Principles of Physics

Change in direction of light as it moves into a new
medium and speed changes
 Only occurs if light enters medium such that 0˚< θi< 90˚
θr = angle of refraction
θi
θr
Refraction can only occur
when:
• there is a medium change
• the light enters at an angle
between 0˚ and 90˚
When a toy car goes from
tile to carpet such that both
front wheels hit the carpet at
the same time, the whole car
slows down at the same
time. A change in direction
does not occur.
When a toy car goes from
tile to carpet at an angle.
Each wheel hits the carpet at
a different time. The first
wheel to hit the carpet,
slows down before the
others causing the car to
turn toward it.
90˚
θi
1. To measure the angle
place the protractor
on the normal line
facing the ray
2. Measure the angle
from the normal line
to that ray
1. To measure the angle
place the protractor
on the normal line
facing the ray
2. Measure the angle
from the normal line
to that ray
90˚
θr
Determine the incident
and refracted angles
Determine the incident
and refracted angles
Determine the incident
and refracted angles

Ratio of the speed of light in a vacuum to the
speed of light in other materials
n 
c
v
n= index of refraction (unitless)
c = speed of light in a vacuum
v = speed of light in material
Greater n →more the light slows down in the
material
Determine the speed of light in water if the index
of refraction for water is 1.33
n 
c
v
1 . 33 
300 ,000 ,000 m / s
v
v 
300 ,000 ,000 m / s
1 . 33
v  226 ,000 ,000 m / s

Relates the index of refraction to the angle of
refraction
n 1 sin  1  n 2 sin  2
Don’t forget ˚ is a unit!!!
Material
Vacuum
Air at STP
Ice
Water at 20 C
Crown glass
Salt
Flint glasses
Sapphire
Diamond
Corn oil
Lucite
Index
1
1.00029
1.31
1.33
1.52
1.54
1.66
1.77
2.417
1.47
1.5
Determine the angle of refraction.
n1 = nair = 1.00
n2 = nglass = 1.52
n 1 sin  1  n 2 sin  2
θi = 30˚
1 . 00 sin 30   1 . 52 sin  2
1 . 00 ( 0 . 5 )  1 . 52 sin  2
crown glass
θr
0 . 5  1 . 52 sin  2
0 .5
1 . 52
 sin  2
0 . 329  sin  2
θ2 ≈ 19˚
θc = critical angle: incident angle that causes
refracted angle to be 90˚ in air
Transparent materials can reflect.
Total Internal Reflection:
i
i
nhigh to nlow
θi > θc
i
θi = θ r




All wavelengths of light travel at 3x108 m/s in
a vacuum
In other materials each wavelength travels at a
different speed
In most materials red travels fastest and violet
travels slowest
Red is refracted least and violet is refracted
most therefore the colors separate