Refraction through a Rectangular Glass Slab

Before looking at Refraction through a Glass slab

We look at what happens when

• Light travels from rarer to denser medium
• Light travels from denser to rarer medium

Rarer and Denser medium we will study in Refractive Index 

Some definitions

• Incident Ray
  Light Ray which travels into a Medium is called Incident Ray
• Refracted Ray
  Light Ray which bends after refracting is called Refracted Ray
• Normal Ray
  Ray which is perpendicular to Surface at Point of Intersection is called Normal Ray

Case 1

Refraction when Light Travels from Rarer Medium towards Denser Medium

Example

When Light travels from Air to Glass (Rarer Medium to Denser Medium)

It does not move in straight Direction

It bends towards the Normal

Case 2

Refraction when Light Travels from Denser Medium towards Rarer Medium

Example

When Light Travels from Glass to Air (Denser Medium to Rarer Medium)

it does not move in straight Direction

It bends away from Normal

Now, lets look at

Refraction through a Glass Slab

In this case, Light First Travels from Air to Glass and then Back from Glass to Air

Hence there are 2 refractions

First, the ray of light travels from air to glass.

Hence , it travels from a rarer medium to a denser medium.

So, the First Refracted Ray bends towards Normal

After travelling in the glass slab

The ray of light travels from glass to air

Hence, it travels from a denser medium to a rarer medium.

Therefore, the Second Refracted Ray (called Emergent Ray) bends away from Normal

Combining the two refractions in the above case, the refraction of light through a glass slab can be represented as

Now, we extend the original incidence ray and bring it towards emergent ray

We observe that, the original incident ray and the emergent ray are parallel to each other.

The angle made by the emergent ray with the normal is called the angle of emergence.

And the Perpendicular distance between the 2 rays is called Lateral Displacement

Why does a light ray incident on a rectangular glass slab emerges parallel to itself?

The ray of light emerges parallel to itself because

the bending of the ray of light on top face AB (air-glass interface) 

is equal and opposite to 

the bending of the ray of light on bottom face CD (glass-air interface)

So, the ray of light emerges parallel to itself

Thus, Angle of incidence = Angle of Emergence