• Textbook sections 26-3 – 26-5, 26-8

Physics 1161: Lecture 22

Refraction

Wave Boundary Behavior

• wave speed and wavelength are greater in less dense medium

• wave frequency is not altered by crossing boundary • reflected pulse is inverted when wave travels from

less dense medium to more dense medium

• incident pulse amplitude is greater than reflected pulse amplitude

Transmission Across a Boundary

• Wave speed & wavelength change

• When the wave approach is perpendicular to the boundary, its speed changes, but there is no bending of the path

Refraction of Light Beam

• Refraction -- bending of light wave path as light passes from one material to another material.

• Refraction occurs at the boundary and is caused by a change in the speed of the light wave upon crossing the boundary.

• Direction of bending depends upon whether light wave speeds up or slows down at the boundary.

Optical Density

• Optical density -- tendency of the atoms of a material to hold on to absorbed energy from a photon in the form of vibrating electrons before reemitting it as a new photon

• The more optically dense a material is, the slower a wave will move through the material.

Index of Refraction

• Index of Refraction is a measure of optical density

• Represented by n

• The higher n is, the more optically dense the material and the slower light travels in the material

Physics 1161: Lecture 17, Slide 7

Indices of Refraction

Law of RefractionSnell’s Law

n1sin1 = n2sin2

Refraction Applets

• Applet by Molecular Expressions -- Florida State University

• Applet by Fu-Kwung Hwang, National Taiwan Normal University

• A ray of light crossing the boundary from a fast medium to a slow medium bends toward the normal. (FST)

• A ray of light crossing the boundary from a slow medium to a fast medium bends away from the normal. (SFA)

FST & SFA

Snell’s Law

n1

n2

When light travels from one medium to another the speed changes v=c/n, but the frequency is constant. So the light bends:

n1 sin(1)= n2 sin(2)

1

2

1) n1 > n2

2) n1 = n2

3) n1 < n2

Preflight 22.1

Compare n1 to n2.

21 %

21 %

58 %

Snell’s Law

n1

n2

When light travels from one medium the speed changes. If the angle of incidence is greater than 0, the light bends. During this process, the frequency remains constant.

n1 sin(1)= n2 sin(2)

1

21) n1 > n2

2) n1 = n2

3) n1 < n2

1 < 2

sin1 < sin2

n1 > n2

Preflight 22.1

Compare n1 to n2.

n1

n2

Snell’s Law Practice

n1sin1 n2 sin2

norm

al

2

A ray of light traveling through the air (n=1) is incident on water (n=1.33). Part of the beam is reflected at an angle r = 60. The other part of the beam is refracted. What is 2?

1 r

Usually, there is both reflection and refraction!

n1

n2

Snell’s Law Practice

n1sin1 n2 sin2

norm

al

2

A ray of light traveling through the air (n=1) is incident on water (n=1.33). Part of the beam is reflected at an angle r = 60. The other part of the beam is refracted. What is 2?

sin(60) = 1.33 sin(2)

2 = 40.6 degrees

1 =r =1 r

Usually, there is both reflection and refraction!

Parallel light rays cross interfaces from air into two different media, 1 and 2, as shown in the figures below. In which of the media is the light traveling faster?

1 2 3

0% 0%0%

1

air air

2

1. Medium 12. Medium 23. Both the same

Parallel light rays cross interfaces from air into two different media, 1 and 2, as shown in the figures below. In which of the media is the light traveling faster?

1 2 3

0% 0%0%

1

air air

2

1. Medium 12. Medium 23. Both the same

The greater the difference in the

speed of light between the two

media, the greater the bending of

the light rays.

Parallel light rays cross interfaces from medium 1 into medium 2 and then into medium 3. What can we say about the relative sizes of the indices of refraction of these media?

1 2 3 4 5

0% 0% 0%0%0%

1

3

21. n1 > n2 > n3

2. n3 > n2 > n1

3. n2 > n3 > n1

4. n1 > n3 > n2

5. none of the above

Parallel light rays cross interfaces from medium 1 into medium 2 and then into medium 3. What can we say about the relative sizes of the indices of refraction of these media?

1 2 3 4 5

0% 0% 0%0%0%

1

3

2

1. n1 > n2 > n3

2. n3 > n2 > n1

3. n2 > n3 > n1

4. n1 > n3 > n2

5. none of the aboveRays are bent toward the normalbent toward the normal when crossing into #2, so nn22 > >

nn11. But rays are bent away from the normalbent away from the normal when going into

#3, so nn33 < n < n22. How to find the relationship between #1 and #3?

Ignore medium #2! So the rays are bent away from the normalbent away from the normal if they would pass from #1 directly into #3. Thus, we have: nn22

> n> n11 > n > n3 3 .

Apparent Depth

• Light exits into medium (air) of lower index of refraction,  and turns left.

Spear-Fishing

• Spear-fishing is made more difficult by the bending of light.

• To spear the fish in the figure, one must aim at a spot in front of the apparent location of the fish.

To spear a fish, should you aim directly at the image, slightly above, or slightly below?

1 2 3

0% 0%0%

1. aim directly at the image

2. aim slightly above3. aim slightly below

To spear a fish, should you aim directly at the image, slightly above, or slightly below?

1 2 3

0% 0%0%

1. aim directly at the image

2. aim slightly above3. aim slightly below

Due to refraction, the image

will appear higherhigher than the

actual fish, so you have to aimaim

lowerlower to compensate.

To shoot a fish with a laser gun, should you aim directly at the image, slightly above, or slightly below?

1 2 3

0% 0%0%

1. aim directly at the image

2. aim slightly above3. aim slightly below

laser beam

light from fish

The lightlight from the laser beam

will also bendbend when it hits the

air-water interface, so aimaim

directlydirectly at the fish at the fish.

Delayed Sunset

• The sun actually falls below below the horizon

• It "sets", a few seconds before we see it set.

Broken Pencil

Water on the Road Mirage

Palm Tree Mirage

Mirage Near Dana – Home of Ernie Pyle