There are three ways that light can interact with matter:
1. it can pass through matter. This occurs in such matter as air, glass or water. If light can pass through matter, the matter is said to be transparent.
2. it can bounce off of matter. If light bounces off of matter, it is said to be reflective. We can see things because light is reflected by matter. Dark colored objects do not reflect as much light as light colored objects. Snow and sand, for example, reflects so much light it can give a bad sunburn. Snow reflects so much light that it can actually damage the eyes causing snow blindness.
3. light can be absorbed by matter. Dark colored objects absorb more light than light colored objects. Because of this, solar collectors are usually black. Because dark colors absorb more light, they also absorb more heat associated with light. This is why black upholstery in a car can get very hot when exposed to the sunlight.
Matter which reflects and absorbs light are said to be opaque. Most solids are opaque. Examples of opaque objects are you and me, books, desks, pencils and mirrors. Matter which allows some light to pass through, but reflects and absorbs some light, is said to be translucent.
A mirror is very opaque. Mirrors reflects so much light, that you can see the image of anything which faces it. Mirrors are usually made of silver, aluminum or other shiny surfaced materials.
A flat mirror will give a true reflection, meaning the reflection is nearly the same as the true shape of the object whose image is reflected.
A convex mirror curves outward in the center. Convex mirrors distort the image by making it appear to be smaller than it really is. However, a convex mirror can be used to see a larger area. Stores often use convex mirrors to observe large areas for shoplifting. Cars which have convex rear view mirrors to give the driver visibility over a larger area have warnings that objects may be closer than they really are.
A concave mirror has a surface that curves inward in the center. Concave mirrors distort the image by making it appear larger that it really is. These mirrors are often used as shaving and make up mirrors, because they magnify the image. However, they reflect a smaller area.
Light travels in a straight line as long as it is moving through one substance. However, when it passes from one substance to another, it changes direction or bends. The bending of light as it moves from one substance to another is called refraction.
Curved transparent objects known as lenses form images through refraction. The light refracts (bends) as is passes from one substance--air, to another--the lens. Lenses are found in eye glasses, cameras, binoculars, telescopes and magnifying glasses.
Convex lenses, like convex mirrors curve outward in the center. However, the effect is just the opposite of a convex mirror. While convex mirrors make things appear smaller, convex lenses magnify things. Concave lenses are lenses that are thinner in the center that at the edges, just like concave mirrors. However, they too are just the opposite of a concave mirror; they make things appear to be smaller than they really are.
We are able to see things because these things reflect light. The reflected light enters the eye by passing through the tough, transparent tissue of the eye called the cornea. The cornea is the part of the eye which one could touch. The cornea refracts the light, bending it inward toward the eyeball. If a person has a warped (irregularly shaped) cornea, they have a visual defect called an astigmatism. From the cornea the light passes through a chamber filled with a clear fluid. This chamber is between the cornea and the lens. This chamber is called the aqueous chamber, and the fluid is called the aqueous humor. The aqueous humor is supplied by a small canal, and drained by another small canal. If the drainage canal gets blocked, the aqueous humor builds up high pressure in the eyeball. This creates the visual condition known as glaucoma. If untreated, glaucoma can produce a gradual loss of vision. Glaucoma is detected by checking the pressure in the eyeball.
After the light passes through the aqueous humor, it passes through the pupillary opening. The pupillary opening is created by the colored part of the eye called the iris. The iris is a smooth muscle which responds to light. If the light gets bright, the iris expands to make the pupillary opening smaller. If the light gets dim, the iris constricts to make the pupillary opening larger. When the pupillary opening is large, the pupil is said to be dilated. The function of the iris, and its pupillary opening, is to regulate the amount of light which enters the eye.
After the light passes through the pupillary opening, it then passes through the lens. The lens is a transparent sac of jelly which focuses the light on the back part of the eyeball. The lens does this by changing its shape--becoming fatter or thinner. This process of changing shape to focus light is called accommodation. In some older people, the lens loses its ability to change shape (accommodate) creating the visual defect called presbyopia.
The lens turns the image upside down. This upside down light image then passes through the large chamber of the eye called the vitreous chamber, filled with a cloudy, translucent substance called the vitreous humor. The vitreous humor occupies more volume of the eye than any other structure.
After passing through the vitreous chamber, the light strikes the back, photosensitive lining of the eye called the retina. The retina is composed of two types of specialized nerve cells: rods and cones. Rods see only blacks, whites and shades of gray. They are used primarily for night vision. The cones see color, and are used primarily for day vision. The rods are concentrated in the periphery of the retina. Therefore, you can spot dim lights at night best if you do not look directly at them. The cones are concentrated in the center of the retina in an area called the fovea centralis. Therefore, you can see things best during the day by looking directly at them.
Where the rods and cones converge to form the optic nerve there are no rods and cones. Therefore we have a blind sport in both of our eyes. However, the blind spot is not in the center of our visual field. Also, because we have two eyes, what is not seen by the blind spot of one eye, will be seem by the other eye, so the blind spot is seldom a factor in vision.
The brain receives an upside down image of the world. It may turn it right side up, but it doesn't really matter because vision is relative. If we never saw right side up, we would not know the difference. Visual images are realized in the occipital lobes of the brain.
Nearsightedness is when the eyeball is too long and the lens focuses the image in front of the retina. A nearsighted person sees close objects clearly, but distant objects are blurred. Nearsightedness often comes with increasing age because gravity tends to flatted and elongate the eyeball. The scientific name for nearsightedness is myopia. Nearsightedness can be corrected with a concave lens.
Farsightedness is when the eyeball is too short. The lens focuses the image behind the retina. The scientific name for farsightedness is hyperopia. Farsighted people can see things clearly at a distance, but things close to them are blurred. Farsightedness can be corrected with a convex lens.
The white layers of tissue which make the eye appear white are called the sclera.
Tear glands are called lacrimal glands. Tears lubricate the eyes, clean the eyes, and they have a substance called lysozyme which is an antibacterial.
A cataract is a disease in which the lens loses its transparency and becomes more opaque. Cataract surgery involves replacing the lens.
Radial Keratotomy, also known as RK, is surgery to change the shape of the eyeball. RK is usually done to correct nearsightedness. This type of procedure can also be done with lasers.
Color blindness is usually a hereditary disease that affects
only males. The genetic defect is carried on the 23rd pair of
chromosomes (sex chromosomes). The trait is carried by a symptom
free female, but the disease occurs only in males. There are,
however, rare forms of color blindness which do occur in females.