Slight AdjustmentFor 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 Can measure the distance and speed of the

traveling object (E&M wave). Sonar. Ultrasound.

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 FrequencyIf the frequency of light is higher than this

frequency, it will get through the metal.Different metal has different plasma

frequencySilver, 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: the direction which is perpendicular to the surface.

Angle of incidence: the angle between normal and the incident ray

Angle of reflection: 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