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Refraction
 Light travels in a straight line, but it bends as it passes from one medium to
another, such as from air to water.
 The bending of light as it passes from one medium to another is called
refraction.
 Refraction causes some interesting visual effects.
Sketch what a spoon looks like in a glass of water
Refraction does more than just cause cool effects, it is required to design and
build camera lenses.
Refraction is due to changes in the speed of light. For example as light moves
from air into water, the speed decreases. Different media (materials) slow down
the light by different amounts.
The more light is slowed down, the more the light is refracted.
Questions: Have you ever stood by a lake or pool, spotted an object in the water
and then dove in to grab it? What happens?
Answer: Yes, the object is not in the same spot.
Reason:
When we view the refracted light with our eyes our brain assumes that light is
coming in a straight direction. The object will appear to have come from
shallower water because of refraction at the surface.
SPEED OF LIGHT
Light travels about 300 million m/s which is fast enough to travel around
the Earth 7 times in one second!. But just as you can’t run or walk as fast in a
crowded hallway, light particles do not always reach their top speed.
Remember, light is electromagnetic radiation, which is transmitted in
waves. The particles in a medium, like the students in a hall, slow down the
passage of waves. Glass is more dense than air, so light travels slower in glass
than in air.
The effects of changes in speed can be seen below.
When light rays enter the glass
they slow down (glass is more
dense), and refracts
When light rays leave the glass
they speed up (air is less dense)
and refract again
 NOTICE THAT REFRACTION ONLY OCCURS at the boundary between 2
different media.
.
 The same thing would occur if the light was shone from the top (reversed)
The Index of Refraction
The amount by which a transparent medium decreases the speed of light is
indicated by a number called the index of refraction or
refractive index.
The larger the refractive index, the more the medium decreases the speed of
light.
Light travels the fastest in a vacuum. The refractive index of light in a vacuum
is assigned a value of 1.00. (This value can also be used for air since the
refractive index is so close to 1.00 (its exactly 1.0003)
Objects that slow down light have a refractive index above 1.0
i.e. diamond = 2.42
water = 1.33
alcohol = 1.36
The refractive index of a medium, n, is determined by comparing the speed of
light in the medium, v, with the speed of light in a vacuum, c. This leads to the
following
Index of refraction = speed of light in vacuum
Speed of light in medium
Or
n= c
v
no units (they cancel)
Sample Problem 1
The speed of light in a sample of glass is 1.91 x 108 m/s. The speed of light
in a vacuum is 3.00 x 108 m/s. What is the refractive index of the glass?
See p. 438 in textbook
Sample Problem 2
What is the speed of light in water given that water has a refractive index of
1.33?
See p. 438 in textbook
Hwk: Read pgs. #434-440 Do Practice Problems#1-2 at top & #1-2 at bottom.