The human eye
lets light in through a hole called the pupil. A lens inside the eye focuses the image, and the retina detects that image—you might picture the retina as the wall a projector puts the image on. The retina contains two light sensitive cells called rods and cones that detect light and send the image to the brain.
While the human eye sees with these two groups of light-sensitive cells, these cells operate very differently. Cones are used during daylight when the visibility level is high, and these provide color perception. Rods are used at night when there are lower visibility levels, but Rods provide no significant color perception.
The eye sees very differently during the day, as compared to the night. The Rods are more numerous, some 120 million, and are more sensitive than the Cones. This contrasts with the 6 to 7 million Cones that provide the eye's color sensitivity.
Rod cells can only see black and white, but remain very sensitive even in very low night light. The lowest level of white light that can be detected by the Rod cells at night must be increased 1,000 times in order for a Cone cell to see it.