Sensory cells of an eye are called as rod cells and cone cells. Why ? What are these concerned with?

The rods and cones are the modified forms of the sensory cells and their names are due to their shapes.
(a) The rods mainly enable the animal to see in the darkness, therefore, rods are present in large number in nocturnal animals.
(b) The cones are chiefly concerned with the distinction
in colour and light vision during day time.