Week 16 - Light Behaves in Predictable Ways

Lesson 8

Lesson 8: Human Eyes and Cameras
Science in Action 8
Read pages 230 - 235
Background Information

In earlier lessons, you learned how light waves are bent or refracted when they travel from one medium to another. You also discovered that a lens is a transparent object with at least one curved side that causes light to bend. The amount of bending can be controlled by making the sides of the lens more or less curved. The more curved the sides, the more a ray of light entering the lens is bent. As you will see in this lesson, the human eye has several structures that refract light.

Your eye is a sense organ that captures the energy of light and converts this light energy into nervous impulses. The nervous impulses travel down a large nerve (the optic nerve) to your brain. The neurons in your brain interpret these nervous impulses into images such as the words and pictures on your computer monitor.

Identify each part of the eyeball in Figure 1 as you read the next paragraph.
Figure 1: Illustration of the eyeball

Light travels in straight lines unless something bends or refracts it. Your eyes are equipped with structures that bend light. As light enters the eye, its waves are first bent by the cornea and then a lens. The lens directs the rays onto the retina. The retina is a tissue at the back of the eye that contains nerve cells that are sensitive to light energy. Two types of light sensitive cells called rods and cones are found in the retina. Cones respond to bright light and colour. Rods respond to dim light and are used to help you detect shape and movement. For example, when you go out at night, you usually do not see the colour of objects. You may make out the size and shape of the object but it is tough to tell the colours of dimly lit objects. This is because there is not enough energy in dim light to stimulate the cones. Light energy stimulates impulses in the rods and/or cones. The impulses pass to the optic nerve, which carries them to the brain. Neurons in the brain interpret the impulses and you "see" what you are looking at.

The iris controls the amount of light entering your eyeball. In dim light, your iris opens allowing more light to enter. In bright light, your iris closes allowing less light to enter your eye. Your pupil is the opening in the iris. It really isn't a structure and what you "see" when you look into someone's pupil is the dark layer at the back of the eyeball.

Light is refracted, or bent when it passes through a lens. Just how it bends depends on the type of lens it passes through. In a previous lesson, you learned that a lens that is thicker in the middle and thinner on the edges is called a convex lens. The lens in your eye is convex. A convex lens causes parallel light rays entering your eye to come together (converge) at a focal point.

A lens that has thicker edges than the middle is called a concave lens. The parallel light waves passing though a concave lens are spread out.

Figure 2. Illustration of eye with image focused on retina

In an eye with normal vision, light rays are focused by the cornea and lens onto the retina. A sharp image is formed on the retina, the brain interprets the signal as being clear and sharp.
Figure 3. Illustration of eye that is too long and has a concave lens correcting the problem.

However, if the eyeball is too long from front to back, light from distant objects is focused in front of the retina. A blurred image is formed. This condition is called nearsightedness because near objects are seen clearly. To correct nearsightedness, eyeglasses with concave lenses are used. Concave lenses spread the light waves out before they enter the eye. This helps focus images on the retina.
Figure 4. Illustration of an eyeball that is too short and has a convex lens correcting the problem.

If the eyeball is too short from front to back, light from nearby objects is focused behind the retina. Once again, the images appears blurred. This condition is known as farsightedness. Convex lenses bend the light before it enters the eye and help to focus the image on the retina.

A typical film camera has a convex lens that forms an image on a section of light sensitive film. Once again, a convex lens forms an image that is smaller and is upside down if the object is more than two focal lengths away. A camera lens is a convex lens used in this way. When the shutter of a camera is open, the convex lens forms an image on a piece of film that is sensitive to light. The film contains chemicals that undergo chemical reactions when struck by light. This causes light areas and darks areas in the image to be recorded. If the film is sensitive to colour, the colours of the object are also recorded.

An image that is too bright may overexpose the film. If there is too little light that reaches the film, the image may be too dark. A diaphragm is a device in a camera that controls the amount of light that reaches the film. The iris (colour part) performs a similar job in your eye.

For more information the eye go to The Human Eye web site.

Exercise 2.4: Eye vs Camera


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