Science Chapter 12 - The Refraction of Light

What is refraction?

The BENDING of change in DIRECTION of LIGHT when it travels from one medium/material into another (e.g. pencil in a cup of water - medium changes from air to water)

What Causes Refraction? Why does the direction change?

• The speed of light changes when in pass through different materials

• Eg. The speed of light is slower when the light passes through water VS in air

How Fast Is the Speed of Light? (Don’t memorize the number, just have an idea of it)

In vacuum/air = 3.00 × 10⁸ m/s

In water = 2.26 × 10⁸ m/s

In acrylic = 1.76 × 10⁸ m/s

(technically the speed of light in air is slower but the difference is so small that we just consider them the same)

What is the angle of refraction? How does it look like on a diagram?

• The angle between the refracted ray and the normal

What are the rules for refraction?

• Incident ray, the refracted ray, and the normal all lie in the same plane. (They all lie in the same line of the surface)

• Incident ray and the refracted ray are on opposite sides of the line that separates the two media

• Light bends toward the normal when the speed of light in the 2nd medium < 1st medium (when the speed of light gets slower)

• Light bends away from the normal when the speed of the light in the 2nd medium > 1st medium (when the speed of light gets faster)

Why does a spoon in a glass of water appear bent?

• Light coming from the spoon below the water’s surface (to your eyes) travel from water into air.

• So, if it hits water-air boundary at an angle (where your eyes are at), light will bend away from the normal.

• BUT, our brain always thinks that light travels in straight line, so it will think that the spoon is behind where it is actually is. (We see a virtual image that is higher than the actual spoon tip)

What is partial reflection and refraction?

• When light is both reflected and refracted at the same time.

• E.g. Mirror sunglasses - special film coating allow some of the light to be refracted and the same amount to be reflected → so you can see through but others can’t see your eyes

What is index of refraction for a medium?

• Speed of light in a vacuum : Speed of light in a medium (e.g. glass, water)

• ** No dinesion (no units)

What is the formula for index of refraction?

n = c/v

where

c = speed of the light in a vacuum (3.0 × 10⁸ m/s)

v = speed of light in a medium

n = index of refraction for the medium

What is the critical angle?

Angle of incidence that results in a 90° angle of refraction

(E.g. critical angle of water is 48.8°)

As the angle of incidence increase, the angle of refraction _____________

increase

What happens if light traveling from more slowly in the first medium than in the second and the angle of incidence is bigger than the critical angle? (Ex. Water → air) What is this phenomenon called?

• The refracted ray will reflect back into the medium

• CALLED: total internal reflection

Optical devices such as periscopes, binoculars, and fiber-optic cables make use of ______ __________ _____.

total internal reflection

• A triangular prism, depending on its orientation, can change the direction of light by 90° (one total internal reflection) or 180° (two total internal reflections). How does this look like?

What is apparent depth? (Optical illusion)

• the depth that an object appears to be

• due to the refraction of light in a transparent medium

• E.g. fish in cup of water seem to be nearer (closer to the surface of the water) than it really is

• E.g. Person’s leg under water appear short than real

The Sun appears flattened near the horizon because light from the ___1___ of the Sun is refracted ___2___ through Earth’s atmosphere than light from the top of the Sun

1. bottom

2. more

What is mirage?

• a virtual image that form when light is traveling from cool air → warmer air

Explain how mirage works.

• As light travels from cool → warm air, index of refraction decreases → light bending farther away from the normal

• But our brain thinks that light travels in a straight line so we see a pool of water (virtual image) on the highway

Shimmering is caused by light travelling at slightly different speeds through air layers of different temperatures. What are the different air layers and temperatures?

• The temperature is different in each air layer

• In the coldest air, light travels more slowly → light bends towards the normal

• In the layer below, the air gets warmer → light bend more and more towards the normal

• Eventually, total internal reflection occurs in the lowest warm air layer → multiple virtual images

What is dispersion?

Separation of white light into its spectrum (colors of the rainbow)

Red → purple (reflected the least → most)

A rainbow is caused by refraction of _____(1)____ and _____ ___(2)___ ______ in water droplets in Earth’s atmosphere.

1. sunlight

2. partial internal reflection