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Refraction of Light
Review: ReflectionLaw of Reflection
The angle of incidence equals the angle of reflection
The incident and reflected rays lie in the same plane with the normal.
RefractionSudden change in direction of a wave as
it changes speed. It must enter obliquely to change direction!
Which direction does it bend as it slows?Which direction does it bend as it slows?
RefractionIn both cases the speed of the wave has
decreased. This is indicated by the decrease in wavelength!
Refraction
In which medium does light travel faster? (glass rod appears bent)
Speed of light
v is the speed of light in the new medium.
c= 3.0 x 108 m/s
Draw a graph for index vs. speed.
What happens to speed of the refracted ray as it hits a medium with n < 1? How about n > 1?
n is the absolute index of refraction. This is a measure of optical density. n is defined as the ratio of the speed of light in a vacuum to the speed of light in a new medium.
Relative Index of Refraction
n is the relative index of refraction. If air is not used, then remember
nrel = n2/n1
What is the relative index when going from diamond into lucite?
If nIf nrelrel < 1 ; < 1 ; speeds up speeds up
If nIf nrelrel > 1 ; > 1 ; slows down slows down
Refraction
Vw = 2.26 x 108m/s
Vg = 2.00 x 108m/s
Calculate the Calculate the speed of light in speed of light in water and glass.water and glass.
nn(water)(water)=1.33; n=1.33; n(glass)(glass)=1.50; n=1.50; n(air)(air)=1.00=1.00
Refraction
When a wave slows down it bends closer to the normal. {less to more – toward} n2>n1
When a wave speed up it bends away from the normal. {BLA – Big ―› Little – Away} n2<n1
nn11- from- from
nn2 2 - into- into
Refraction
If light rays bend closer to the normal when slowing down, why does the glass rod seem to bend away form the normal?
Apparent Depth
Diverging rays enter your eyes.
You “think” in Straight Lines.
A virtual image appears to come from point y
R – Real Depth
A – Apparent Depth
Apparent Depth
If the chest is 20 m below the surface at what depth will the image appear? Assume nsea water = 1.34
Snell’s Law
n1sinθ1 = n2sin θ2
v1/v2 = λ1 / λ2
Example
A monochromatic light ray f= 5.09 x 1014 Hz is incident on medium X at 55˚. The absolute index of refraction for material X is 1.66
1.1. What is material X?What is material X?2.2. Determine the angle Determine the angle
of refraction.of refraction.3.3. Determine the Determine the
speed of light in speed of light in medium X.medium X.
Ex: Solution
The index of 1.66 is Flint Glass
To find the angle of To find the angle of refraction use refraction use Snell’s Law.Snell’s Law. θθ22= 30˚= 30˚
To find the speed use To find the speed use n=c/v.n=c/v.
v = 1.8 x 10v = 1.8 x 108 8 m/sm/s
Dispersion
The breaking up of white light into its component frequencies.
Refractive Index & Frequency
The refractive index depends on the medium & the frequency of light.
Each frequency “color” propagates at a different speed and bends a different amount.
Critical Angles
Def: The angle of incidence when the angle of refraction is 90 degrees.
When a light ray exceeds the critical angle it undergoes total internal reflection (TIR)
Snell’s Law Lab
What factors effect the index of refraction of a liquid?
Write up an experiment to test this question.
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