An optician prescribes a power = -0.5 dioptre. The corresponding lens must be a

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Q: 88 (NDA-I/2013)
An optician prescribes a power = -0.5 dioptre. The corresponding lens must be a

question_subject: 

Science

question_exam: 

NDA-I

stats: 

0,40,36,12,12,40,12

keywords: 

{'convex lens': [0, 0, 0, 4], 'concave lens': [0, 1, 6, 5], 'focal length': [1, 0, 5, 7], 'lens': [0, 0, 0, 1], 'cm': [2, 0, 7, 20]}

The optician prescribes a power of -0.5 dioptre, which means that the lens should have a focal length of -2 m.

Option 1: A convex lens of focal length 2 m. Convex lenses have positive focal lengths, so this option is incorrect.

Option 2: A convex lens of focal length 50 cm. The focal length should be -2 m, so this option is incorrect.

Option 3: A concave lens of focal length 2 m. Concave lenses have negative focal lengths, so this option is correct.

Option 4: A concave lens of focal length 50 cm. The focal length should be -2 m, so this option is incorrect.

The correct answer is option 3 because a concave lens with a focal length of -2 m is required to produce a power of -0.5 dioptre.