Question

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Answer 1

The main concept involved here is that of Total Internal Reflection (TIR).



As light moves from an optically denser medium to an optically rarer medium, it moves away from the normal.

So, as the angle of incidence increases, the angle of refraction also increases.

For a specific angle of incidence (called the Critical Angle), the angle of Refraction becomes 90°

At angles of incidence above this critical angle, light gets reflected back into the denser medium.



Here, in the question, a glass slab is placed on a point light source.

A light source emits light in all directions.


So there are light rays coming to the other surface of the slab at every angle.


However, we just saw that if the angle of incidence at the other surface is greater than the critical angle, light won't come out from the surface.

This implies that all light doesn't escape.


The light, whose angle of incidence is less than critical angle can only escape.

Thus, the light comes out only from a circular region on the other surface of the slab.


The boundary of this circle defines the critical angle of incidence.


The solution is shown in the image.

Final Answer:

Radius of Circle is 6 cm.

Answer 2

.$\textsf{\red{Answer}}$

tan C = R8                         ......................(i)tan⁡ C = R8                         ......................(i)

   .    43 sin C = 1.sin90o43 sin⁡ C = 1.sin⁡90o

          sinC=35sin⁡C=35

          C= 37oC= 37o

          34=R834=R8
 
         R=6 cm..