Huygens wave theory is used

Explanation

Laws of refraction
First law of refraction. Let XY be a plane refracting surface and AB be a plane wavefront incident on the surface as shown in the figure.

According to Huygens. principle, every point on wavefront AB is a source of secondary wavelets and the time during which wavelet from B reaches C, the refracted wavelet from A would arrive at D.
i.e. `t=(BC)/(v_(1))=(AD)/(v_(2))` ...(i)
where `v_(1)` is the velocity of light in medium 1 and `v_(2)` is the velocity of light in medium II.
Eq (i) can be written as
`(BC)/(AD)=(v_(1))/(v_(2))` ...(ii)
In rt `/_ Delta ABC`,
`sin i=(BC)/(AC)`
or BC = AC sin i
Similarly in rt `/_ Delta ADC`,
AD = AC sin r
So Eq (ii) becomes
`(AC sin i)/(AC sin r)=(v_(1))/(v_(2))=(cv_(1))/(cv_(2))=(c)/(v_(2)).(v_(1))/(c)`
or `(sin i)/(sin r)=(mu_(2))/(mu_(1))=mu`
This is Snell.s law and is called first law of refraction.
Second law of refraction. From the figure, we find that, the incident ray and the refracted ray and the normal at the point of incidence, all lie in the same plane XY. This proves second law of refraction.