5.20 Eyes 1 - Structure & Iris Reflex

Question 1

What type of organ is the eye?

Answer 1

A sense organ

Question 2

The cornea

Answer 2

The cornea is the transparent (so has no blood vessels) outer layer found at the front of the eye. This means that all the oxygen it needs has to diffuse into it from the outside air, rather than being supplied from the blood like everywhere else in the body. It refracts light into the eye.

Question 3

The iris

Answer 3

The iris contains muscles that allow it to control the diameter of the pupil (the hole in the middle) and therefore how much light enters the eye.

Question 4

The lens

Answer 4

The lens also refracts light, focusing it onto the retina (which contains receptor cells sensitive to light). However, importantly, the lens can change its shape, which allows it to control how strongly it refracts light; this helps it to always focus the light perfectly onto the retina at the back of the eye.

Question 5

The retina

Answer 5

The retina is made up of two different types of receptor cells:
- Cone cells are sensitive to the colour of light and so allow us to see in colour. However, they don’t work very well in low light conditions.
- Rod cells are more sensitive to light but only allow us to see in black and white. This explains why we can’t see colour in low light conditions, as it is only our rod cells that are working.

Question 6

The fovea

Answer 6

The fovea is the point where light focuses on the retina. This region contains the highest concentration of cone cells and gives the sharpest image.

Question 7

What is the shape of the lens controlled by?

Answer 7

The ciliary muscles and suspensory ligaments

Question 8

The optic nerve

Answer 8

The optic nerve takes all the impulses generated by the receptor cells and transmits them to the brain.

Question 9

Describe the iris reflexes

Answer 9

Very bright light can damage the retina - so you have a reflex to protect it.
- When light receptors in the eye detect very bright light, a reflex is triggered that makes the pupil smaller. The circular muscles in the iris contract and the radial muscles relax. This reduces the amount of light that can enter the eye.
- The opposite process happens in dim light. This time, the radial muscles contract and the circular muscles relax, which makes the pupil wider.

Question 10

What do we call the pupil when it is smaller?

Answer 10

Constricted

Question 11

What do we call the pupil when it is bigger?

Answer 11

Dilated

Question 12

What is the pupil?

Answer 12

The gap through which light passes to reach the lens

Question 13

The eye is a sense organ. Which two stimuli are the receptor cells of the eye sensitive to?

Answer 13

• Colour
• Light intensity