Snells Law 2013.notebook
October 28, 2013
Refraction
When light travels from one medium to another it appears to bend due to the fact that the speed of the light waves have changed. The light bends toward the normal when it travels into a
medium with greater opitical density (ex. air to plastic)
The light bends away from the normal when it travles into
a medium with less optical density (plastic to air)
Oct 14­11:50 AM
Index of Refraction
"n"
measurement of optical denisty: A vaccuum has the smallest density n=1
n=c
vmedium
ratio of the speed of light in vacuum (c) to
the speed of the light in given medium(v).
Nov 8­6:57 AM
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Snells Law 2013.notebook
October 28, 2013
The index of refraction in air is also 1
V air is 3.00x108
n = c/v = 1
air offers very little resistance.
Oct 14­11:49 AM
Table of indices of Refraction
page 79
Example: Speed of light in a liquid is 2.25 x 108 m/s.
What is the a) refractive index of the liquid?
a) c = 3.00 x 108 m/s, v = 2.25 x 108 m/s
Formula: n = c/v
Calculation: n = 3.00 x 108 m/s/2.25 x 10 8 m/s = 1.33
b) look on page 79 of text, what is the liquid according to the
table of indices of Refraction?
Oct 14­4:57 PM
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Snells Law 2013.notebook
October 28, 2013
Java applet for
refraction and reflection
http://lectureonline.cl.msu.edu/~mmp/kap25/Snell/app.htm
Oct 14­11:57 AM
Snell's law is written
n 1 sinθi = n 2 sinθR
or n 1sinθ1 = n 2sinθ2
n1 is the index of refraction for the first medium
n2 is the index of refraction for the second medium
Oct 14­12:01 PM
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Snells Law 2013.notebook
October 28, 2013
Use subscripts to indicate the different mediums.
Example
Light travels from crown glass (g) into water (w). The angle of
incidence in crown glass is 40.0o. What is the angle of refraction
in water?
(use the table to get indices of refraction p.79)
n gsinθg = n wsinθR
1.52 sin 40.0 o = 1.33 sin θR
sinθR = (1.52)(0.643)/1.33 = 0.735
θR = Sin -1 (0.735)
θR = 47.3 o
Oct 14­7:48 PM
Label the diagram
air n= 1.00
water n = 1.33
If θi = 200 what is θR
Nov 10­10:29 PM
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Snells Law 2013.notebook
October 28, 2013
Nov 10­10:35 PM
Total Internal Reflection and the Critical Angle
As the angle of incidence increases, the intensity of a
reflected ray becomes progressively stronger. And...
intensity of a refracted ray becomes progressively weaker.
θR
θr
θi
Oct 14­7:58 PM
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Snells Law 2013.notebook
October 28, 2013
Total internal reflection when light does not refract it only reflects back into the medium
the critical angle: The angle of incidence when the light ray refracts at an angle of 90o
That means that the light is travelling along the interface (the line that separates the 2 mediums). It does not travel into the new medium Oct 14­8:05 PM
Total internal reflection The critical angle is a constant between two media. ­ occurs when light is leaving a denser medium o
and is travelling into a less
dense medium and
θ i when θR = 90
It is the value of θ i > the critical angle. θ i < the critcial angle : a refracted ray (partial reflection). a) b) θ i = the critical angle : 90o (to the normal). c) θ i > the critical angle : totally reflected ray (no refraction)
Oct 16­3:42 PM
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Snells Law 2013.notebook
October 28, 2013
To calculate the critical angle Total internal reflection for any substance set the angle of refraction to 90 o . ­ occurs when light is leaving a denser medium Example: The light ray is travelling from diamond (n
= 2.42) to air (n = 1.0003) and is travelling into a less
dense medium and
1
2
n1sin ic = n 2 sin R θ
> the critical angle. θi = n2 sinθR
n isin
1 2.42 (sin
θi ) = 1.0003 ( sin 90 o ) 2.42 (sin θi ) = 1.0003 (1.0) 2.42(sin θi ) = 1.0003 sin θi = 0.4133 θi = sin­1 (0.4133) i = 24.4 o
Oct 16­3:42 PM
Any angle of incidence ( ) greater than the critical angle will only be reflected
Oct 28­10:45 AM
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Snells Law 2013.notebook
October 28, 2013
Practice in textbook
P. 78‐79 # 16, 17, 23, 30, 36, 37, 38, 39, 40,42,45, 50 (3 mediums therefore 2 steps)
Note the answers to many of the questions are in the back.
Oct 16­4:19 PM
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