Reflection and Refraction
Reflection
The Law of Reflection
i   r
Law of Reflection – the angle of incidence and
the angle of reflection are equal
The Law of Reflection
 Reflection - a wave bounces back into the first
medium when hitting a boundary of a second
medium
 Our brain thinks light travels in straight-lines.
 Angle of Incidence – angle made by the incident ray
and the normal
 Angle of Reflection – angle made by the reflected ray
and the normal
Reflection of Sound
 An echo is reflected sound
 Sound reflects from all surfaces of a
room
 Acoustics is the study of the way sound
reflects off of objects in a room
 Reverberations – Multiple reflections of
sound within a room
 The walls of concert halls are designed
to make the reflection of sound diffuse
Ex: Sitting in her parlor one night, Gerty sees the reflection of her
cat, Whiskers, in the living room window. If the image of
Whiskers makes an angle of 40° with the normal, at what angle
does Gerty see him reflected?
Reminder: Speed of Light
What speed does light travel at in a vacuum? In air?
3 x 108 m/s
Does light speed up, slow down, or stay the same
speed when it travels through different media?
Light travels fastest in a vacuum and
slows down in water, glass, and other
media
Refraction
 Refraction: Bending of waves as
they change speeds.
 This change in direction is caused
by the fact that light travels at
different speeds in different media.
 The imaginary line (perpendicular
to surface) to which we measure
the angles is called the normal line.
Refraction
Snell’s Law
 Relationship between the angle of incidence (θ1) and
angle of refraction (θ2) can be explained by Snell’s
Law.
sin 1 v1

sin 2 v 2
 Speed of light is greatest in a vacuum (c = 3 x 108
m/s), so it is convenient to compare to this value.
This ratio is called the index of refraction (n).
c speed _ of _ light _ in _ vacuum

n 
v
speed _ of _ light _ in _ medium
Snell’s Law and Refraction
 Another way to write Snell’s Law:
n1 sin 1  n2 sin 2
 Crossing a boundary, the frequency of a wave does
not change (to conserve energy). θ, v, and λ do
change.

 Our brain thinks light travels in straight lines. Ex:
We see the fish shallow and far from the boat.
Refraction
Ex: Hickory, a watchmaker, is interested in an old timepiece that’s
been brought in for a cleaning. If light travels at 1.90 x 108 m/s in
the crystal, what is the crystal’s index of refraction?
Ex: While fishing out on the lake one summer afternoon, Amy
spots a large trout just below the surface of the water at an angle
of 60.0° to the vertical, and she tries to scoop it out of the water
with her net. a) Draw the fish where Amy sees it. b) At what
angle should Amy aim for the fish? (nwater = 1.33).
 Ex: Water has an index of refraction (n) of 1.33.
a) Find the speed of light in water.
b) Light from the tuna’s snout heads to the surface at 380 and refracts to
Carl’s eye. Find θ.
c) Extend Carl’s line of sight and sketch an imaginary tuna at the location
that he envisions it.
Mirrors and Lenses
Lenses
 Lens – a piece of glass which bends parallel rays
so that they cross and form an image
 Lenses work because light bends as it goes from
the air into the glass. (THINK: Refraction)
 Eye glasses and telescopes use lenses to help us
see better.
Eyes
 Your lenses in your eyes change size all the time.
 When you look at objects real close up, the lens gets
thicker. If you look at objects far away, it gets thinner. It
does this to help you focus the correct image on the
retina.
 After light passes through the lens it shines through the
vitreous humor to the back of the eye where it hits the
retina. The retina takes the light and changes it into
nerve impules so the brain can understand what the eye
sees.
 In both the camera and the eye, the image is upside
down, our brain flips the image right-side up for us.
Mirrors
 Your eye cannot tell the difference between an
object and its virtual image
 Virtual Image – the point located behind a mirror
where an object appears to originate
 The image is as far behind a mirror as the object is
in front of the mirror
Mirror Image