Psychology: A Self-Teaching Guide

Waves a little longer than 750 nanometers are called infra-red rays.Waves a

little shorter than 400 nanometers are called ultra-violet rays.Both of these

kinds of waves are invisible to the naked eye.

(a) The part of the electromagnetic spectrum we see is called the.

(b) Waves 750 nanometers in length are associated with what color?

Answers: (a) visible spectrum; (b) Red.

Light is necessary for vision, but it is not sufficient. In order to see it is neces-

sary to have a sense organ that can convert light waves into useful neurological

information. This organ is, of course, the eye. In the front of the eye is the

cornea,a kind of window that allows light to enter the eye. Because the cornea

has a convex shape, it also is somewhat responsible for bending light waves and

making them converge on the lens.

The lensis used to focus light waves, and it produces an inverted, or upside-

down, image on the retina. The retinais a photosensitive neurological structure.

Think of it as a target. The center of the target is called the fovea,and it plays a

dominant role in visual acuity and color vision. The outer rim of the target, the

periphery,plays an important part in signal detection and brightness vision. The

neurons in the retina are called photoreceptorsbecause they are light sensitive.

The optic nerveconveys the retina’s activity pattern to the brain.

The two kinds of photoreceptors are the cones and the rods. They have been

given these names because of the shapes of their cell bodies. The conesare

located primarily in the fovea. The rodsare located primarily in the periphery. As

already indicated, color vision is associated with the fovea, suggesting that the

cones have a lot to do with this particular quality of sensation.

(a) The retina is a photosensitive.

(b) The two kinds of photoreceptors in the retina are the.

Answers: (a) neurological structure; (b) rods and cones.

A leading theory of color vision is the trichromatic theory.This theory is

also known as the Young-Helmholtz theory in honor of the scientists who first

introduced it. The trichromatic theory hypothesizes that we have three kinds of

cones. These are differentially sensitive to three wavelengths of light: (1) 750

48 PSYCHOLOGY

Violet

Ultra-

violet

rays

Infra-red

400 450 500 550 600 650 700 750 rays

Indigo Blue Green Yellow Orange Red

The visible portion of the electromagnetic spectrum.