Refraction
NOTE
REFRACTION





the bending of light at the
boundary of two transparent
substances (or “media”)
occurs because different
materials slow the speed of
passing light at different rates
e.g. the speed of light in glass is
less than the speed of light in air
Light travels fastest in a vacuum
at 3.00 x 108 ms-1.
We’ll assume air is approximately
the same as a vacuum.
2
3
FYI
LIGHT SPEED IN DIFFERENT MEDIA
MEDIUM
air
ice
water
vegetable oil
glass
ruby
diamond
SPEED (km/s)
300 000
229 000
226 000
204 000
197 000
170 000
124 000
4
NOTE
INDEX OF REFRACTION (n)
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
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number which indicates how much the speed of light
will decrease when entering a certain medium
i.e. it is impossible for light to move at top speed
when particles get in the way
the larger the “n”, the more the medium decreases
the speed of light
since speed of light is fastest in the vacuum of space
(where there are very few particles), n = 1.00 (no
units)
speed of light in vacuum (c)
Index of Refraction (n) =
speed of light in medium (v)
5
NOTE
n FOR SELECTED MEDIA
vacuum
air
CO2 gas
water
alcohol
Pyrex glass
Plexiglas
table salt
flint glass
sapphire
cubic zirconia
diamond
gallium phosphide
1.00
1.0003
1.0005
1.33
1.36
1.47
1.49
1.51
1.61
1.77
2.16
2.42
3.50
Index of Refraction Trends
• The materials in which light travels fastest
are the least optically dense materials.
• The materials in which light travels slowest
are the most optically dense materials.
• As the index of refraction value increases,
the optical density increases, and the
speed of light in that material decreases.
7
FYI
LESS
DENSE
MORE
DENSE
TOWARDS
NORMAL
When light travels from a less dense medium to a more
dense medium it travels towards the normal
8
FYI
MORE
DENSE
LESS
DENSE
AWAY FROM
NORMAL
When light travels from a more dense medium to a less
dense medium it bends away from the normal
“Broken Pencil”
• What happens when a pencil is moved across
the middle of a glass of water?
• Why is this phenomenon observed?
• How does the refraction of light cause the
pencil to appear fatter and shifted to the
side?
Broken Pencil Conclusion
• Answer: The light rays leaving the pencil are
broken at the water surface. Our eye is
unaware of this, so it sees the rays running
straight forward. So we think the pencil lies in
the extension of the straight line from our eyes
Snell’s Law
• Snell was a wave theorist and he discovered
that the bending of light followed a pattern:
𝑣1 𝜆1
𝑠𝑖𝑛𝑖 𝑛2
𝑣2
=
𝜆2
=
𝑠𝑖𝑛𝑅
=
𝑛1
• Where i is the angle of incidence and R is the angle of
refraction, v is the speed of light, λ is the wavelength
and n is the index of refraction.
Examples
1) When light passes from air into water at an
angle of 60 from the normal, what is the angle of
refraction?
Examples
2) When light passes from water into a diamond
at an angle of 45 from the normal, what is the
angle of refraction?
Examples
3) A ray of light approaches a jar of honey at an
angle of 30. If the angle of refraction is 19.5,
what is the refractive index of honey?
Examples
4) The refractive index of the lens in the human
eye is 1.41. If a ray of light goes from the air
into the lens at an angle of 55, what is the angle
of refraction?
Examples
5) A block of amber is placed in water and a
laser beam travels from the water through the
amber. The angle of incidence is 35 while the
angle of refraction is 24. What is the index of
refraction of amber?
Examples
6) When light passes from air into water at an
angle of 30 from the normal, what is the angle
of refraction?
Examples
7) A red laser beam travels from flint glass into
lemon oil. The angle of incidence is 40 and the
angle of refraction is 44.4. What is the
refractive index of lemon oil?
Examples
8) In an experiment, a block of cubic zirconia is
placed in water. A laser beam is passed from the
water through the cubic zirconia. The angle of
incidence is 50, and the angle of refraction is
27. What is the index of refraction of cubic
zirconia?
Critical Angle and TIR
• A special angle exists when light moves from
a more dense to a less dense medium.
• As the angle of refraction moves away from
the normal, the critical angle yields an angle
of refraction of 90 degrees!
• The is the property that allows total internal
reflection to occur, which is used in fibre
optics.
• If the critical angle is exceeded, the light ray
reflects and obeys the law of reflection: i = r
Example
• Determine the critical angle of diamond (n =
2.42).
Let’s See it Again!
• http://www.youtube.com/watch?v=xwzw5Ghp
C_0&NR=1