Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

White light is always said to contain all the different wavelengths of light. Why, then, can we 'make' new colors simply by adding wavelengths? Is it just a matter of our perception, that, when two colors are added together, they appear to be the wavelength of another color. Or are the waves actually forming into another, differently 'waving' wave, with a different wavelength?

share|cite|improve this question

It is simply a matter of perception.

Human eyes have four types of receptor

  • "cones" that come in three varieties each with a response curve that peaks at a different frequency (but which have considerable spread and overlap)
  • "rods" that work well in lower light than cones do and effectively provide monochrome vision at night

So, to understand human colour perception in daylight, you only need to work with a model of light as a mixture of different frequencies which pass through one another losslessly.

If a particular source of light is strong at frequencies that mainly stimulate two types of cone, your brain interprets this as an intermediate "colour" between the peaks of each types response curve.

See Wikipedia - Color Vision

Note that TV screens exploit the specific mechanisms of human colour perception. They don't really need to produce an arbitrary range of frequencies, they just need to produce three specific colours at the right mix of intensities to stimulate your receptors to the same degree that an intermediate frequency would.

share|cite|improve this answer
The response curve or rods is no different in quality than the response curve of any single type of cone receptor. (Did you ever notice that red looks black at night?) It's just that at night, you have only one type of receptor working, and you need to compare the output of at least two types to discriminate between frequencies. – Peter Shor Aug 6 '13 at 11:58
@Peter: thanks for the correction, I'll update my answer. – RedGrittyBrick Aug 6 '13 at 12:49

In my answer to this question, I explain how white light is composed of all of the wavelengths output by the Sun in the visible spectrum. I also go on to explain how we can add specific wavelengths to "simulate" white light due to the way our eyes have evolved. The specialized cone cells in our eyes are tuned to different wavelengths and by stimulating them at specific ratios, we can simulate any visible wavelength. I recommend giving it a read as it should perfectly answer your question and it has some really nice and colourful pictures.

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.