Imaging physics: inverse square law: practice question

A Radiology 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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Radiology · Imaging physics: inverse square law

Sample item

The radiation intensity at 1 metre from a point source is 400 units. What will it be at 2 metres, ignoring attenuation?

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

The answer and the reasoning

A. 100 units

Intensity is inversely proportional to the square of the distance. Doubling the distance reduces the intensity to one fourth, so 400 divided by 4 gives 100. This principle underlies the practice of standing as far as possible from the source.

Option by option

  • A. 100 units (correct)

    Correct. Doubling the distance divides intensity by 4.

  • B. 200 units

    That would be a halving, as with a linear relationship.

  • C. 50 units

    That would be a division by 8, as with a cube law.

  • D. 800 units

    Intensity falls with distance and does not rise.

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 Radiology question

What is the answer to this Radiology question?

The best answer is A, 100 units. Intensity is inversely proportional to the square of the distance.

Why is "200 units" not the answer?

That would be a halving, as with a linear relationship.

Why is "50 units" not the answer?

That would be a division by 8, as with a cube law.

Why is "800 units" not the answer?

Intensity falls with distance and does not rise.

What concept does this question test?

It tests Imaging physics: inverse square law in Radiology. Doubling the distance reduces the intensity to one fourth, so 400 divided by 4 gives 100. This principle underlies the practice of standing as far as possible from the source.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 Imaging physics: inverse square law?

Learn signs with the image, and the disease they point to. Then use the flashcards and the other Radiology 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.