為何光波在反射後會有180度的相位改變?

為何光波在反射後會有180度的相位改變?(由光密界質去光疏界質)
Light wave is electromagnetic wave. It composes of time varying electric and magnetic field components. When light wave incident onto a surface, the electric filed component causes electrons of atoms in the reflecting surface to vibrate and absorb energy from the light wave. The vibrating eelctrons then re-emit the absorbed energy in the form of electromagnetic wave. This is the reflected light.

When light is incident onto an optically dense medium, the electrons are more reluctant to vibrate than when light incident onto an optically less dense medium, because of elecrons are more tightly held in the optically dense medium. As a result, there is a phase difference between the vibration of the electrons and the incident light wave. Hence, the reflected light wave would experience a phase lag with the incident wave. This is observed as a phase difference of 180 degrees in the reflected wave.

聲波在反射後又會不會有相位改變呢?
The phenomenon of phase shift occurs when waves are reflected by the surface of a medium where the speed of the wave in it is lower than that in the incident medium.

For light, an optically dense medium has speed of light lower than that in an optically less dense medium. Hence, there is a phase shift on reflection. Such phenomenon is true for any waves. That is to say, it applies to sound wave, wave on a atretched string etc.

有的話, 是否由光疏界質去光密界質?
The propagation of sound is not related to the optical densityof a medium. Sound and light are waves of two completely different nature. Sound wave is a kind of mechanical waves.

As said above, the phase shift phenomenon applies to sound wave when it is reflected from a medium within which the speed of sound is lower than in the incident medium. Since the speed of sound depends on the density of the medium, hence there is a phase shift to the sound wave when it is reflected from hot air (lower density, higher speed of sound) onto cold air (higher density, lower sound speed).

thats right....

when a pulse directly to the fixed point,
the pulse will have 180 degrees phase difference in reflection

http://www.youtube.com/user/makseyuen2#p/u/39/75bHcNPz2e0
http://www.youtube.com/user/makseyuen2#p/u/39/75bHcNPz2e0

2010-02-28 21:38:32 補充：
http://www.youtube.com/user/makseyuen2#p/u/41/drWy5a5EB5M

p3004962, this is a "dead end" reflection
which introduces a phase shift of pi...
it is essential for forming a standing wave in some cases....

照你講應該係折射(refraction)，而唔係反射(reflection)。