrating electron emits light that is vertically

polarized.

– A horizontally vibrating electron emits horizontally polarized

light.

• Common light sources emit light that is not polarized

because the electrons vibrate in random directions.

• Light that is not polarized can be polarized by a

polarizing filter.

Polarized Light

• Light that reflects at glancing angles from nonmetallic surfaces vibrates mainly in the plane of the reflecting surface.

– Which pair of glasses would best block glare from the road?

• Light can also be polarized by refraction.

– the mineral calcite refracts incident light into two different paths.

– Both refracted light beams are polarized - one in a direction parallel to the surface and the other in a direction perpendicular to the surface.

Polarized Light

• Light will pass through

a pair of polarizing

filters when their

polarizing axes are

aligned, but not when

they are crossed at right

angles.

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Polarized Light and 3-D Viewing

• Vision in 3 dimensions depends on the fact that each eye views a scene from a slightly different angle.

• Slide shows or movies achieve a 3-D effect by projecting a pair of views that are polarized at right angles to each other.

• Polarizing eyeglasses ensure that each eye sees a separate picture.

• “Sixty Symbols- How 3D Glasses work”, very cool video on the use of polarization

• http://www.physicsclassroom.com/class/re

frn for most of the unit

• http://www.physicsclassroom.com/class/lig

ht for polarization and double slit experiment

Stop here for Spring 18