Bending of light when it travels from one medium to another Speed of light differs in different mediums: Vacuum = 3.0 x 108
Download ReportTranscript Bending of light when it travels from one medium to another Speed of light differs in different mediums: Vacuum = 3.0 x 108
Bending of light when it travels from one medium to another Speed of light differs in different mediums: Vacuum = 3.0 x 10 8 m/s Air ~ Vacuum Water = 2.26 x 10 8 m/s Acrylic = 1.76 x 10 8 m/s
Rules of Refraction: 1.
Incident ray, refracted ray and normal lie on the same plane all 2.
Light bends TOWARDS the normal when speed of light in 1 st medium > 2 nd medium (i.e. Slows down) 3.
Light bends AWAY from the normal when the speed of light in 1 st medium < 2 nd medium (ie. Speed up)
If
SLOWING
down when entering another medium bend
TOWARDS
the normal Normal Line Incident Ray
If
SPEEDING UP
when entering another medium bend
AWAY
from the normal Normal Line Incident Ray
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
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
= n
where n = Index of Refraction
Example: A light ray travelling from air into acrylic has an angle of incidence of 48 0 . The angle of refraction is determined to be 30 0 . What is the index of refraction of acrylic?
Normal Line n = sin 48 0 sin 30 0 Incident Ray n = 0.743
0.5
< i = 48 o n = 1.49
Air (Faster ) Acrylic(Slower) 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 10 8 m / s v NB: Index of refraction for air is almost the same as for vacuum. For our calculation purposes they are the same. Example 1: Speed of light in air is 3.00 x 10 8 light in salt is 1.96 x 10 8 m/s. Speed of 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 10 8 m/s = 1.53 v 1.96 x 10 8 m/s Example 2: Index of refraction of quartz is 1.46. If the angle of incident ray from air is 60 0 , what is the angle of refracted ray in quartz? Recall that refraction is often accompanied by reflection = Partial Refraction & Reflection Normal Line Refracted ray Air (FASTER) Water (SLOWER) Incident ray Reflected ray Q: What happens when we keep increasing the angle of incidence? Refracted rays Normal Line Air (FASTER) Water (SLOWER) Reflected rays Incident rays Critical angle = when • • Occurs when
NO light is being refracted. Air (FASTER) Water (SLOWER) ONLY occurs under 2 conditions: 1. Light is travelling more slowly in 1 st medium than in 2 nd medium (going from SLOWER to FASTER) 2. Angle of incidence is GREATER than critical angle 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 = 24.4 0 Any light coming in at angle greater than 24.4 0 will be internally reflected “sparkling” effects Apparent Depth Mirage Apparent Depth To draw diagram to show this phenomena: 1. 2. Draw the 2 different mediums & eye Draw the object in slower medium AIR WATER 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 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 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 Eventually once you reached the lowest (and hottest) air layer, total internal reflection occurs Light ray now travels UP through the layers. 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