Biology 12

SKILL FOCUS

Predicting

Performing and recording

Analyzing and interpreting

Conducting research

Investigation 5•A

154 MHR • Unit 2 Homeostasis

Pre-lab Question

What is the advantage of the pupillary reflex?

Problem

Does the colour of light affect the speed of the pupillary

reflex?

Prediction

Predict whether the colour of light affects the speed of

the pupillary reflex and, if it does, which colour will have

the greatest effect.

Materials

stop watch

flashlight

pieces of coloured cellophane (red, yellow, green, blue)

large enough to cover the flashlight lens

The Effect of Light on Pupil Size

The size of the pupil is controlled by ciliary muscles of the iris. The size of the pupil

changes with the amount of light. This reaction is called the pupillary reflex.

cones, but they are unable to distinguish colours.

Cones require more light to stimulate them, but

they are able to detect red, green, and blue.

The eye has two chambers divided by the lens.

The anterior chamber between the cornea and the

lens is filled with the fluid aqueous humour. The

cornea and anterior chamber act as a pre-lens to

initiate the process of focussing an image on the

retina before it encounters the lens. The lens

completes the process. The posterior chamber,

behind the lens, is filled with a clear gel

(vitreous humour) that helps maintain the shape

of the eyeball.

How the Eye Functions

As light enters the eye, the pupil dilates if there is

insufficient light or constricts if there is too much.

The pupil also constricts when you focus on

something close, to reduce the distortion that

occurs around the outside field of view.

The shape of the lens changes in response to

your distance from the object being viewed.

Figure 5.16 illustrates that if you are looking at

something far away, the ciliary muscles relax. The

suspensory ligaments, which are attached to the

ciliary muscles, become taut and the lens flattens.

When you focus on something close, the ciliary

muscles contract, pulling forward and releasing the

tension on the suspensory ligaments. The lens

then becomes more rounded. This adjustment is

called accommodation.

The image is focussed on the retina, which is

composed of three layers — the ganglion cell layer,

the bipolar cell layer, and the rod and cone cell

layer (see Figure 5.17 on page 156). Bipolar cells

synapse with rods or cones and transmit impulses to

the ganglion cells. The ganglion cells join together

and form the optic nerve as they exit the eye. The

retina is composed of approximately 150 million

rod cells and 6 billion cone cells. Both function

using a purple pigment called rhodopsin. When

light strikes this pigment, rhodopsin breaks down

into two proteins — retinal (which is formed from

vitamin A) and opsin (which releases the energy

Some evidence shows that the pupil of the eye dilates when

the person is interested in what he or she is viewing.

BIO FACT