Penetrative pictures (1-4)
I believe the answer is:
x-rays
'pictures' is the definition.
(I know this)
I can't explain the remainder of the clue.
Can you help me to learn more?
(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" .)