12.1 What is Refraction?

What is Refraction?In a vacuum (air) light travels in straight lines.

But what happens when light travels from one material into another?

Like for example, from air to water?

What Happens When Light hits a Medium

The straight-line path of light CHANGES!

Light bends (REFRACTS!!)

Your brain THINKS that the light should travel in

a straight line. But it doesn’t.

Try This! Exploring With Light - Page 515

1. Place a coin in the middle of a beaker. Fill the beaker with water.

2. Look at the coin from the edge of the beaker. Be sure that you are looking ABOVE the beaker THROUGH the surface of the water

3. Aim a stir stick just inside the outer edge of the coin so that it looks as though you are going to just touch the coin. Place the stir stick in the water and attempt to touch the coin.

4. Now look at the beaker from the side and notice the position of the stir stick and the position of the coin.

Why does this happen?!?

REFRACTIONThe bending or change in direction of light when it travels from one medium into another.

What Causes Refraction?A light ray bends because light

travels at different speeds in different mediums.

In a vacuum, light travels at a speed of 3 x 108 m/sec

But when light travels through a material, it is absorbed and re-emitted by each atom or molecule it hits.

What Causes Refraction?

This absorption and emission slows down the light’s rays

The more light is slowed down the more it bends

Light only refracts at the boundary

Speed of Light

Medium Speed

Vacuum 3.00 x 108m/s

Water 2.26 x 108m/s

Acrylic 1.76 x 108m/s

Light slows down when it travels from air into another medium.

Rules of Refraction

1. The incident ray, the refracted ray, and the normal all lie in the same plane.

The incident ray and refracted ray are on opposite sides of the line that separates the two media

Angle of Refraction: the angle between the refracted

ray and the normal

Rules of Refraction2. Light bends toward the normal when the speed of light in the second medium is less than the speed of light in the first medium (when light slows down).

In other words...Light bends TOWARDS the normal when entering a more dense medium

Rules of Refraction2. Continued...

Light bends away from the normal when the speed of light in the second medium is greater (when light speeds up).

In other words...Light bends AWAY from the normal when entering a less dense medium

Partial Reflection & Refraction•Light can partially REFLECT and REFRACT at the same time

•Example: You can stand in front of a clear window and (1) see your faint reflection in the window (partial reflection) and (2) see through the window at the objects on the other side (refraction)

Coating a glass surface with a

special film coating creates

two-way mirrors. This is used in

making mirrored sunglasses and

windows of many buildings

12.7 Phenomena Related to Refraction

Apparent Depth:

The depth that an object appears to be at due to the refraction of light in a transparent medium.

• Refraction causes the pencil to appear closer to the surface than it actually is.

Objects under water always appear to be nearer to the surface than they actually are. Apparent depth is an optical illusion. This is what makes fish in water appear to be closer to the surface than they actually are.

For the same reason, the legs of someone standing

in water appear to be shorter

Flattening Sun

When the Sun is close to the horizon, light from the bottom of the Sun is refracted more than the light from

the top of the Sun. The air is more dense near the Earth’s surface than higher up in the atmosphere. So the

increased density of air closer to Earth results in a greater bending of the Sun’s rays.

The air is more dense near the Earth’s

surface than higher up in the

atmosphere. So the increased density of air closer to Earth results in a greater

bending of the Sun’s rays.

Flattening Sun

Mirage

Shimmering on a lakeOccurs when light travels at different speeds through air layers of different

temperatures.

Rainbow - Dispersion Dispersion: the

separation of white light into its spectrum.

Dispersion occurs because each colour of visible light travels at slightly different speeds when it goes through a glass prism

RainbowTh rainbow is caused by a combination of dispersion and partial internal reflection in water droplets in the atmosphere.

Millions of raindrops are necessary to produce a rainbow

Page 519 #1-7

1. Refraction is the change in direction of a light beam that occurs when light enters a new medium.

2.a) When light enters a new medium, its speed changes causing the light to travel in a new direction.

Light bends toward the normal if its speed is slower in the new medium and away from the normal if its speed is greater in the new medium.

2.b) In order for light to be refracted, it must travel from one transparent medium into another, and light must have a different speed in each medium.

3. a) Medium A is air and medium B is ice.

When light passes from one medium into another, its path will be closer to the normal in the medium where lights speed is slower, so medium B must be ice.

3.b) The diagram does not show the direction in which the light is traveling, but this does not matter because the light would follow the same path going either way.

4.a) Away form the normal

b) Toward the normal

5. Partial reflection and refraction are illustrated in Figure 11.

6. Mirrored sunglasses, energy saving window coatings, domes around ceiling-mounted security cameras, security windows in stores

7. One application of partial reflection and refraction is the use of two-way mirrors that are used in places where people on one side need to be able to see through the mirror but do not want people on the other side to be able to see through.

This is the case for situations such as police lineups, focus groups or other observations of behavior in which those being observed might behave differently if they could see the people who are observing them.

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