Camera and Human Eye: We know that if an object is lying at a distance more than the focal length of a convex lens, its real and inverted image is formed on the other side of the lens. The image through the eye and camera is formed in the same way. Let us compare the structure and function of both of them.

Camera

Camera is a kind of box to which a convex lens is mounted on the front side. The lens forms a real and inverted image of an object on the sensitive film placed behind it. A system is provided in the camera to move the lens back and forth so that sharp image is obtained on the film. There is shutter behind the lens that remains close normally. When the button is pressed, the shutter opens for a while. Light coming from the object enters the camera during this interval and images formed on the film. The amount of light entering into the camera depends upon the size of aperture. Aperture is an opening in the diaphragm behind the lens. This can be made smaller or larger as required. The picture is obtained by developing the image on the film.

Camera diagram

 

Human Eye

The human eye also works like a camera. Different parts of eye.

Different parts of eye

 

The eye is almost a sphere of diameter about 2.5cm. Its outer boundary called the sclera is thick and hard. At the front of the eye , there is a transparent hard skin known as cornea.

Behind the cornea there is iris and after that there is convex lens. The inner layer of the back wall of eye is called retina. The retina of eye and the film of camera serve the same purpose. Like camera, the eye lens forms a real and inverted image of the object on retina.the optic nerve carries it in the form of signals to the brain. Although the image formed on the retina is inverted, but our brain interprets this correctly I.e. the right way up.

ciliary muscle relaxed, lens flattened for distant vision

ciliary muscle relaxed, lens flattened for distant vision

The iris acts like the diaphragm of camera. The opening at the center of iris is called pupil which is just like aperture of a camera. When light outside is dim, the iris contracts to make the pupil larger so that more light can enter the eye. In bright light, the iris makes the pupil smaller.

In a camera, lens is moved back and forth to focus the image on the film, but the eye lens does not move. Instead, the ciliary muscles make the lens thick or thin due to which its focal length changes. When you are looking at distant object, the ciliary muscles are is relaxed position and the image is formed on the retina. To look at something closer to the eye , these muscles make the lens thicker. This makes its focal length shorter and the image is again formed on the retina instead of forming at a point beyond it.