Why does a red apple turn black under a green light?
After you watchWhy does a red apple turn black under a green light?
The short answer
A red apple turns black under a green light because the color you see is the light an object bounces back, not a color baked inside it. A red apple only bounces red light and soaks up the rest. Pure green light has no red in it at all, so the apple absorbs the green and reflects almost nothing — with no light reaching your eye, it looks black.
Try this next
- What if you shine pure red light on the red apple instead of green? Sweep the lamp to red before you watch — predict first: does the apple glow brighter, stay the same, or go dark?
- What about a white shirt or a yellow banana under that green lamp? A white surface bounces every color and a banana bounces both red and green — predict first whether each one glows green, glows yellow, or goes dark before you reason it out.
- What if the light has two colors mixed in, not just one? Picture a lamp with both red and green in it and predict whether the apple's red comes back before you decide.
The whole story
How it works
White light is really a mix of every color, including red, green and blue, all travelling together. When light lands on a surface, the surface soaks up some colors and bounces the leftover ones back to your eye, and those bounced colors are the color you see. A red apple's skin absorbs most colors and reflects mainly red. Under white or red light there is red available for it to bounce, so it looks red. Under pure green light there is no red to reflect, the apple absorbs the green, and it sends back almost nothing, so it appears black.
What people get wrong
Many people think an object's color lives inside the object and stays the same no matter what light hits it. In fact color depends on the light: the color you see is whichever colors the object reflects, and an object can only reflect colors that are actually present in the light shining on it. Take red out of the lamp and a red apple has nothing red to bounce, so its red disappears.
The catch
White light carries every color, so every object can show its true color, but that only works because all the colors are present — remove one and any object that depends on it goes dark. Colored light, like a pure red or green spotlight, can make matching colors glow or create a dramatic mood, but it hides the true colors of everything else, turning many objects dull or black.
Questions kids ask
Does the apple actually change color under the green light?
No. The apple's skin is exactly the same. What changes is the light. Its skin can only bounce back red, and pure green light has no red in it, so there is nothing for the apple to reflect and it looks black even though it never changed.
What color does a green leaf look under pure red light?
It looks black or very dark. A leaf bounces back green and soaks up other colors, but pure red light has no green in it, so the leaf has nothing to reflect and appears black, just like the apple does under green light.
Why do all colors look right under sunlight or a normal bulb?
Sunlight and good white bulbs contain every color at once, so whatever an object is built to bounce back is available in the light. That is why reds look red and greens look green outdoors, and why some cheap bulbs make colors look slightly off if they are missing part of the mix.
If the apple soaks up the green light, where does that light go?
The absorbed light turns into a tiny bit of heat in the apple's skin. Absorbed light energy does not vanish; it warms the object very slightly instead of bouncing back to your eye, which is also why dark objects heat up faster in the sun.
Talk about it
- Guess first: if we turned our living room into one big green light, which of your toys would disappear?
- Where do you think the color of an apple really lives — in the apple, or in the light?
- Why do you think food can look weird under the lights in some stores or parties?
For grown-ups
A surface's color comes from selective reflection: pigment molecules absorb most wavelengths and reflect a particular band, such as the long (red) wavelengths from an apple's skin. You only perceive a color when that wavelength is both present in the illuminating light and reflected by the surface. Under a narrow-band green source there are no red wavelengths to reflect, the green is absorbed, and the apple reflects almost nothing, so it reads as near-black. This is why perceived color depends on the light source's spectrum, the idea measured by a bulb's colour-rendering index (CRI).
Keep going
What else makes you wonder?
- If a green leaf goes black under red light, what color would a banana look under blue light?
- When the room is lit by one colored lamp, why do some things still glow while others vanish?
- Where does all that soaked-up light energy actually end up?