Invisible Light
This page is the CodyCross answer for the clue below from Physics, Group 1889, Puzzle 4 (Space-Time Continuum).
Question is: Invisible Light
and the answer is: INFRARED
Word Meaning:
INFRARED — (uncountable) The electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation, having a wavelength between 700 nm and 1 mm.
Definitions from Wiktionary.