Physics 1230: Light and Color - University of Colorado … · 1.5 2 2.5 3 3.5 s Score Exam 2 Histogram Average: 82+/-8 of 100 points. 3 Physics 1230: Light and Color Chuck ... Refraction

$Page 1: Physics 1230: Light and Color - University of Colorado … · 1.5 2 2.5 3 3.5 s Score Exam 2 Histogram Average: 82+/-8 of 100 points. 3 Physics 1230: Light and Color Chuck ... Refraction$
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Physics 1230: Light and Color

Chuck Rogers, [email protected] Heinemann, [email protected]

www.colorado.edu/physics/phys1230

Projects

First list of proposed project topics and group

members is due tomorrow (Friday).

Exam 2 solutions are on D2L.

Good Job! 82+/-8 of 100 points

HWK 6 is due at 5PM today.

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Exam 2 HistogramAverage: 82+/-8 of 100 points

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Lecture 9

Refraction and how light behaves at

boundaries between materials.

Where are we at?

Unit 1: What is light?Unit 2: Light as a waveUnit 3: Color in nature and technology (light sources and the spectrum)Unit 4: ReflectionUnit 5: RefractionUnit 6: LensesUnit 7: Eye and cameraUnit 8: Color perceptionUnit 9: Visual perception, illusion, artUnit 10: TBD

Light travels in straight lines from a source

Until it hits a mirror…Or a different medium

incident

Transparent material (e.g., water, glass)

What happens to light when it hits a boundary between materials?

incident reflection

incident reflection

?

refraction

incident reflection

refraction

incident reflectionθinc

0

θreflected

θrefracted

Medium 2

Air

Air MaterialRay bends towards the normal

Medium 2

Air

Air MaterialRay bends towards the normal

0

Medium 2

Air

Material - AirRay bends away from the normal

Medium 2

Air

Material - AirRay bends away from the normal

0

Why does the light ray bend?

“Light slows down inside materials”: Light waves travel more slowly when they are inside stuff. That makes the light wave bend.

Here is one way to say it:

Index of refraction, n:

OR c c

n vv n

MaterialRefractive

Index

Air 1.0008

Water 1.330

Glass 1.5

Diamond 2.417

Ruby 1.760

But,… I thought light traveled at 3x108 m/s always!

That is the speed of light in VACUUM, where everyone always agrees on the speed! Inside stuff, light slows down (always slows down).

Index of refraction, n:

OR c c

n vv n

MaterialRefractive

Index

Air 1.0008

Water 1.330

Glass 1.5

Diamond 2.417

Ruby 1.760

A light beam goes from air (n = 1) to glass (n = 1.5) and is observed to bend

v c n

The speed of the light inside the glass is:

A) 3x108 m/s. Duh!

B) 4.5x108 m/s. Duh!

C) 2x108 m/s. Duh!

D) Something else (duh!).

A light beam goes from air (n = 1) to glass (n = 1.5) and is observed to bend

For a wave, we know that speed, frequency,

and wavelength are all related by: v fFrequency can’t be different inside glass, so

if the wave is slower, what happens to inside the glass compared to in the air?

A) Nothing, wavelength is fixed too

B) Must get larger

C) Must get smaller

Index of Refraction

If the wavelength in air is 600 nm, what will be the

wavelength in the glass? Assume the index of

refraction of glass is 1.5

A) 800 nm

B) 400 nm

C) 1200 nm

D) 200 nm

Index of Refraction

If the wavelength in air is 600 nm, what will be the

wavelength in the glass? Assume the index of

refraction of glass is 1.5

A) 800 nm

B) 400 nm

C) 1200 nm

D) 200 nmYep! Wavelength

got smaller.


“Light slows down inside materials”: Light waves travel more slowly when they are inside stuff. That makes the light wave bend.


Yeah, OK… but,

• 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.

Wavefronts bend when hit slower medium

slower medium

faster medium

Refraction…

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

PhET bending light

θI

θII

I

II

In which medium is light traveling slower?A. Medium IB. Medium IIC. Light travels the same speed in both

θI

θII

I

II


θI

θII

I

II


θI

θII

I

II


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




Where does this ray appear to have come from?



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




c) Of natural size;

Good place for a break!

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Air

Water

Refracted

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!

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





c) Of natural size;


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

b) No.


• 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 above

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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

Raindrop

Dispersion occurs

here during refraction

white lightcomes in

Reflections

Dispersion occurs


A spectrum ofcolors comes out

Rainbow: Colors spread within the raindrop

Raindrop

Dispersion occurs


white lightcomes in

Reflections

Dispersion occurs


A spectrum ofcolors comes out

Rainbow: Colors spread within the raindrop

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big

raindrops

Sun

(behind you)

these rays are seen

How we see a rainbow

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big

raindrops

Sun

(behind you)

this ry not seen

these rays are seen


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big

raindrops

Sun

(behind you)

these rays are seen


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big

raindrops

Sun

(behind you)

this ray not seen

this ray not seen

these rays are seen


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Waterfall droplets create rainbows

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• Light travels in straight lines at speed “c”

• Unless it enters another material (in which case it changes direction and slows down)

• Light of different colors bends different amounts

So…

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Projects

First list of proposed project topics and group

members is due Friday.

Exam 2 Good Job!

Physics 1230: Light and Color - University of Colorado … · 1.5 2 2.5 3 3.5 s Score Exam 2 Histogram Average: 82+/-8 of 100 points. 3 Physics 1230: Light and Color Chuck ... Refraction

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