eli5:How come one end of light wavelengths is red and one violet, not black and white?
In: Physics
Because red and violet is how our vision interprets frequencies of light, and black and white is how it interprets light intensity.
Because white is a combination of all of the visible wavelengths and black isn’t any light at all.
Colors are the result of different wavelengths of light. White and black are all or nothing. It makes zero sense to talk about black and white when discussing wavelength
White isn’t a single color, it’s what we see when red, green and blue (the primary spectral colors that our eyes detect) are mixed together.
Now off the red end of the spectrum is infrared, and off the violet end is ultraviolet. Our eyes can’t see those wavelengths of light, so you could argue that the visible spectrum actually starts and ends with black…
Color is how our brain interprets the presence and absence of various frequencies of light.
Some colors correspond to light of a certain frequency, while others do not.
If you go though all the frequencies of the visible spectrum you will see it start with red at one end and end with violet at the other.
If you have something that emits or reflects all the frequencies of light we can see in more or less equal measure it will appear white.
If you have something that doesn’t emit or reflect any visible light at all it will appear black.
to simplify all that. White is what you get when you have light in all colors and black if what you get if you light of no color and the spectrum from red to violet is what you get if you sort all the colors my frequency/wavelength.
Visible light is the same form of radiation as x-rays, radiowaves, microwaves etc- electromagnetic radiation (EMR). The frequency of the wave is what determines its wavelength and each type of EMR is associated with different ranges of wavelength. Visible light falls in ~400-700nm range, with violet/blue light at the lower end and red light at the upper end as you mentioned.
Waves of light do not inherently possess color. Color is a biological phenomenon. In humans, our visual systems have evolved to best detect EMR in the visible light range (hence the name). Cone cells in our retinas have special proteins called opsins that respond to specific wavelengths (frequencies) of light within visible range and each corresponds to a different spectrum of the visible range (ie blue, green, red). After a photon (light wave) of the correct frequency hits an opsin, it triggers a structural change that allows it to interact with neurons in the eye and transmit a signal to the occipital (visual) cortex of your brain. Our brains have evolved to associate ~400nm light as blue, ~500nm light as green and ~600nm light as red.
But we can distinguish ~1mil different colors of light instead of just blue green and red. The different colors we see are a result of mixed input from each of the different opsins/cones and the strength of those signals relative to one another. Black is the absence of color – black objects absorb most frequencies of light in the visible spectrum instead of getting reflected into our eyes. White objects reflect most frequencies of visible light and when those reflected waves hit our cones the combination is balanced out and we do not see color. This is why sunlight appears white but in a rainbow (where white light is refracted into narrower ranges of wavelength by water droplets in the air) we see several colors.
If you have a pocket microscope or a dslr camera, put it up against your TV or computer monitor and focus it. You should be able to see individual units (pixels) that contain red green and blue diodes. Instructions from the computer change the voltage applied to those diodes to change the mixture/intensity of each diode which changes the *perceived* color. A white screen will have equal intensity of each diode.
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