Assertion : In optical fibre, the diameter of the core is kept small. Reason : This smaller diameter of the core ensures that the fibre should have incident angle more than the critical angle required for total internal reflection.

Step-by-step solution

Total internal reflection will be for rays incident at an angle greater than C . If the fibres are thin, only rays almost parallel will pass, further for nonparaxial rays, the angle of incidence is large and therefore they also will be totally internally reflected. Both are right.