7th Grade Science Student ebook

18.2 VISION

CHAPTER 18: VISION AND HEARING

18.2 Vision

In Chapter 4, you learned how a microscope uses light to form magnified images. The
human eye also uses light to form images. Every time you see something, light is
involved. In complete darkness, you cannot see anything! In this section, you will
explore how human vision works.

The human eye

You see the world
by reflected light

Figure 18.4 shows what happens when you see this page. Light

rays in the room reflect off the page and into your eyes. The

reflected light carries information that allows your brain to form an

image of the page. If you were in a room with no light, you would

not be able to see this page because it does not give off its own light.

You see many objects because they reflect light

How the eye

works

The eye is the sensory organ used for vision. You learned about the

mammalian eye in Chapter 15. The structures of the human eye

are similar to the eyes of other mammals.

Light passes through the cornea and

enters the eye through the pupil. It

passes through the lens and is refracted

to a focal point on the retina.

The retina contains light-sensitive cells

called photoreceptors. Photoreceptors

convert light into nerve impulses that

travel through the optic nerve to the

visual cortex of the brain. The visual

cortex interprets the light as an image.

Figure 18.4: What happens when

you see this page.

Review Section 4.3 to refresh your

memory about light. Write down

the following terms and their

meanings: light ray, reflection,

refraction, lens, focal point,

focal length.

photoreceptors - light-sensitive

cells of the retina that convert light

into nerve impulses.

optic nerve - a nerve that carries

nerve impulses from the eyes to

the brain.