Optics: Prentice rule: practice question

A Ophthalmology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Ophthalmology · Optics: Prentice rule

Sample item

A patient wears -6.00 D lenses and looks through a point 8 mm from the optical centre. What is the prismatic effect?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. 4.8 prism dioptres

Prentice rule gives prism in dioptres as decentration in centimetres multiplied by lens power in dioptres. Here 0.8 cm multiplied by 6 D gives 4.8 prism dioptres. Using millimetres directly gives an error by a factor of ten.

Option by option

  • A. 4.8 prism dioptres (correct)

    Correct. 0.8 cm multiplied by 6 D is 4.8 prism dioptres.

  • B. 0.75 prism dioptres

    0.75 results from dividing the power by the decentration.

  • C. 48 prism dioptres

    48 results from using 8 mm instead of 0.8 cm.

  • D. 14 prism dioptres

    14 results from adding the numbers.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Ophthalmology question

What is the answer to this Ophthalmology question?

The best answer is A, 4.8 prism dioptres. Prentice rule gives prism in dioptres as decentration in centimetres multiplied by lens power in dioptres.

Why is "0.75 prism dioptres" not the answer?

0.75 results from dividing the power by the decentration.

Why is "48 prism dioptres" not the answer?

48 results from using 8 mm instead of 0.8 cm.

Why is "14 prism dioptres" not the answer?

14 results from adding the numbers.

What concept does this question test?

It tests Optics: Prentice rule in Ophthalmology. Here 0.8 cm multiplied by 6 D gives 4.8 prism dioptres. Using millimetres directly gives an error by a factor of ten.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Optics: Prentice rule?

Group the red eye by what to examine and what the pupil does. Then use the flashcards and the other Ophthalmology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.