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