Extended · Practice questions · Correcting sight

Where does the focus land?

Six original Cambridge-style questions. Answer where the light focuses, and the correcting lens follows. They drill the two defects, their causes, and the lens that fixes each.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

The logic that never fails

Find the focus first. The lens follows.

Short sight: distant blurred, focus in front of the retina, corrected with a diverging lens.
Long sight: near blurred, focus behind the retina, corrected with a converging lens.
Cause: short sight, eyeball too long or lens too strong. Long sight, eyeball too short or lens too weak.

[3 marks]

A person is short-sighted.

(a) State whether they see near or distant objects clearly. [1] (b) State where light from a distant object is focused relative to the retina. [1] (c) Name the type of lens used to correct it. [1]

(a) Near objects clearly; distant objects are blurred. ✓

(b) The light focuses in front of the retina. ✓

[3 marks]

A person is long-sighted.

(a) State whether they see near or distant objects clearly. [1] (b) State where light from a near object is focused relative to the retina. [1] (c) Name the type of lens used to correct it. [1]

(a) Distant objects clearly; near objects are blurred. ✓

(b) The light would focus behind the retina. ✓

Analysis

[2 marks]

Explain, in terms of the shape of the eyeball or the strength of the eye lens, two possible causes of short sight.

The eyeball is too long, so the retina is further back than the focus point. ✓
The eye lens is too strong (too powerful), so it bends the light too much and focuses it too soon. ✓

Either cause puts the focus in front of the retina.

Diagram

[3 marks]

The diagram shows parallel light from a distant object entering an eye.

Light from a distant object entering an eye.

(a) State the sight defect shown. [1] (b) Explain how you can tell from the diagram. [1] (c) State the lens needed to correct it. [1]

(a) Short sight. ✓

(b) The light is focused in front of the retina, so the image on the retina is blurred. ✓

Analysis

[2 marks]

A student writes that long sight is corrected with a diverging lens. Explain why this is wrong, and give the correct lens with a reason.

In long sight the light focuses behind the retina, so a lens that spreads light out further would only make it worse. ✓
A converging lens is needed, because it brings the light together sooner so it focuses on the retina. ✓

Focus too far back, pull it forward with a converging lens. Focus too far forward, push it back with a diverging lens.

Analysis

[3 marks]

An elderly reader finds they have to hold a newspaper at arm's length to read it clearly, but they can see across the street without difficulty.

(a) Name the sight defect. [1] (b) Explain, in terms of the focus, why the newspaper is blurred when held close. [1] (c) State the lens that would correct it. [1]

(a) Long sight. ✓

(b) Light from the close newspaper is focused behind the retina, so its image on the retina is not sharp. ✓

Holding it at arm's length pushes the object far enough that the eye can just focus it. The convex lens removes the need.

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.