Refraction & Lenses
Chapter 18
Refraction of Light
 Look
at the surface of a
swimming pool
 Objects look distorted
 Light bends as it goes from
one medium into another\
 Why?
 When
a medium causes a
wave to slow down it is more
optically dense
Entering more optically
dense
 Waves
moving into a more optically
dense medium will cause the wave
speed to slow down and then bend
toward the normal
 Therefore, the angle of refraction is
smaller than the angle of incidence
Entering less optically
dense
 Waves
moving into a less optically
dense medium will cause the wave
speed to increase and then bend
away from the normal
 Therefore, the angle of refraction is
larger than the angle of incidence
Snell’s Law
 Dutch
Scientist Willebrord Snell
discovered that a ray of light
bends in such a way that the ratio
of the sine of the angle of
incidence to the sine of the angle
of refraction is a constant, index
of refraction (n)
 ni sin i nr sinr
 ni
index of refraction for the
incident medium
  i angle of incidence
 nr index of refraction for the
refracting medium
  r angle of refraction
Index of refraction
 ratio
of speed of light in a
vacuum, c, to another medium, v
 n=c/v
 index of refraction in a vacuum is
1.00
 table on page 486
Problem Solving Strategy
 Draw
a diagram showing the
two media. Label them,
indicating the two indices of
refraction, ni and nr
 Draw a normal to the surface
 Use Snell’s law to calculate the
angle of refraction
Total Internal Reflection
 If
ray travels from a more optically
dense into a less optically dense
medium the angle of refraction is
larger than the angle of incidence
 Eventually the incidence angle is
so great that all the light reflects
back into the medium
 No refraction takes place
Critical Angle
 The
incident angle that
causes the refracted ray to
lie right along the boundary
of the substance
 Unique to each substance
Critical Angle
air
critical ray
water
c
Calculate the critical angle
Total Internal Reflection
 When
light enters a thin glass
rod light is internally reflected
(fiber optics)
 Telephone, computer and video
signals
 Explore the human body
 Plants use internal reflection
Effects of Refraction
 Mirages
 Summertime
- the sun hits to road
and causes the air above the road
to heat up
 The index of refraction for warm
air is 1.00026
 The index of refraction for cool air
is 1.00028
 This
small change in index of
refraction causes the rays to
bend
 This bending makes the road
look like there is a puddle on
it
 Because
light travels slightly
slower in Earth’s atmosphere than
in outer space the sun rays bend
causing the sun to reach us before
the sun is actually above the
horizon
 Same in the evening, the rays
bend and reach us after the sun
has actually set
Dispersion of Light
 Light
of all wavelengths travels at
the same speed in a vacuum
 Other media causes the speed to
slow down
 The wavelength also determines
the speed & index of refraction
 In
most materials red light
travels fastest
 It also has the smallest index
of refraction
 Violet is the slowest and has
the largest index of refraction
 Red is bent the least, violet
the most
Dispersion
 The
separation of light into a
spectrum by refraction
 Diamond
 Rainbow (water)