Refraction
And you
Parallel situation
• Imagine a cliff diver leaping off a tall cliff
• As they move through the air they move
fast
• As they hit the water their speed slows
down because the density of the water is
greater than the density of the air
Light
• Think as light as a cliff diver. As it moves
through a vacuum it moves fast
(3.0x108m/s)
• As it moves though air, it is about the
same speed
• As it moves through more dense materials
it slows down (just like the cliff diver)
What happens to the light as it
slows down???
• As you recall speed is distance/time
• Therefore, if light slows down, it travel a
smaller distance per unit of time
• This results in the light changing direction
so that it can travel a shorter path
Shorter distance in glass = bend
toward normal
To clarify:
• When moving from a low to high density
material light will slow down and bend
toward the normal (less distance per unit
of time)
• When moving from a high to low density,
the light will move faster = longer distance
per unit of time = bend away from the
normal
Index of refraction (n)
• = the ratio of the speed of light in a
vacuum to the speed of light in the
material it is passing through
Examples please
• The index of refraction for air =
• n air = 3x108 / 3x108 = 1
• Wow that is really easy!!!!!!!!
Other materials
• n water = 1.33
• Therefore the speed of light as it travels
through water would be found by:
• Rearranging the equation:
And the answer is:
More materials please
• n ethanol= ? If C ethanol = 2.21x108m/s
• Here we simply use our equation:
• n ethanol = C vac/ C ethanol =
`
•1.36 !!!!!!!!!!!!!!!!!!!!!
But why does the light bend?
• Rather than just slow down when it
reaches the interface of different media.
• Think of a car that is moving down the
road at a fast speed. If one of the front
wheels slows down, the car will turn in the
direction of that wheel
Some valuable values
•
•
•
•
•
n air = 1.0003
n water = 1.33
n diamond = 2.42
n glass = 1.5
n plastic = 1.46
Snell’s law
• n = sin θi / sin θr
Therefore: ni sin θi = nr sin θr
One last thing
• When light strikes the more dense material
the frequency of the waves stays the same
– but it is moving slower – so the
wavelength must change.
Think about it like this:
• You have a stream of people going into a
building at a constant speed. The people
are spaced 1 m apart. When they enter
the building it is dark so they slow down.
There will be a back up of people at the
door unless the people inside get closer to
each other as they move through the
building.