Optional Unit VI: Optics
A. Applications

1. Human Vision

Key Concepts

The convex-shaped cornea causes light to refract at the surface of the eye.

The lens, whose curvature is controlled by ciliary muscles and suspensory ligaments, causes light to undergo further refraction. The lens in the eye is a converging lens.

The change in the shape of the lens, called accommodation, allows objects located at different distances to be seen clearly. The more convex the lens the shorter the focal length becomes.

The near point (about 25 cm for adults with normal vision) is the closest an object can be placed to the eye and still remain sharply in focus.

The iris, located behind the aqueous humour and in front of the lens, contracts or dilates to alter the size of the pupil.

The size of the pupil determines the amount of light which can enter the eye. (Doubling the radius of the pupil permits four times as much light to enter the eye.)

The vitreous humour behind the lens helps to maintain the shape of the eye.

The tough white surface of the eyeball is called the sclerotic.

The retina is a light sensitive area at the back of the eye. It consists of different types of light-sensitive receptors called rods and cones. Rods are sensitive to dim light and do not respond to colour .
Cones (located closer to the fovea) are sensitive to bright light and colour. (The Young-Helmholtz theory, which suggests that there are different cones to respond to each of the three primary colours of light, may be useful but it fails to account for certain things such as colour constancy.)

An image is formed on the retina. The image is inverted, real, and diminished. The ability of the eye to form an image on the curved surface of the retina is extremely important in human vision.

The blind spot, located where the optic nerve meets the eye, contains no photo-receptors.

The optic nerve transmits impulses to the brain. A crossover takes place. The optic nerve of each eye sends the information to the opposite hemisphere of the brain.

One eye is usually dominant, controlling perspective. The use of two eyes (binocular vision) is necessary to obtain the correct perception of depth.

In spite of the poor optical qualities in the human eye, (compared to other types of lenses and optical systems), the brain is capable of making adjustments and corrections. No optical system yet devised can rival the superb qualities of human vision.

Some defects in human vision can be corrected.

(The following is provided as background information on defects in human vision. It is not available in some key resources.)

Myopia (nearsightedness) is the inability of the eye to focus distant objects on the retina. The lens is too convex, or the eyeball is too long to allow proper focusing to take place. The image is formed in front of the retina. (The light striking the retina, no longer sharply focused, is blurred.) Diverging meniscus lenses (i.e., on glasses or contact lenses) can correct the problem.

Hypermetropia (farsightedness) is the inability of the eye to focus nearby objects on the retina. The lens is too flat or the eyeball is too short. The image would ordinarily be focused behind the retina. (An image does not form there, of course, because the converging light rays strike the retina first. Some physics texts illustrate this incorrectly by showing light rays passing right through the eye to form the image behind the retina. Dotted lines should be used behind the retina when illustrating this, to convey the idea that the light rays never actually reach that position.) Converging meniscus lenses can correct the problem.

Lens accommodation, occurring in elderly people, is caused by a gradual loss in the elasticity of the eye's lens. It results in presbyopia (a type of farsightedness). Converging meniscus lenses correct the problem. Distant vision is usually unaffected. Bifocals, special glasses with a converging lens in the lower portion of the frame, are often worn, so that the eyes can look down (when reading, for instance) to focus properly on nearby objects.

Astigmatism results if the cornea or the lens of the eye are not perfectly spherical. Light from different planes can not be simultaneously focused clearly. A numbered, spoked wheel is used to test for astigmatism. It can be corrected by wearing glasses whose lenses have slightly different radii of curvature in different planes (cylindrical lenses).

Colour blindness results in the inability to distinguish between certain colours, such as red and green (daltonism). Seeing things only in shades of grey is very rare (literal colour blindness). Defects in colour vision are primarily hereditary. Men are affected more often than women. There is no known treatment for colour blindness. The reasons why colour blindness occurs are not clearly understood.

Some animals have literal colour blindness. They do not have cones on their retinas.

Cataracts are formed due to a gradual loss of transparency of the eye's lens. They occur quite commonly in elderly people, or due to reasons other than aging. Surgical procedures can be performed to replace the lens with an artificial, interocular lens implant made from synthetic material. The artificial lens can not accommodate for near and far objects, so bifocals or trifocals are usually worn by patients after a cataract operation.

Learning Outcomes

Students will increase their abilities to:

1. Define the following terms: accommodation, near point.

2. Describe the function of the following parts of the human eye: cornea, lens, ciliary muscles, suspensory ligaments, iris, aqueous humour, vitreous humour, sclerotic, retina, rods, cones, optic nerve.

3. Research the causes and known treatments of any of the following defects in human vision: myopia (nearsightedness), hypermetropia (farsightedness), presbyopia, astigmatism, colour blindness, cataracts.

4. Describe how the rods and cones on the retina respond to light differently.

5. Explain why binocular vision is necessary for correct depth perception.

6. Explain the differences between regular eye glasses, bifocals, and trifocals.

Teaching Suggestions, Activities and Demonstrations

1. Perform an activity to test vision by using eye charts or colour blindness charts.

2. Label the important parts of the human eye on a diagram or identify the important parts of the human eye from a diagram.

3. Perform an activity to observe the results of the eye's "blind spot" and differences in perspective formed by the dominant eye.

4. Research defects in human vision and known ways they can be corrected.

5. Apply an understanding of lenses and refraction to explain how an image is formed on the retina.

6. Explain how a change in the shape of the lens, through accommodation, changes the focal length.

7. Students should recognize the need to wear protective eye wear and observe other important safety precautions in laboratory and other potentially hazardous environments. As an activity, students could develop an instructional video on lab safety to be used in lower grades. Specific attention could be placed on proper eye protection, and situations which could potentially cause eye injury.

8. Local optometrists often have models of the eye and various optical tests (such as colour blindness charts) that they may be willing to lend to schools. They may also be interested in speaking to students about career opportunities in their field of work.