10.5 The Eye

A free GCSE Biology lesson for AQA (8461) from OBA School: structures of the eye and their functions, the pupil reflex and accommodation. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Label the main structures of the eye and give the function of each
• Explain how the iris controls the amount of light entering the eye
• Describe accommodation: how the eye focuses on near and distant objects
• Relate the structures of the eye to their functions in exam-style questions

Key words

• Retina: Light-sensitive layer at the back of the eye. Contains receptor cells sensitive to light intensity and colour.
• Cornea: Transparent front of the eye. It refracts (bends) light.
• Iris: Coloured ring of muscle that controls the size of the pupil.
• Ciliary muscles: Ring of muscle that changes the shape of the lens.
• Suspensory ligaments: Fibres connecting the ciliary muscles to the lens.
• Accommo- dation: Changing the shape of the lens to focus on near or distant objects.

Structures of the eye

The eye is a sense organ containing receptors sensitive to light. Light is refracted by the cornea and lens and focused on the retina, which sends impulses along the optic nerve to the brain.

• Cornea; Function Transparent; refracts light into the eye
• Iris; Function Controls the diameter of the pupil, so how much light enters
• Pupil; Function Hole in the iris that lets light through
• Lens; Function Changes shape to focus light on the retina
• Ciliary muscles and suspensory ligaments; Function Work together to change the shape of the lens
• Retina; Function Contains receptors (rods for dim light, cones for colour) that detect light
• Optic nerve; Function Carries impulses from the retina to the brain
• Sclera; Function Tough white outer layer; protects the eye

The pupil reflex

Too much light can damage the retina; too little and you cannot see. The iris contains two sets of muscles: circular muscles (round the pupil) and radial muscles (like spokes). This is a reflex action.

• Circular muscles; Bright light Contract; Dim light Relax
• Radial muscles; Bright light Relax; Dim light Contract
• Pupil; Bright light Gets smaller (constricts); Dim light Gets bigger (dilates)
• Effect; Bright light Less light enters; protects retina; Dim light More light enters; you can see better

Accommodation

To focus on objects at different distances, the eye changes the shape of the lens. Light from a near object spreads out more, so it must be refracted more.

• Ciliary muscles; Near object Contract; Distant object Relax
• Suspensory ligaments; Near object Loosen (slacken); Distant object Pulled tight
• Lens shape; Near object Thicker, more rounded; Distant object Thinner, flatter
• Refraction; Near object Light refracted more; Distant object Light refracted only slightly

Worked example: explaining a change

Rosa looks up from her phone to a bus far away. Describe how her eye adjusts.
Her ciliary muscles relax, so the suspensory ligaments are pulled tight.
This pulls the lens thinner, so light is refracted less and the image of the bus is focused on the retina.

Answer: Ciliary muscles relax → ligaments tighten → lens thinner

Exam tip: it seems backwards, but when the ciliary muscles contract the ligaments go slack. The ring of muscle gets smaller, so it stops pulling on the lens.

Common mistakes

• Saying the lens does all the focusing. The cornea does most of the refraction; the lens fine-tunes it.
• Mixing up the iris and the pupil. The iris is the coloured muscle; the pupil is the hole.
• Saying the circular muscles contract in dim light. They contract in bright light to make the pupil smaller.
• Saying the ligaments tighten for near objects. They slacken, so the lens becomes fatter.

Revision questions

Try these, then check your answers below each one.

Question 1. In bright light, what happens in the iris? (Choose from: Ciliary muscles contract, pupil gets smaller, Circular muscles contract, radial muscles relax, pupil gets smaller, Radial muscles contract, circular muscles relax, pupil gets bigger, Both muscles relax, pupil gets smaller.)

Answer: Circular muscles contract, radial muscles relax, pupil gets smaller

Working: Circular muscles contract and radial muscles relax, so the pupil constricts and less light enters.

Question 2. When you look at a near object, what happens? (Choose from: Ciliary muscles contract, ligaments tighten, lens thinner, Iris muscles contract, lens thicker, Ciliary muscles relax, ligaments tighten, lens thinner, Ciliary muscles contract, ligaments slacken, lens thicker.)

Answer: Ciliary muscles contract, ligaments slacken, lens thicker

Working: Ciliary muscles contract, suspensory ligaments slacken and the lens becomes thicker, refracting light more.

Question 3. What is the name for changing the shape of the lens to focus on near or distant objects?

Answer: accommodation

Working: This process is called accommodation.

Related lessons

• Previous lesson: 10.4 The Brain
• Next lesson: 10.6 Common Problems of the Eye
• In this chapter: 10.1 Principles of Homeostasis
• In this chapter: 10.2 The Structure and Function of the Human Nervous System
• In this chapter: 10.3 Reflex Actions

Continue in the full course

This lesson is section 10.5 (AGCBH1005) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.