The eyeIB MYP Biology: Revision notes
Section 1
Structure of the eye and what each part does
The eye detects light and sends information to the brain. Light passes through the following parts in order:
- Cornea: the transparent front surface. It does most of the refraction (bending) of light.
- Iris: a ring of muscle that controls the size of the pupil.
- Pupil: the hole in the centre of the iris where light enters.
- Lens: a transparent, flexible structure that bends light further and changes shape to focus on near or distant objects.
- Retina: the layer at the back containing light-sensitive cells (receptors) that detect light and turn it into electrical impulses.
- Optic nerve: carries the impulses from the retina to the brain, which produces the image we see.
Light is refracted by the cornea and the lens, but only the lens can change shape to adjust the focus.
Section 2
The pupil reflex
The pupil reflex is an automatic response that protects the retina and helps us see in different light levels. It is a reflex, so it is fast and does not involve thinking.
- Bright light: the circular muscles of the iris contract and the radial muscles relax. The pupil gets smaller, so less light enters and the retina is protected from damage.
- Dim light: the radial muscles contract and the circular muscles relax. The pupil gets larger, so more light enters and we can see more clearly.
The pupil is not a structure that moves: it is a hole. Say that the iris changes the size of the pupil.
Section 3
Accommodation: focusing on near and distant objects
Accommodation is the way the lens changes shape so that light from objects at different distances is focused on the retina. The lens is held in place by suspensory ligaments, which are attached to the ciliary muscles.
Near object: the ciliary muscles contract, the suspensory ligaments slacken and the lens becomes thicker (more curved), so it refracts light more.
Distant object: the ciliary muscles relax, the suspensory ligaments are pulled tight and the lens becomes thinner, so it refracts light less.
Remember the pair: near means ciliary muscles contract and the lens is fat. Distant means they relax and the lens is thin.
Section 4
Short sight and its correction
A short-sighted (myopic) person can see near objects clearly but distant objects look blurred.
- Cause: the eyeball is too long, or the lens is too strong (too curved).
- Effect: light from distant objects is focused in front of the retina.
- Correction: a concave (diverging) lens spreads the light slightly before it enters the eye, so the focus moves back onto the retina.
Section 5
Long sight and its correction
A long-sighted person can see distant objects clearly but near objects look blurred.
- Cause: the eyeball is too short, or the lens is too weak and cannot become thick enough.
- Effect: light from near objects is focused behind the retina.
- Correction: a convex (converging) lens brings the light together slightly before it enters the eye, so the focus moves forward onto the retina.
The fault in short and long sight is where the light is focused, not damage to the retina or optic nerve.
Must Know
- Cornea and lens refract light; the retina detects it; the optic nerve carries impulses to the brain
- Bright light: pupil smaller. Dim light: pupil larger (iris muscles)
- Near object: ciliary muscles contract, lens thicker. Distant object: ciliary muscles relax, lens thinner
- Short sight: focus in front of retina, concave lens
- Long sight: focus behind retina, convex lens
That's the notes covered.
Carry on to the next subtopic.
Exam questions on The eye
- Amara walks from a bright street in Lagos into a dimly lit cinema. For a few moments she can see very little, then her vision gradually improves. When she leaves after the film, the bright sunlight is uncomfortable at first.Explain how Amara's pupils respond when she leaves the cinema, and why this response is useful.2 marks
- Kenji is reading a message on his phone, held close to his face. He then looks up to read a road sign on the other side of the street.Explain how the eye changes so that Kenji can focus on his phone.2 marks
- A class investigates the pupil reflex. One student sits in a room where the light intensity can be changed. After one minute of adjusting to each light level, a partner holds a ruler beside the student's eye and estimates the pupil diameter. The results were: 7.5 mm at 10 lux, 5.0 mm at 100 lux, 3.0 mm at 1000 lux and 2.0 mm at 10 000 lux.Describe the trend in the results and use the data to support your answer.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).