Optics

The light ray model

A light ray is a line in the direction that light energy flows. My laser beam for example, is really a bundle of many parallel light rays.

The ray model says

1. Light travels through a transparent medium in straight lines, called light rays at speed v = C/n, where n is the index of refraction of the medium.

Why does light travel faster in a vacuum?

The ray model also says…

2. Light rays do not interact with each other.

3. A light ray continues forever unless it has an interaction with matter that causes the ray to change directions or to be absorbed.

The ray model also says…

4. Light has four different ways in which it can interact with matter.

Medium 1 Medium 2

Reflection

4. Light has four different ways in which it can interact with matter.

Medium 1 Medium 2

Refraction

4. Light has four different ways in which it can interact with matter.

Medium 1 Medium 2

Scattering

4. Light has four different ways in which it can interact with matter.

Medium 1 Medium 2

Absorption

An object is a source of light rays. We make no distinction between self luminous objects and reflective objects. Rays originate from every point of the object, and each point sends rays out in all directions.

To simplify the picture, we use a ray diagram that shows only a few important rays. A ray diagram does not imply that these are the only rays.

Luminous objects give off an infinite number of light rays in all directions.

Our minds locate the object by sensing the point where the lines diverge from.

Reflective Objects

Reflection

Reflection off Paper Reflection off a Mirror

Regular ReflectionDiffuse Reflection

Law of Reflection

i

rNormal - an imaginary line drawn perpendicular to the surface

Reflection

Reflections in a Mirror

Q

An observer observes a light source S in a mirror. Where does he perceive the mirror image of S to be located?

S

●1

●2

●3

●4

An observer observes a light source S in a mirror. Where does he perceive the mirror image of S to be located?

S

●1

●2

●3

●4

How big does a mirror have to be in order to see your entire length in it?

Half His Height!

To see more of her head in the mirror, she …

1. should hold the mirror closer

2. should hold the mirror farther away

3. needs a bigger mirror

Another way to tell when you’re dealing with someone who does not have a grasp of even the most rudimentary concepts of physics.

Why does light from the sun or moon appear as a column when reflected from a body of water?

How would it appear if the water surface were perfectly smooth?

If the water were perfectly smooth, a mirror image of the round sun of moon would be seen in the water. If the water were slightly rough, the image would be wavy. If the water were a bit more rough, little glimmers of the sun or moon would be seen above and below the main image.

This is because the water waves act like an assemblage of small flat mirrors. For rougher waves, there is a greater variety of mirror facets properly tilted to reflect the light into your eye. The light then appears smeared into a long vertical column.

Calmer Water

Calm Water

                                                                                            

Very Calm Water Q

Refraction

i

R

Refraction

As light travels from one medium to another, the speed of light changes and the light bends accordingly.

Direction of refraction can be predicted by thinking about chariot wheels.

Must remember Snell’s Law

i

R

Refraction

i

R

Refraction

Hits the water first, and slows

down

i

R

Refraction

i

R

Refraction

i

R

Refraction

n1 sin I = n2 sinR

Snell’s Law: The Law of Refraction

Index of Refraction of the media in which the angle of incidence is

Index of Refraction of the media in which the angle of refraction is

Index of Refraction

mediainlightofspeed

spaceinlightofspeedrefractionofindex

Cannot be less than 1.00

Air = 1.00 Glass = 1.5

Water = 1.33

Q

Bruno wishes to “spear” a fish with a laser. Should he aim the laser beam…

1. above 2. below 3. directly at

the observed fish to make a direct hit?

Light is incident on a block of glass as shown. Which ray is the refracted ray?

1.2.

3.

4.

Consider a fish with a flashlight doing experiments….

•All the light will reflect from the surface.

•None of the light will refract (escape)X

100%

Total Internal Reflection

The angle of incidence where total internal

reflection first takes place is called the critical angle.

critical

refraction = 90o

n1 sin critical = n2 sin refraction

(1.33) sin critical = (1.00) sin 90o

sin critical = 1/1.33

critical = 48.8o

Whenever the angle of

incidence is greater than 48.8o, total

internal reflection takes

place.

Example: Calculate the critical angle for water to air.

Example: O.K. Let’s Calculate the critical angle for air to water.

criticalrefraction = 90o

n1 sin critical = n2 sin refraction

(1.00) sin critical = (1.33) sin 90o

sin critical = 1.33/1.00

Clearly, this can’t be done. You can only have total internal

reflection going from a high index of

refraction to a low index of refraction.

Q

Calculate the critical angle θ from glass (n=1.5) to air.

θ

1. 52.9o 2. 41.8o 3. 32.2o

4. 22.5o 5. 12.3o

Calculate the critical angle θ from glass (n=1.5) to air.

θ

1. 52.9o 2. 41.8o 3. 32.2o

4. 22.5o 5. 12.3o

n1 sin I = n2 sinR

1.5 sin I = 1.0 sin 90

sin I = .667

I = 41.8

Fiber Optics

Fiber optics uses total internal reflection

Apparent Depth

Refraction

Should really be…