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


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


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


Subscribe to our Youtube Channel -

  1. Class 10
  2. Chapter 10 Class 10 - Light - Reflection and Refraction

About the Author

Davneet Singh's photo - Teacher, Computer Engineer, Marketer
Davneet Singh
Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 9 years. He provides courses for Maths and Science at Teachoo.