In an optical fibre, is the refractive index of the core greater than that of the cladding, or smaller?

Assertion: Diamond glitters brilliantly.
Reason: Diamond does not absorb sunlight.

$A$ ray of light passes from a denser medium to a rarer medium at an angle of incidence $i$. The reflected and refracted rays make an angle of $90^{\circ}$ with each other. The angles of reflection and refraction are respectively $r$ and $r^{\prime}$. The critical angle is given by:

Brilliance of diamond is due to

$A$ transparent block $A$ having refractive index $\mu_2 = 1.25$ is surrounded by another medium of refractive index $\mu_1 = 1.0$ as shown in the figure. $A$ light ray is incident on the flat face of the block with incident angle $\theta$ as shown in the figure. What is the maximum value of $\theta$ for which light suffers total internal reflection at the top surface of the block?

$A$ diver in a swimming pool wants to signal his distress to a person lying on the edge of the pool by flashing his waterproof flashlight. In which direction should he direct the beam?