9/12/2012

1

Notes on Homework

• Please tear the "ruffle edges" off of homework done in a notebook and torn out

• If the homework is two pages long, staple the pages together

• If you have a problem with the grading, you think something was graded incorrectly, points were added incorrectly, or something was entered wrong into D2L, put your homework and a written explanation in Molly’s mailbox.

9/12/2012

2

Chapter 2: Geometrical Optics

• Reflection

– Half-silvered mirrors

– Locating an image

– Multiple reflections

• Refraction

– Total internal reflection

– Fiber Optics

– Mirages

• Dispersion

– Prisms

– Rainbows

– Diamonds

Transmission and Reflection

Incident ray Transmitted ray

Reflected ray

The incident ray causes electrons in the material to oscillate, which generates a new wave. Some of this new wave travels backwards as the reflected wave, some travels forward, and combines with the incident wave as the transmitted wave.

9/12/2012

3

Half-silvered mirrors Silver is a good mirror because of all the oscillating charges. But what if the layer is too thin to allow the electrons to move enough to fully cancel the forward going wave?

This is called a “half-silvered” mirror, because half the light is transmitted and half is reflected. Often called “one way mirrors”, this is not technically correct. Light passes both ways through the mirror, but typical use leads them to appear to be only one way mirrors.

Half-Silvered Mirrors: Application

9/12/2012

4

Clicker Question

If someone turns the light on in the policeman’s room, what does the suspect see? A. Only his reflection B. Only the policeman C. Both his own reflection

and the policeman

??

Law of Reflection

Ray tracing:

1. Draw the incident ray

2. Draw the “normal”, a line at right angles to the surface, at the point where the incident ray hits

3. The reflected ray will be at the same angle to the normal as the incident ray

incident ray

normal to surface

reflected ray

reflective surface (mirror)

𝜃𝑖 𝜃𝑟

This law is independent of the reflective surface material. It only matters that the surface is reflective.

9/12/2012

5

Notes on Ray Tracing

• There are an infinite number of rays that you can draw, but some of them are more useful than others:

Pinhole camera

Image

Object

Notes on Ray Tracing

• There are an infinite number of rays that you can draw, but some of them are more useful than others:

Pinhole camera

Image

Object

We only NEED these two rays to find the position and size of the image

9/12/2012

6

Specular and Diffuse Reflections

From a mirror or mirror-like surface

From a rough or highly textured surface, like a sheet of paper, cloth, or fur

Clicker Question

• Where are you most likely to see a good reflected image of yourself?

A. A pool of water

B. A polished metal sheet

C. A piece of window glass

9/12/2012

7

Reflections from the surface of water

Reflected Image Formation

9/12/2012

8

Virtual Images

• The virtual image is in the same place regardless of the location of the viewer

• The image is called virtual because no real rays reach the image, and it cannot be seen by putting a screen at its position

Image Formation in Flat Mirrors

9/12/2012

9

Multiple Mirrors

B

A virtual image can act as an intermediate image and generate its own virtual image

A

C

Multiple Mirrors

The mirrors do not have to be at right angles To avoid ray-tracing at weird angles, you can extend one of the mirrors and position the image in the appropriate spot

9/12/2012

10

Retroreflectors

• Retroreflectors are a special set of multiple mirrors that reflect incoming rays back the way they came, parallel to the incoming ray

• They are used in bike reflectors and other safety equipment

Refraction • When going from a fast to a

slow medium, light bends towards the normal to the surface

• When going from a slow to a fast medium, light bends away from the normal to the surface

• The speed of the medium is related to its index of refraction , n – n = 1.3 for glass – n = 1.5 for water

• Remember there is still a reflection as well

Air (fast medium)

Glass or water (slow)

Normal

Glass or water (slow)

Normal Air (fast medium)

9/12/2012

11

Refraction Analogy

Incident ray

Rows of soldiers before they hit rough (muddy) slow terrain

Rows of soldiers after they hit slow terrain

Normal to front edge of slower terrain

• The rows of marching soldiers are analogous to the wave fronts of light

• When the soldiers hit muddy terrain they slow down

• This causes the rows or parts of rows in the mud to move less far in a given time.

