Refraction
The bending of a wave as it enters a
different medium due to a change in
velocity
__________
If a wave enters a more dense medium, it bends
towards the normal; if a wave enters a less dense
medium, it bends away from the normal
For light, the optical density of the
medium is indicated by the index of
refraction (n)
n = c/v
Where:
n = index of refraction
c = speed of light in air (3.0 x 108 m/s)
v = speed of light in the medium
How fast does light travel in glass (n=1.58)?
Given:
c = 3.0 x 108 m/s
n = 1.58
Equation : n = c/v
1.58 = 3.0 x 108 / v
v = 3.0 x 108 / 1.58
v = 1.9 x 108 m/s
Snell’s Law
n1 (sin θ 1) = n2 (sin θ 2)
Where:
n1 = index of refraction of the medium light is leaving
θ1 = angle of incidence (with respect to the normal)
n2 = index of refraction of the medium light is entering
θ2= angle of refraction (with respect to the normal)
Practice Problems:
Light in air (n = 1.00) is incident on a piece
of glass (n = 1.58) at 42o. What is the angle
of refraction?
Given:
n1 = 1.00
n2 = 1.58
θ1 = 42o
42o
air
glass
Equation : n1 (sin θ1) = n2 (sin θ2)
o
1.00 (sin 42 ) = 1.58 (sin θ2)
sin θ2 = 1.00 (sin 42 )  1.58
o
sin θ2 = 0.424
θ2 = 25
o
Practice Problems:
Light in air (n = 1.00) is incident on a piece of
plastic at 55o. The angle of refraction is 37o.
What is the index of refraction of plastic?
Given:
55o
air
n1 = 1.00
θ1 = 55
plastic
o
θ2 = 37o
37o
Equation : n1 (sin θ1) = n2 (sin θ2)
o
o
1.00 (sin 55 ) = n2 (sin 37 )
n2 = 1.00 (sin 55 )  (sin 37 )
o
n2 = 1.36
o
Total Internal Reflection
When light is incident upon a medium of lesser
refraction
index
of _______________,
__________
____
the ray is bent
away
from
__________
____________
the normal so the exit
greater _____
than the angle of incidence.
angle is _________
90° for some
The exit angle will then approach ________
critical angle, θc, and for incident angles greater than
Total _________
Internal __________
Reflection
the critical angle _______
will occur (NO REFRACTED RAY)
n1 (sin θc ) = n2
where: n1 > n2
Practice Problems:
Calculate the critical angle for the crown glass (n=1.52) air (n=1.00) boundary
Equation : n1 (sin θc) = n2
1.52 (sin θc) = 1.00
θc = 41.1
o