Laws of Reflection and Refraction

Laws of refraction:

(i) The incident wave front AB, the refracted wave front CD and the refracting surface XY all lie in the same plane.

(ii) Angle of incidence

i = ∠PAN =− ∠ NAB = ∠BAC

Angle of refraction

r = ∠N₁AD = − ∠ DAC = ∠ACD

₁μ₂

₁μ₂ is called the refractive index of second medium with respect to first medium. This is Snell’s law of refraction.

If ₁μ₂ > 1, the first medium is rarer and the second medium is denser.

Then . This means that the velocity of light in rarer medium is greater than that in a denser medium.

Refractive index of a medium μ_m is given by

Since only wavelength changes on refraction,

where and are the wavelengths in medium and air respectively.

Laws of reflection:

(i) The incident wavefront AB, the reflected wavefront CD and the reflecting surface XY all lie in the same plane.

(ii) Angle of incidence

i = ∠ PAN = − ∠ NAB = ∠BAC

Angle of reflection

r = ∠ NAD = − ∠ DAC = ∠DCA

For right angled triangle

∠B = ∠ D =

BC = AD and AC is common

∴ The two triangles are congruent

∠ BAC = ∠DCA

i.e. i = r

Thus the angle of incidence is equal to angle of reflection.