Eye Structure

• Your eyes are powerful devices that send messages to your brain so that you can see what is in front of you. The entire process of how vision works is very complex, but there are some basics that you should know. The first part of the vision process begins with light. Light is required to bounce off an object and into your eyes. So it is not actually the object you see, but the light that reflects off it. The structure of the eye is maintained by the sclera, which makes up the white of your eye. It holds everything in place and protects the delicate inner workings of the eye.
  
  

Light Enters the Eye

• The inner workings of the eye National Eye Institute/US gov public domain
  
  Light enters the eye through the cornea, which is the clear, rounded circle on the surface of the eyeball. The rounded surface of the cornea allows the light to bend and enter the eye. Next, the light travels through the pupil, the black circle that is in the middle of the eye, underneath the cornea. It looks like a spot but is really a hole. This hole is surrounded by the colored circle, called the iris. The iris can expand and contract the size of the pupil, controlling how much light goes in. In dim light, the pupil opens larger to let more light in. When light is too bright, the iris expands, making the pupil smaller and letting in less light.
  
  

From Light to Images

• Once the light goes through the pupil, it passes through the lens, which is behind it. The lens focuses the light and makes things look sharper and clearer. Light continues to travel toward the back of the eye, through the vitreous cavity, which is the jelly-like substance that transmits light and helps the eye maintain its shape. The light then travels to the back of the eye, where it is met by the retina. The retina forms a thin tissue inside of the eyeball and is filled with photoreceptors that receive light signals. These photoreceptors are called rods and cones. Light also hits the macula, a nerve-dense part of the retina that helps you see fine detail. The retina converts light into electrical signals. It then relays these signals to the optic nerve, the bundle of nerves that connects the eyeball to the brain. The retina actually perceives the image upside down, but the brain flips it back over so you see things right side up.