Biology 11–12 · Year 12

Near point, amplitude of accommodation and why reading glasses become necessary

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

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

The lens changes shape to focus near objects, and the closest point a person can focus (the near point) measures how much it can change, an amount that falls with age until reading needs added lens power.

What you need

Metre rule or tape measure; a card with a line of small print; a cover card for one eye
Results table; optional volunteers of other ages (with consent) to widen the data set

How to do it

Cover one eye; hold the ruler's zero at the outer corner of the open eye and slide the print card slowly toward the eye along the ruler until the print first blurs (the push-up method).
Record that distance as the near point; repeat three times for each eye and take the mean.
Calculate the amplitude of accommodation A = 1 / near point (metres), assuming distant objects are seen clearly with any usual spectacles or lenses worn.
Compare your amplitude with Hofstetter's average, 18.5 - 0.3 x age, and minimum, 15 - 0.25 x age, in dioptres.
Use the model to find the age at which the average near point passes 25 cm and 40 cm, and the extra lens power needed to bring the near point back to 25 cm.
Link the fall in amplitude to presbyopia and to the use of spectacles.

What you should see

For a 16-year-old Hofstetter's average is 13.7 dioptres (near point 7.3 cm) and the minimum 11.0 dioptres (9.1 cm); at 45 the average is 5.0 dioptres (20 cm) and at 55 it is 2.0 dioptres (50 cm) (all computed). A review of 21st-century studies (the PMC source) found pooled amplitudes significantly below Hofstetter's average for children aged 6, 9, 10 and 11, so a measured value under the average is not unusual. The learner knows it worked when repeated measurements for the same eye agree within about 1 cm and older volunteers show longer near points.

What changes

What you change: age (across volunteers) or eye tested
What you measure: near point and amplitude of accommodation
What you keep the same: print size and lighting
speed of approach
measurement point at the eye
number of trials

Common misconceptions

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

Needing reading glasses in middle age means the eye is diseased; the lens stiffens with age in everyone.
Short-sighted people cannot focus close up; they see near objects clearly and distant ones poorly.
The eye focuses by moving the lens back and forth like a camera; it changes the lens's curvature.

Safety card

Low riskLearners carry it out

Hazards

Card or ruler edge near the eye

Controls

Move the card slowly and stop at the first blur; never touch the eye

Note

Learners are participants: participation is voluntary, anyone may opt out or withdraw their data, and results are recorded without names, following the briefing and consent approach in the practicalbiology.org human-subject protocols. This is not an eye test; anyone with concerns about their vision is referred to an optometrist.

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 BIO11/12-5
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities