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Penetrative pictures (1-4)

Ross

I believe the answer is:

x-rays

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'pictures' is the definition.
(I know this)

I can't explain the remainder of the clue.

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(Other definitions for x-rays that I've seen before include "Short-wavelength radiation" , "Medical photographs" , "'Electromagnetic waves, for medical diagnosis say (1-4)'" , "Penetrating radiation" , "Photos or exams made with kind of radiation" .)

I've seen this clue in the Evening Standard.
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