1. Refraction
Changing of speed of a wave (light) when it travels from one medium to another.
When light travels from one medium into another at an angle, the difference in speed causes a change in direction of the light.
When light travels from air into this glass block, it bent towards the normal. What do you think will happen when light travels from the block back into air?

2. For media of different optical densities,
Refraction
Angle of
Incidence
Incident Ray
For media of different optical densities,
θ(r) ≠ θ(i); for θ(r) ≠ 0 º θi
Medium 1
Medium 2
Angle of
Refraction θr
Refracted Beam
Example:
Water is optically denser than air. This means that light travels more slowly in water than in air.
Light travelling from air into water with an incident angle of 56º will bend, towards the normal, with an angle of refraction of 38º.

3. Refraction
How can you predict whether light will bend towards or away from the normal?
Light traveling from a less dense medium to a more dense medium, light bends TOWARDS the normal
Less Dense
More Dense
Light traveling from a more dense medium to a less dense medium, light bends AWAY from the normal
More Dense
Less Dense

4. Index of Refraction (n)
Ratio of speed of light in a vacuum to the speed of light in a medium.
The higher the index of refraction:
The higher the medium’s optical density
The slower light travels in the medium
Speed of light in a vacuum (c)
Speed of light in medium (v)
Example: The speed of light in a solid is 1.24 × 108 m/s.
Determine the solid’s index of refraction.
Identify the material, using Table 1 on pg. 524 of your textbook.

5. Refraction Effects: Apparent Depth
Objects that are under water appear to be closer to the surface than they actually are.
This is due to the refraction of light that travels up through the water, and out into the air towards your eyes.

6. Refraction Effects: Lateral Displacement
Refraction can also shift light rays over to one side.
When light passes from air into water then back into air, the emergent ray is parallel to the incident ray, but no longer moves in the same path.
This stick appears “broken”: can you explain why?

7. Applications of Refraction
Inferior Mirages
Occur when cooler layers of air lie above warmer air (close to ground!)
The index of refraction of air decreases from sky to ground
As light travels downwards, it refracts away from the normal
Light is eventually totally internally reflected upward from one of the layers
We perceive a reflective (shiny) surface, which is actually an image of the sky reflected upward “from the road”.
Why does the road look wet in this photograph?

8. Applications of Refraction
Superior Mirages
Occur when denser layers of air lie below less dense air
The index of refraction of air increases from sky to ground
As light travels downwards, it refracts towards the normal
We perceive a virtual image
A photo of a superior mirage taken in Finland.

9. Applications of Refraction
Sunsets
The index of refraction of the atmosphere increases moving from the outermost layer to the ground
Light travelling at a large angle to the layers of the atmosphere causes greater refraction than when the sun is directly overhead
Refraction causes the sun to appear flattened when it is close to the horizon
Refraction also prolongs sunsets

10. Applications of Refraction
More on sunsets…
Images of the rising sun near Colorado Springs, Colorado.
Earth Science Picture of the Day,

11. Snell’s Law
SNELL’s LAW
n1 sin θ1 = n2 sin θ2

12. Snell’s Law n1 sin θ1 = n2 sin θ2
Predicts the amount that light bends due to a change in speed θ1
n1 sin θ1 = n2 sin θ2 n1 n2 θ2
Example: Light travels from air into an unknown solid at an angle of incidence of 45˚. The angle of refraction is 27˚.
Determine the index of refraction of the solid.
Identify the unknown solid.