Physics, Astronomy & Cosmology / Re: If Red Light cures everything, why aren't hospital lightbulbs red?« on: 27/02/2024 15:12:33 »
Red light does indeed penetrate the epidermis to a greater extent than blue, but all the "red light therapy" installations I have seen (I get involved with a lot of "fringe" stuff!) also produce a fair bit of radiant heat, free coffee, and pleasant music. Plus handsome, enthusiastic, young therapists.No. It is because red light doesn't trigger the transition from night vision to day vision. Night vision uses the rods of our eyes, and they start activating due to the production of rhodopsin, which takes a good bit of time become fully effective. It reacts with and decays rapidly when exposed to high enough levels of light. Thus when your eyes have become dark adaptive, and you look at a bright light, this neutralizes the rhodopsin, shuts down the rods, and you lose your dark adaptation.
The dark-adapted human eye is more sensitive to red light, which is why it is used in cockpits and ships' bridges at night.
Rhodopsin is not sensitive to red light*, so it doesn't break down, thus preserving your dark adaptation. The red light in ship bridges and cockpits at night is to preserve the occupants' night vision, allowing them to see outside better.
* The photosensitive chemicals used for black and white photography are the same way, which is why you can use a red "safe-light" in a darkroom while developing.