Transcript Homework Set 2:

Slight Adjustment
For Homework Set 2
From “Seeing the Light” Chapter 2:
(starting page 68)
P2, P5, P12, P14,
P18, P21, P23, PM2
Wave Reflection
When a wave travels, it reflects
whenever there is a change of
medium. The speed of wave will
also change.
 When a rope is tied to a wall, a
traveling wave gets totally reflected,
with 180 degree phase change.

Hard Reflection. When a rope is
connected to a heavier rope, some
wave travels through (small
transmission). The reflection is again
180 degree inverted.
 Soft Reflection. When a rope is tied
to a thin rope, the reflected wave
has 0 degree phase difference.
There is also a transmitted wave.
 The amount of reflection depends on
how much the media have changed.

Examples of Wave Reflection

Radar


Sonar. Ultrasound.


Can measure the distance and speed of
the traveling object (E&M wave).
Used by Bats, Dolphins,Seals….(sound)
Ionosphere
Plasma created by sun light, Reflects
radio waves
 High at night, low during the day.

Reflection from metal
Have a lot of free electrons.
 When E&M waves incident on the
metal, the electrons respond,
reflecting almost all of them
(opaque).
 If the frequency is too high and the
electron cannot move fast enough,
the waves do get through.


Plasma Frequency
If the frequency of light is higher than
this frequency, it will get through the
metal.
Different metal has different plasma
frequency
Silver, Gold, Copper…
Mirrors
Silver has almost 100% reflection.
 But exposed silver tarnishes due to
chemical reactions.
 Plate silver on the back of glass:
mirror, the glass protects the silver
surface. There is some reflection
also from the surface of the glass.

Half-Silvered Mirror
If the silver is made thin enough,
some of the light will penetrate
through: You can see through a thin
layer of silver!
 Half-silvered mirror: half of the light
get through and another half get
reflected. (two-way mirror).

The law of reflection

Normal of the surface:

Angle of incidence:

Angle of reflection:
the direction
which is perpendicular to the surface.
the angle between
normal and the incident ray
the angle between
normal and the ray of the reflect light

Law of reflection:
Angle of incidence = Angle of
reflection (optical board)
Law-of-reflection diagram
Θi = Θr
Animation:
When a ray of light
strikes a plane mirror,
the light ray reflects off
the mirror. Reflection
involves a change in
direction of the
light ray. According to the
law of reflection, the angle
of incidence equals the
angle of reflection.
Plane Mirror Image
You stand in the front of a mirror
and you see yourself in the mirror.
What you see is the reflected light
from your body by the mirror, going
through your eyes.
 How does the light get into your
eyes? One can determine this by the
law of reflection.
 The technique is called ray-tracing.

Interpretation of your brain!
How are Images formed?
Man and Image
Plane Mirror Image: play
with a java applet
http://www.phy.ntnu.edu.tw/java/optics/mirror_e.html