Under what circumstances can a red shadow be formed?
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Your retina, which covers the back of the eye, contains light receptors called rods and cones. Rods are used for night vision and they only let you see in shades of gray. You have only one type of rod but three types of cones. Cones let you see in color as long as it's not very dark.
All three types of cones respond to a wide range of wavelengths, but one type is the most sensitive to long wavelengths (the red end of the spectrum), one to medium wavelengths, and one to short wavelengths (the blue end of the spectrum). With just these three types of cones, we are able to perceive more than a million different colors.
When a red light, a blue light, and a green light are all shining on the screen, the screen looks white because these three colored lights stimulate all three types of cones in your eyes approximately equally, creating the sensation of white. Red, green, and blue are therefore called additive primaries of light.
With these three lights you can make shadows of seven different colors—blue, red, green, black, cyan, magenta, and yellow—by blocking different combinations of lights (click to enlarge diagram below). When you block two lights, you see a shadow of the third color—for example, block the red and green lights and you get a blue shadow. If you block only one of the lights, you get a shadow whose color is a mixture of the other two. Block the red light and the blue and green light mix to create cyan; block the green light and the red and blue light make magenta; block the blue light and red and green make yellow. If you block all three lights, you get a black shadow.
All three types of cones respond to a wide range of wavelengths, but one type is the most sensitive to long wavelengths (the red end of the spectrum), one to medium wavelengths, and one to short wavelengths (the blue end of the spectrum). With just these three types of cones, we are able to perceive more than a million different colors.
When a red light, a blue light, and a green light are all shining on the screen, the screen looks white because these three colored lights stimulate all three types of cones in your eyes approximately equally, creating the sensation of white. Red, green, and blue are therefore called additive primaries of light.
With these three lights you can make shadows of seven different colors—blue, red, green, black, cyan, magenta, and yellow—by blocking different combinations of lights (click to enlarge diagram below). When you block two lights, you see a shadow of the third color—for example, block the red and green lights and you get a blue shadow. If you block only one of the lights, you get a shadow whose color is a mixture of the other two. Block the red light and the blue and green light mix to create cyan; block the green light and the red and blue light make magenta; block the blue light and red and green make yellow. If you block all three lights, you get a black shadow.
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