2. Vision Those of us fortunate enough to have good vision usually consider information from vision to be more essential than our other senses, no other sense allow us to gather so much information from nearby and distant sources. To understand vision it is important to know how light works and how our eyes function

3. Light Light is electromagnetic energy, it is described in wavelengths. Not all light is visible to humans; in fact the light that we see makes up only a small part of the spectrum of electromagnetic energy. You have probably seen sunlight broken down into colors as it filters through water vapor, this what makes a rainbow. sunlight can also be broken down into colors by means of a glass structure called a prism. The main colors of the spectrum from shortest to longest wavelengths are: red, orange, yellow, green, blue indigo and violet.

5. The Eye When you take a picture with a camera, light enters through an opening and is focused onto a sensitive surface, the film. Chemicals on the film are changed by the light and create a lasting impression of the image that entered the camera. Your eye is very similar, as in a camera, light enters the eye and then is proyected on a surface. The amount of light that enters the eye is determined by the size of the opening in the colored part of the eye. That opening is called pupil (the black circles you see in the middle are the pupils, they may look solid for you but they are openings.

7. The size of the pupil adjusts automatically to the amount of light entering the eye. When the light is bright your pupils become small because the light is so bright that we only need a bit of it, on the other hand if there is not enough light the pupils become bigger to let in as much light as possible. Pupil size is also sensitive to our emotions, we can be literally “wide-eyed with fear”, that means that the pupils open widely when we are afraid.

8. Once light has entered the eye, it encounters the lens, the lens adjusts to the distance of objects by changing its thickness. Have you ever seen that people squint to look at something, that’s because they are adjusting the thickness of the lenses in their eyes.

9. These changes project a clear image of the object onto the retina. The retina is the sensitive surface in the eye that acts like a film in a camera. However the retina consists of neurons-not film. Neurons that are sensitive to light are called photoreceptors. Once the light hits the photoreceptors, a nerve carries the visual input to the brain, in the brain the information is relayed to the visual area in the occipital lobe.

10. Rods and Cones Rods and cones are two kinds of photoreceptors, rods are sensitive only to the brightness of light, they allow us to see in black and white, cones provide color vision. Dark and Light Adaptation Imagine that you enter a movie theater, it may look dark at first, as time passes you will be able to see the seats and other people more clearly. This adjustment to lower lightning is called dark adaptation, your ability to see in low light continues to improve for up to 45 minutes, on the other hand if you are in the darkness and move into the light, at first your eyes hurt, but a little while later, you get adapted, adaptation to bright light happens much more quickly than adaptation to the dark.

11.  Visual Acuity The sharpness of vision is called visual acuity. Visual acuity is determined by the ability to see visual details (in normal light). When people have their eyes checked, they have to read a chart called the Snellen Vision Chart. It is used to measure visual acuity. If you have the need to get particularly close to an object to see its details you are nearsighted. A person who is farsighted on the other hand, needs to be far away from an object than a person with normal vision. Have you seen that older people often hold newspapers or books at more of a distance from their eyes than younger people? As people reach middle age, their lenses become relatively brittle, therefore it is more difficult for them to focus, especially on nearby objects, as a result many people are farsighted