Isaac Newton passed sunlight through a glass prism and saw it spread into a rainbow of colors, then used a second prism to combine them back into white. White light is a mix of all the colors of the visible spectrum. What color an object looks depends on which of those colors it reflects, absorbs or transmits.
Key idea
An object's color is the color of the light it reflects into your eyes; it absorbs the rest. A colored filter transmits light of its own color and absorbs (blocks) the others. With no light of its color to reflect, an object looks black.
Why it works
A red apple in white light absorbs most of the colors and reflects mostly red, so red light reaches your eye. Put the same apple under pure blue light: there is no red light to reflect, and it absorbs the blue, so it looks black. A white object reflects every color, so it takes on the color of whatever light shines on it.
Worked example: A filter
White light shines through a green filter onto a white wall. What color is the spot?
- The filter transmits green and absorbs the other colors.
- Only green light reaches the white wall, which reflects it.
Answer: Green
Worked example: A shirt under colored light
A red shirt is lit only by green light. What color does it look?
- A red shirt reflects red and absorbs other colors.
- There is no red light to reflect, and it absorbs the green.
Answer: Black (very dark)
Common mistake
Thinking a red filter adds red to the light, or "dyes" it. A filter can only take colors away: white light contains red already, and the filter blocks everything else. Check: a red filter in front of a blue light gives darkness, not purple.
Split white light with a prism on the Optics Bench.