Let's understand how the human eye helps us in seeing objects. The eye is roughly spherical in shape.
- Light mainly falls on the cornea - the white clear front part of the eye. It is tough and protects the interior of the eye from accidents.
- The iris (a muscular structure behind the cornea) of the eye controls the amount of light reaching the interior of the eye by automatically adjusting the size of the pupil. (The iris is that part of the eye which gives it the colour such as black, brown, green eyes commonly called cat-eyes etc)
- The eye's crystalline biconvex shaped lens is located behind the pupil. It further focuses the light on the retina. The retina contains several nerve cells which convert the optical images to signals to the brain via the optic nerve.
- There are 2 types of nerve cells. (i) Cones : which are sensitive to bright light and help us recognize color (ii) Rods: which are sensitive to dim light.
- The brain then decodes this signal as the image we see!
Note : The light is focused by the cornea, then the iris by controlling the opening of the pupil and then the lens. In dim light, your pupil expands to allow more light to enter your eye. In bright light, it contracts.