Why does red light seem darker?


I’ve got a colour changing light that cycles through all the colours. Whenever red shines on its own, the whole room goes a bit darker. When blue or green light is added, the whole room instantly gets a bit brighter. Is this a trick of the eye or something to do with the light itself? (best seen when it is the only light source)

In: Physics

So all energy is on a spectrum right, visible light is just one portion of that spectrum and that spectrum is arranged by the amount of energy associated with them. Red is the lowest energy of the visible light spectrum, then progressively as you work through the rainbow the different colours increase in energy, and that carries on into many different forms of energy that we can’t see like infrared, x-rays, gamma rays, etc. So the way energy is arranged makes red light slightly dimmer. If you had a red laser pointer and green one with the same power, the green would appear to travel further

Take a look at this image: https://upload.wikimedia.org/wikipedia/commons/thumb/0/04/Cone-fundamentals-with-srgb-spectrum.svg/1200px-Cone-fundamentals-with-srgb-spectrum.svg.png

These are the spectra of our cones, meaning, this shows the different wavelengths and the cone response they evoke. As you can see, red, around 620 to 750 nm, elicits a much weaker response than other colors. Of course, some other colors also have a weak response, you can even see it by eye and it corresponds to the spectra.

However, this is just part of the story. You also have the purkinje effect, where luminescent sensitivity of the eyes shifts towards blue in darker environments, it’s an adaptive response. The mechanism for that is simple: cones are less sensitive than rods when it comes to light (these are the two classes of photoreceptors in your retina), and as light dims, the sensitivity shifts to that of the rods peak wavelengths, which happens to be in the blue-green region (507 nm). I mean if you try to look at a bright red object in daylight, it’s pretty bright, only when it’s a bit darker does it seem dimmer than other colors.

So we likely perceive red as dimmer due to a combination of the two reasons.