Post on 04-Jan-2016
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/sAir ~ VacuumWater = 2.26 x 108 m/sAcrylic = 1.76 x 108 m/s
Refraction
Rules of Refraction:
1. Incident ray, refracted ray and normal all lie on the same plane
2. Light bends TOWARDS the normal when speed of light in 1st medium > 2nd medium (i.e. Slows down)
3. Light bends AWAY from the normal when the speed of light in 1st medium < 2nd medium (ie. Speed up)
Refraction
If SLOWING down when entering another medium bend TOWARDS the normal
Normal Line
Incident Ray <i
Medium A (Faster)
Medium B (Slower) <R
Refraction
Refracted Ray
If SPEEDING UP when entering another medium bend AWAY from the normal
Normal LineIncident Ray <i
Medium A (Slower)
Medium B (Faster)
<R Refracted Ray
Refraction
Partial Reflection & Refraction
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
Partial Refraction & Reflection
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
Calculating Index of Refraction (n)
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 LineIncident Ray
<i = 48o
Air (Faster)
Acrylic(Slower)
n = sin 480 sin 300
n = 0.743 0.5
n = 1.49
<R = 30o
Refracted Ray
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?
Calculating Index of Refraction (n)
Total Internal Reflection
Normal Line
Air (FASTER)
Water (SLOWER)
Recall that refraction is often accompanied by reflection = Partial Refraction & Reflection
Incident ray
Refracted ray
Reflected ray
Normal Line Air (FASTER)
Water (SLOWER)
Critical angle = when <R is equal to 900
Q: What happens when we keep increasing the angle of incidence?
Incident rays
Refracted rays
Reflected rays
Air (FASTER)
Water (SLOWER)
Total Internal Reflection
• Occurs when <i is GREATER than critical
angle.
• NO light is being refracted.
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
Total Internal Reflection
Effects of refractions
Apparent DepthMirage
Apparent Depth To draw diagram to show this phenomena:1. Draw the 2 different mediums & eye2. Draw the object in slower medium
AIR
WATER
Apparent Depth3. 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
Apparent Depth5. 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
Apparent Depth
7. Where the two dashed lines meet is where your image will be.
AIR
WATER
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
Water on pavement - Mirage
Refracted ray reaches your eyesYour brain automatically extends the ray BACKWARDS. Image of the sky appears to come from the ground ahead of you