Unit 29

Sight & Sound

The best way to understand the behavior of light through a curved lens is to relate it to a prism. A prism is thicker at one end, and light passing through it is bent (refracted) toward the thickest portion.

A lens can be thought of as two rounded prisms joined together. Light passing through the lens is always bent toward the thickest part of the prisms. To make a minus lens (above on the left), the thickest part, the base, of the prisms is on the outer edges and the thinnest part, the apex, is in the middle. This spreads the light away from the center of the lens and moves the focal point forward. The stronger the lens, the farther the focal point is from the lens.

To make a plus lens (above on the right), the thickest part of the lens is in the middle and the thinnest part on the outer edges. The light is bent toward the center and the focal point moves back. The stronger the lens, the closer the focal point is to the lens.

When light rays reach an angulated surface of a different material (like glass, lens, water), it causes the light rays to bend. This is called refraction.

Regular light from the sun or from a light bulb really contains all the colors of the rainbow. But you have to split it up to see this.

When light rays reach a convex lens the light rays bend away from the center: they diverge.

When light reaches a convex lens the light rays bend towards the center: they converge.

The main function of the sun at the center of our solar system is to provide light. Light is what drives life. It’s hard to imagine our world and life without light. The sensing of light by living things is almost universal. Plants use light through photosynthesis to grow. Animals use light to hunt their prey or to sense and escape from predators.

On the back of your eye is a complex layer of cells known as the retina. The retina reacts to light and sends that information to the brain. The brain translates all that activity into an image. Because the eye is a sphere, the surface of the retina is curved.

When you look at something, three things must happen:

The image must be reduced in size to fit onto the retina.
The scattered light must come together -- that is, it must focus -- at the surface of the retina.
The image must be curved to match the curve of the retina.

To do all that, the eye has a lens between the retina and the pupil (the "peep hole" in the center of your eye that allows light into the back of the eye) and a transparent covering, or cornea (the front window). The lens and the cornea work together to focus the image onto the retina.

When light enters the eye it first passes through the cornea, then the aqueous humor, lens and vitreous humor. Ultimately it reaches the retina, which is the light sensing structure of the eye. The retina contains two types of cells called rods and cones. Rods handle vision in the low light, and cones handle color vision and detail. When light contacts these two types of cells, a series of complex chemical reactions occurs. The chemical that is formed creates electrical impulses in the optic nerve. Generally the outer segment of rods are long and thin, whereas the outer segment of cones are more cone shaped. Below is a example of a rod and a cone:

The retina contains 100 million rods and 7 million cones. The retina is lined with black pigment called melanin – just as the inside of a camera is black – to lessen the amount of reflection. The retina has a central area that contains a high concentration of only cones. This area is responsible for sharp detailed vision.

There are 3 kinds of color sensitive pigments:

red sensitive pigment
green sensitive pigment
blue sensitive pigment

Each cone cell has one of these pigments so it is sensitive to that color. The human eye can sense almost any gradation of color when red, green and blue are mixed. Color blindness is the inability to differentiate between different colors. The most common type is red-green color blindness. This occurs in 8% of males and 0.4% of females. It occurs when either the red or green cones are not present or not functioning properly. What can you see in this picture?

Normally your eye can focus an image exactly on the retina:

Nearsightedness and farsightedness occur when the focusing is not perfect.

When Nearsightedness (myopia) is present, a person is able to see near objects well and has difficulty seeing objects that are far away. Light rays become focused in front of the retina. This is caused by an eyeball that is too long, or a lens system that has too much power to focus. Nearsightedness is corrected with a concave lens. This lens causes the light to diverge slightly before it reaches the eye. When farsightedness (hyperopia) is present, a person is able to see distant objects well and has difficulty seeing objects that are near. Light rays become focused behind the retina. This is caused by an eyeball that is too short, or by a lens system that has not enough focusing power. This is corrected with a convex lens.

Some sights of interest are:-

http://www.exploratorium.edu/learning_studio/cow_eye/index.html

http://micro.magnet.fsu.edu/primer/lightandcolor/index.html

http://www.internz.com/walton/Room2/Eyes/eye.html

where you learn how to cut up an eye!

The Eye

Retina - has rods which detect brightness and cones which detect colour.

Yellow spot - when you focus on an object, this is where the image falls. It is highly concentrated with rods and cones.

Blind spot - where the optic nerve leaves - there are no rods or cones so you can't see this point.

Optic nerve - takes impulses to the brain.

Cornea - protects the eye and also starts bending the light.

Pupil - small in bright light and large in dim. Controlled by muscles in the iris. This protects the retina from very bright light which could damage it.

Lens - bends the light and turns it upside down so it can be focused on the retina.