Eye contain receptors called photoreceptors viz rods and cone cells, which convert the energy of specific wavelength into action potential of nerve fibre. The retina has about 6 million cones and 120 million rods.

rod and cone
Cones are cone shaped structures and are required for bright light (day light) vision. Rods are rod like structures and are required for dim light (twilight/ night) vision. Both rods and cones contain light sensitive pigments. Rod cells contain a purplish pigment known as visual purple or rhodopsin and it is formed from vitamin A. They function in dim light and at night as bright light bleaches rhodopsin, making  rods ineffective. On the contrary, cones contain a violet coloured pigment said to be visual violet or iodopsin. They function in day light and produce detailed images and give colour vision. Stimulation by light causes some photochemical reaction within the rods and cones.
Rods vs Cones
Rod cells are far more numerous than cone cell. Cones cells are far fewer than rod cells.
Usually located around the periphery of retina. Usually located in the centre of retina.
All rod cells are alike, and do not give colour vision. Cones are of three types: blue, green and red, and give colour vision.
Sensitive to dim light and give twilight vision. Sensitive to bright light, and give daylight vision.
Outer segment is cylindrical and contains rhodopsin. Outer segment is conical and contains iodopsin.
Inner end has a small knob. Inner end is branched.
Insufficient rhodopsin  results in night blindness. Insufficient iodopsin results in colour blindness.
Arranged in functional units served by one bipolar neuron, therefore, acuity low. Each cone served by its own bipolar neuron, therefore , acuity high.
Rapid regeneration of light sensitive pigment, therefore, can perceive flicker well. Slower regeneration of light sensitive pigment, therefore, less response to flicker.

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