alleryIn a young eye, the focal length range of the eye lens is approximately:
A~ 2.5 cm fully relaxed to slightly less when contracted
BVariable from 5 m to 1 m
CFixed at 1 mm
DVariable from 1 km to 100 m (option)
Answer & Solution
Correct answer: A. ~ 2.5 cm fully relaxed to slightly less when contracted
1. The eyeball is ~2.3 cm in diameter, and the relaxed lens has focal length close to this so that infinity focuses on the retina.
2. Accommodation can shorten this slightly to focus near objects.
3. Hence the focal length is around 2.5 cm with a small downward adjustment range.
_Source: syllabus Science, Ch 10 "The Human Eye and the Colourful World", §10.1_
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