Refraction

Refraction Bending of light when it travels from one medium to another Speed of light differs in different mediums: Vacuum = 3.0 x 108 m/s Air ~ Vacuum Water = 2.26 x 108 m/s Acrylic = 1.76 x 108 m/s

Refraction Rules of Refraction: Incident ray, refracted ray and normal  all lie on the same plane Light bends TOWARDS the normal when speed of light in 1st medium > 2nd medium (i.e. Slows down) Light bends AWAY from the normal when the speed of light in 1st medium < 2nd medium (ie. Speed up)

Refraction Refracted Ray Normal Line Incident Ray <i If SLOWING down when entering another medium  bend TOWARDS the normal Normal Line Incident Ray <i Medium A (Faster) Medium B (Slower) <R Refracted Ray

Refraction Normal Line Incident Ray <i <R Refracted Ray If SPEEDING UP when entering another medium  bend AWAY from the normal Normal Line Incident Ray <i Medium A (Slower) Medium B (Faster) <R Refracted Ray

Partial Reflection & Refraction

Partial Refraction & Reflection Refraction is often accompanied by reflection = partial refraction & reflection Ex: Transparent window Silvered two-way mirror: Ex: mirrored sun glasses, Glasses on buildings Here most lights are reflected, while few are refracted

Calculating Index of Refraction (n) If ray of light is going from vacuum (or air) to a slower medium, then…. We can use the angles given to calculate index of refraction using the formula sin <i .= n sin <R where n = Index of Refraction

n = sin 480 sin 300 n = 0.743 0.5 n = 1.49 Normal Line <i = 48o Example: A light ray travelling from air into acrylic has an angle of incidence of 480. The angle of refraction is determined to be 300 . What is the index of refraction of acrylic? Normal Line Incident Ray <i = 48o Air (Faster) Acrylic(Slower) n = sin 480 sin 300 n = 0.743 0.5 n = 1.49 <R = 30o Refracted Ray

Calculating Index of Refraction (n) Another way to calculate n is by using the ratio between the speed of light in vacuum (c) and the speed of light in the medium (v) n = c . Where c = 3.00 x 108 m/s v NB: Index of refraction for air is almost the same as for vacuum. For our calculation purposes they are the same.

Calculating Index of Refraction (n) Example 1: Speed of light in air is 3.00 x 108 m/s. Speed of light in salt is 1.96 x 108 m/s. Light enters from air to salt. Which way does the light bend? What is the index of refraction? Answer: Light travels from air (faster) to salt (slower), thus refracted ray will bend TOWARDS the normal. Index of refraction (n) = c = 3.00 x 108 m/s = 1.53 v 1.96 x 108 m/s

Calculating Index of Refraction (n) Example 2: Index of refraction of quartz is 1.46. If the angle of incident ray from air is 600, what is the angle of refracted ray in quartz?

Total Internal Reflection

Recall that refraction is often accompanied by reflection = Partial Refraction & Reflection Normal Line Air (FASTER) Water (SLOWER) Refracted ray Reflected ray Incident ray

Q: What happens when we keep increasing the angle of incidence? Normal Line Air (FASTER) Water (SLOWER) Refracted rays Reflected rays Incident rays Critical angle = when <R is equal to 900

Total Internal Reflection Air (FASTER) Water (SLOWER) Occurs when <i is GREATER than critical angle. NO light is being refracted.

Total Internal Reflection ONLY occurs under 2 conditions: 1. Light is travelling more slowly in 1st medium than in 2nd medium (going from SLOWER to FASTER) 2. Angle of incidence is GREATER than critical angle

Total Internal Reflection Critical angle is a physical property of medium High index of refraction means a very small critical angle. Example: Diamond  n = 2.42  critical angle = 24.40 Any light coming in at angle greater than 24.40 will be internally reflected  “sparkling” effects

Total Internal Reflection

Effects of refractions Apparent Depth Mirage

To draw diagram to show this phenomena: Apparent Depth To draw diagram to show this phenomena: Draw the 2 different mediums & eye Draw the object in slower medium AIR WATER

4. EXTEND the refracted ray BACKWARDS Apparent Depth 3. Draw an incident ray coming from a point on the object, the normal line and the refracted ray. 4. EXTEND the refracted ray BACKWARDS AIR WATER

6. EXTEND the refracted ray BACKWARDS Apparent Depth 5. Draw a second incident ray from the same point, but at different angle, show the new normal line and refracted ray. 6. EXTEND the refracted ray BACKWARDS AIR WATER

7. Where the two dashed lines meet is where your image will be. Apparent Depth 7. Where the two dashed lines meet is where your image will be. AIR WATER

Water on pavement - Mirage The mirage is a virtual image of the sky. n of cold air > n of warm air Light bends AWAY from normal as it goes from cold to warmer air

Water on pavement - Mirage Eventually once you reached the lowest (and hottest) air layer, total internal reflection occurs Light ray now travels UP through the layers.

Water on pavement - Mirage Refracted ray reaches your eyes Your brain automatically extends the ray BACKWARDS. Image of the sky appears to come from the ground ahead of you