Biology 11–12 · Year 12

A lens-and-screen model eye: myopia, hyperopia and how each is corrected

Module 8: Non-infectious Disease and Disorders (Technologies and Disorders: visual disorders)

Practical, model not builtLow risk

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The idea

A sharp image needs the eye's focusing power to match its length, so an eye that is too long or too powerful (myopia) needs a diverging lens and one that is too short (hyperopia) needs a converging lens, while laser surgery changes the cornea's power instead.

What you need

Converging lens of focal length +10 cm (+10 dioptres) in a holder, standing for cornea and lens
White card screen (the retina) on a holder that slides along a metre rule optical bench
Diverging lens -20 cm (-5 dioptres) and converging lens +20 cm (+5 dioptres) for correction
A distant bright scene through a window (or an illuminated object more than 3 m away); darkened room

How to do it

Focus the distant scene on the screen with the +10 D lens; record the lens-to-screen distance (the normal eye, about 10 cm).
Move the screen to 20 cm (a long, myopic eye) and note the blur; place the -5 D lens against the +10 D lens and check that the image is sharp again.
Move the -5 D lens 1.5 cm in front of the main lens (spectacle position) and find where the sharp image now forms; compare with the model's prediction.
Move the screen to 6.7 cm (a short, hyperopic eye) and correct it with the +5 D lens.
Use the model to show laser surgery on the myopic eye: lower the main lens power until the image is sharp at 20 cm without any added lens.
Explain which disorder each correction treats and why contact lens and spectacle powers differ.

What you should see

The +10 D lens images a distant scene about 10 cm behind it. With the screen at 20 cm a sharp image needs a total power of 5 dioptres, so a -5.00 D lens in contact corrects it, while the same correction worn 15 mm in front needs -5.41 D; with the screen at 6.7 cm the eye needs 14.93 dioptres, corrected by +4.93 D in contact or +4.59 D at 15 mm, and the +5 D lens in contact focuses at 6.67 cm, 0.3 mm short of the screen; a -5.00 D lens moved to 15 mm in front focuses the scene at 18.7 cm, short of the 20 cm screen (all computed). Laser surgery in the model lowers the main power from 10 to 5 dioptres. The learner knows it worked when each correcting lens restores a sharp image of the distant scene on the moved screen.

What changes

What you change: screen distance (eye length) and correcting lens power and position
What you measure: sharpness of the image of a distant scene
What you keep the same: main lens
object distance
lighting

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Short-sightedness is corrected with a magnifying (converging) lens; it needs a diverging lens.
Spectacles and contact lenses for the same eye have the same power; the distance from the eye changes the power needed.
Laser surgery changes the lens inside the eye; it reshapes the cornea.

Safety card

Low riskLearners carry it out

Hazards

Looking at the sun through lenses
Glass lens edges

Controls

Never point the lenses at the sun; use a scene away from direct sunlight
Handle lenses by their holders

Note

No hazardous materials.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Biology Stage 6 Syllabus (2017), current: Year 11 taught to the end of 2026 and Year 12 to Term 3 2027BIO12-15 BIO11/12-6
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities