Question

During refraction of light through the glass slab ,incident ray and emergent ray are parallel to each other."explain.

Answer 1

when the incident Ray enters the glass slab it bents towards the normal as glass slab is denser than air . And when the Ray enters air again it bents away from the normal and parallel to the incident ray

Answer 2

Let us consider the light was initially in air medium.

The light incidents on the glass-air refracting surface .Let the angle of incidence is i .

Let the angle of refraction is r .

As per snell's law,    $\frac{sin i}{sin r} =\mu_{g}^a$   [1]

Here, $\mu_{g}^a$ is the refractive index of glass with respect to air.

The light will be refracted by the first surface of the glass. It will travel inside the glass. After travelling inside the glass, it will again undergo refraction. Now, the refraction will occur from glass to air.

The angle of incidence for glass to air will be equal to  the angle of refraction of air to glass.

Hence, i = r for glass to air boundary .

Let the ray is emerged at an angle e .

Putting Snell's law again, we get-

                         $\frac{sin r}{sin e} =\mu_{a}^g$    [2]

Here, $\mu_{a}^g$ is the refractive index of air with respect to glass.

We know that $\mu_{2}^1\times \mu_{1}^2=\ 1$

Multiple 1 with 2 .

                        ⇒ $\frac{sin i}{sin r}\times \frac{sin r}{sin e} =\mu_{g}^a\times \mu_{a}^g$

                        ⇒ $\frac{sin i}{sin e} =1$

                        ⇒ $sin i\ =\ sin e$

                        ⇒ i = e

Hence, incident ray and refracted ray are parallel to each other.