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Physics 1230: Light and Color

Chuck Rogers, Charles.Rogers@colorado.eduRyan Henley, Valyria McFarland, Peter Siegfried

physicscourses.colorado.edu/phys1230

Lecture 16:

Refraction in more complex cases!

Online and Written HW9 due THURSDAY

Last Time: Refraction…

… is the bending of light rays due to the slowing of light in a medium.

PhET bending light

• Wavefronts illustrate peaks and troughs in wave

• Right part of the wavefront hits the medium first and is slowed down first.

• Causes the wave to bend.

• Also makes wavelength shorter (since v = f )

Last Time: Wavefronts bend at faster/slower medium

slower medium

faster medium

Concrete Sand

v

v

Last time: Parts of wave-front hitting an interface

turn, like a car turns when it hits sand. Wheels on

right hit sand first, slowing down, and turning.

(like glass)

slower

speed

(like air)

faster

speed

Last Time: What does my eye see when an object is within or behind medium?

Try sketching this: Draw refracted rays from bottom of pencil, towards the observer

Try sketching this: Draw refracted rays from bottom of pencil, towards the observer

Try sketching this: Draw refracted rays from bottom of pencil, towards the observer

Try sketching this: Draw refracted rays from bottom of pencil, towards the observer

Where does this ray appear to have come from?

Try sketching this: Draw refracted rays from bottom of pencil, towards the observer

Try sketching this: Draw refracted rays from bottom of pencil, towards the observer

The observer will see the underwater part of body being

a) Shorter than it really is;

b)Taller than it really is;

c) Of natural size;

Feet look like they’re here

The observer will see the underwater part of body being

a) Shorter than it really is;

b)Taller than it really is;

c) Of natural size;

Good place for Questions!

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Air

Water

Refracted out to Air

Reflectedinternally

Case 1near normal incidence (light comes out)

Case 2, far from normal incidence(internal reflection)

There is a critical angle at which this happens

Refraction out OR Total internal reflection!

Last Time: Light coming out of water: 2 possibilities

I

II

I

II

θcritical

I

II

θ1 =90

θ>θcritical

I

II

Total Internal Reflection!

Law that allows quantitative understanding of all these angles:Snell’s Law – we won’t cover, involves ratios of Sine of angles

Feet look like they’re here

The observer will see the underwater part of body being

a) Shorter than it really is;

b)Taller than it really is;

c) Of natural size;

Feet look like they’re here

• If the critical angle condition is satisfied, will the snorkler see the upper part of the swimmer’s body?a) Yes;

b) No.

Feet look like they’re here

• If the critical angle condition is satisfied, will the snorkler see the upper part of the swimmer’s body?a) Yes;

b) No.

Legs up and down!

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Total internal reflection makesfiber optic communication possible

In a high “n” material, light bounces around inside and doesn’t exit until the end of the fiber. Demo: lucite light pipe water pipe

Good place for a demo!

Prisms demonstrate refraction and dispersion

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Reflection at a transparent surface occurs because the n values are different. Only a few percent of the light is reflected this way.

Prisms demonstrate refraction and dispersion

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Dispersion = different colors refracted at different angles.

Different colors bend at different angles. Why could this be?

A. Different colors travel at different speeds through the material

B. Different colors have different values of “n” in the material

C. Both A and B

D. None of the aboveIndex of refraction, n:

OR c c

n vv n

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color n (index of refraction)

(blue) 1.523 (bent more)

(yellow) 1.517

(deep red) 1.514 (bent less)

Ordinary glass

Called “dispersion”

Both “n” and speed varies with color

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180 degree rainbow is possible. Double rainbow (woah!) is possible.

Both together is very rare.

Rainbow: Dispersion via water droplets

Good place for a demo!

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180 degree rainbow is possible. Double rainbow (woah!) is possible.

Both together is very rare.

Rainbow: Dispersion via water droplets

Notice: Things get more complex

and interesting as you increase

the number of surfaces…

Water drops, prisms, and more.

Refraction all the way through block

Refraction all the way through block

What was happening in Activity 8?

U2L05 35

U2L05 36

PHET simulation

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• First draw the normal to each surface• Then start with how the light leaves the source and hits the first

surface, then the second surface, then leaves the block• Where does the eye think it came from?

Ray tracing

U2L05 38

Ray tracing

U2L05 39

Ray tracing

U2L05 40

Ray tracing

U2L05 41

Ray tracing

U2L05 42

Ray tracing

Apparent Position is (below actual position)

Sight line

U2L05 43

Apparent Position is (below actual position)

Sight line

Ray tracing

The observer perceives an image of the source

below the actual source position. The image is:

A) Real B) Virtual

Where are we at?

Unit 1: What is light?

Unit 2: Light as a wave

Unit 3: Color in nature and technology (light sources and the spectrum)

Unit 4: Reflection

Unit 5: Refraction

Unit 6: Lenses

Unit 7: Eye and camera

Unit 8: Color perception

Unit 9: Visual perception, illusion, art

• Mirrors reflect light and do not transmit light –glass coated with silver at back.

• Lenses are made of materials that transmit light, e.g., glass.

Examples of lenses: eyeglass, amplifying glass, reading glass, camera, ...

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Lenses

Convex and concave lenses

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• Each of the two surfaces has a spherical shape.

• Light can penetrate through the lenses and bend at the air-lens interface.

Concave and convex lenses

Light (the red ray) enters (and exits!) a thin window

pane (with parallel edges)

Which ray continues the red ray?

A B

C

D

A B

C

D

Light (the red ray) enters (and exits!) a thin window

pane (with parallel edges)

Which ray continues the red ray?

But what if the sides are NOT parallel? The ray bends which way:

A

B

C

A

B

C

But what if the sides are NOT parallel? The ray bends which way:

Glass

We build lenses out of glass with non-parallel sides

Which ray of light will have changed direction the most upon exiting the glass?

Glass

A

C

B

If slabs aren’t parallel - lens

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We build lenses out of glass with non-parallel sides

Put film, Retina here!

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• Light rays bent towards each other… CONVERGING LENS. • The less parallel the two sides, the more the light ray changes

direction.• Rays from a single point, converge to a single point on the other

side of the lens (and then start diverging again).

We build lenses out of glass with non-parallel sides

Put film, Retina here!

Definition: Focal pointWhen light rays approach a lens, parallel to the axis of a lens, they come to a focus at the focal point.

Parallel light rays coming in from an object

Converging (convex) lens

http://www.colorado.edu/physics/phet/dev/html/optics-lab/1.0.0-dev.9/optics-lab_en.html

PHET

Focus

f

optical axis

Light rays coming in parallel focus to a point, called the focal point

Converging (convex) lens

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A good light collector or solar oven; can also fry

your hand very nicely (but please don’t do that)

and please do not look at the Sun (YIKES!)

Light focusing properties of converging lens

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The “backwards” light collector:

Create a collimated light beam like in a

flashlight or headlight on a car.

Light focusing properties of converging lens

Good place for a break!