Transcript refl and refr, mirrors

Electromagnetic Waves
Physics 202
Professor Vogel
Lecture 13
Reflection and Refraction
Angles measured from
normal !!
Angle of incidence (q1)
Angle of reflection (q1’)
Angle of refraction (q2)
Law of Reflection
q1’ = q1
Law of Refraction
n2 sin q2 = n1 sin q1
General Cases
 n2 > n1
 Light is bent towards the normal
 q2 < q1
 e.g. air to glass
 n2 < n1
 Light is bent away from the normal
 q2 > q1
 e.g. glass to air
Index of Refraction
Every material has an index of
refraction that determines its optical
properties
Indices of refraction determine bending,
by law of refraction
Index of refraction also determines the
speed of light within the material
v=c/n
generally, v<c
Consequences of Laws
Law of Reflection
The angle of reflection is equal to the angle
of incidence (q1’ = q1)
Light changes direction when it reflects
but our vision expects light to travel on a
straight line
we see images where light appears to come
from
Plane Mirror
q1’ = q1
we see images where
light appears to come
from
Consider how light
from your eye reflects
from a mirror to get to
your eye, so you can
see it
repeat for light from
your feet
Images in Plane Mirror
Upright
Same distance behind mirror as object is
in front
Generally “front” is side light comes from.
Same size as object
Left and right are reversed.
Cardinal directions aren’t reversed.
Images in Plane Mirror
Based on depth perception.
Where do
both the
purple
and the
green rays
appear to
come
from?
Images in 2-D corner Reflector
Can see an
image in each
mirror
Light can also
reflect from both
mirrors, then
back to you.
You see image
of image 1
Also see image
of image 2
Both should be
same place
Images in 2-D corner Reflector
Image of image
reflected twice
left and right
exchanged twice
If you are righthanded, so is that
image!
Images in Curved Mirrors
Concave mirror
think “cave”
like bowl of spoon
image is upright and
larger – if object is close
image is inverted and
smaller – if object is far
Convex mirror
like back of spoon
image is upright and
smaller – any distance
Consequences of Laws
Law of Refraction
n2 sin q2 = n1 sin q1
Light changes direction when it refracts
but our vision expects light to travel on a
straight line
we see images where light appears to come
from
Demo
Chromatic Dispersion
 The index of refraction
depends on the
wavelength of light
 In general, n is larger for
shorter wavelengths
 Blue light bent more than
red
 Incident white light is
spread out into its
constituent colors
 Chromatic dispersion
with raindrops causes
rainbows
Chromatic
Dispersion