• Another way of saying this is that the ray perpendicular to the rows of soldiers outside the mud bends towards the normal to the muddy region for rows in the mud

Refraction Analogy

Incident ray

Rows of soldiers before they hit rough (muddy) slow terrain

Rows of soldiers after they hit slow terrain

Normal to front edge of slower terrain

• You can imagine that the muddier the terrain, the more the soldiers will slow down, and the more the “ray” will bend towards the normal.

• Different optical materials are differently “muddy”, which we identify using the index of refraction.

• Higher index materials are “muddier” and bend the ray more than lower index materials

n = 1.3 for glass n = 1.5 for water n = 2.4 for diamond

9/12/2012

12

Locating an Image Underwater

• The ray from the fish’s fin is bent away from the normal as it exits the water

• We interpret the ray as having arrived in a straight line.

• Adding one more ray from the fish’s fin (the ray normal to the surface is chosen because it is not bent) allows us to locate the image of the fish

There is still a reflection here as well

Clicker Question

• Where does the fish see the fisherman?

A. Where he is

B. Not at all

C. Higher than where he is

D. Lower than where he is

9/12/2012

13

Clicker questions Ch. 2

10.The light and fish are both underwater in the lake. Where does the fish see the light and/or the image of the light?

a) Higher

b) Lower

c) Where the light actually is

d) (c) and (a)

e) (c) and (b)

Top of lake

Total Internal Reflection

• Recall that rays moving from glass (slow) to air (fast) bend away from the normal

• The amount by which they bend away depends on the material

• For any material, we can find an angle where there is no transmitted ray, called the critical angle

Glass

Glass

Glass

9/12/2012

14

Fiber Optics

Mirages

• Occur when the index of refraction increases as you travel into the medium

• In our analogy with the marching soldiers this means they go slower and slower as they advance.

• Imagine the mud getting deeper and deeper as they advance.

• The ray keeps bending more and more towards the normal as the mud deepens

• In the limit as the muddier transitions become more gradual the refracted ray looks curved

Incident ray

9/12/2012

15

Mirages In

creasing

temp

erature

Hot air is less dense, and is thus a “faster” medium than cold air. This leads to the ray bending away from the normal. Because we interpret rays as traveling in straight lines, we see a “reflection” in the road, and assume it is wet.

(color indicates temperature, not a sunset or sunrise)

“Flying Islands” Mirage

9/12/2012

16

“Flying Islands” Mirage D

ecreasing

temp

erature

Cold Ocean

The temperature near the surface of the ocean is often cooler than the air above it, we see the upside down version of the road mirage, as the ray travels from cool to warm air, and is bent away from normal. Again, we interpret rays as traveling in straight lines, and thus the island appears to float above the ocean

(color indicates temperature, not a sunset or sunrise)

Dispersion

So far, what we’ve learned about refraction means that in a prism would refract like this:

But we’ve seen that what really happens is this:

9/12/2012

17

Dispersion

• Dispersion means that different colors (frequencies/wavelengths) of light have different speeds through a material (different index of refraction), and thus refract at different angles.

• In most transparent optical materials, blue (high frequency) light has a higher index than red (low frequency), and thus blue light is bent more than red.

Clicker Question

• Where does blue come out of prism, compared to red?

red

blue

red

blue

A)

B)

C) Somewhere in the middle

9/12/2012

18

What is the normal to a curved surface and how is it used to find rays?

• To find the normal to a curved surface at a point where a ray hits that surface (and will be reflected or refracted) – First draw a tangent line to the

curve (or tangent plane to the surface)

– The normal is perpendicular to that line or plane and going through the point

– Once you have drawn the normal you can draw the reflected or refracted ray

Rainbows

9/12/2012

19

Diamonds • Diamond has a very high

index of refraction leading to a small critical angle for total internal reflection

• Diamonds are cut such that most light entering will hit a back facet at more than the critical angle, reflecting many times before returning out the front

• This long path through the diamond leads to a lot of dispersion, spreading the colors out

24.5°

Diamond: n=2.4