refractive index

1. Snell's law 係唔係只用於light??? (n1sin θ1 = n2 sin θ2 )

Light is electromagnetic wave in nature. Snell's Law applies not only to light, but to all waves in the electromagentic spectrum. However, you need to note that the refractive index of a medium is a function of frequency. That is, the value of refractive index for light cannot be used at other frequencies.

The basic principle of Snell's Law can also apply to other waves, e.g. sound wave.

2. 係唔係所有waves(包括光)穿過medium 1 同medium 2 都follow以下:
speed1 / speed 2 = sin θ1 / sin θ2 = n2/n1

Yes.

3. 我知道當waves由medium 1 去到medium 2, 會有個:
n(1->2) = speed1 / speed 2 = λ1 / λ2, 其實n(1->2) 係唔係等於n2/n1 ??? 同埋n(1->2) 係唔係等於sin θ1 / sin θ2 ???

If medium 1 is air, its refractive index n1 = 1. The your general equation:
speed1 / speed 2 = sin θ1 / sin θ2 = n2/n1
reduces to: speed1 / speed 2 = sin θ1 / sin θ2 = n2

In other words, n2 is just n(1-->2), the refractive index from air to the medium.

You should note that when we say the refractive index of a medium (e.g. water), this means that it is the refractive index for light to travel from air to the medium.