Definition
Light absorption is what happens when a material neither bounces light back nor lets it pass through, and instead takes the light's energy into itself. That absorbed energy makes the molecules inside the material move more actively, so it mostly turns into heat.
Why Is an Apple Red and a Black Shirt Hot?
The white light from the sun or a lamp is really a mix of every color, like a rainbow: red, orange, yellow, green, blue, indigo, and violet. When this light hits an object, some colors bounce off and some get sucked in, depending on what the material is made of.
An apple looks red, but not because its skin gives off red light. It absorbs most of the other colors, like blue and green, and mainly bounces red light back toward your eyes.
A black shirt, on the other hand, absorbs almost every color of visible light, with nothing left over. No light bounces back, so it looks black to your eyes. All that light energy piles up in the fabric and turns into heat. That is why a black shirt feels extra hot in the midsummer sun.
A Bit More Precisely: How Electrons Swallow Light
Zoom in on a material all the way down to its atoms. At the center is a nucleus, and electrons circle around it in fixed energy levels. Think of these levels as steps on a staircase. An electron can stand on a step, but it can't hover in the gap between two steps.
Light arrives in small packets of energy called photons. If a photon's energy exactly matches what an electron needs to climb to the next step, the electron absorbs the photon and hops up to the higher step. This is called an excited state.
Every material has its own staircase, with steps of different heights. So each material can only absorb light whose color matches the height of its energy steps. Light that doesn't match is not absorbed. It either passes through or bounces off. In molecules and solids, though, the steps overlap very closely. So they absorb a whole range of colors at once, not just one. That is why black fabric can absorb almost every color.
From Plants Making Food to Sunscreen
Nature and people both put light absorption to good use. A leaf looks green because chlorophyll, its pigment, strongly absorbs the red and blue light that photosynthesis needs. Green light is absorbed less, so more of it bounces back. The plant turns the light energy it absorbs into chemical energy and makes its food.
The organic compounds in the sunscreen you put on in summer work in a similar way. They soak up strong ultraviolet rays that could damage your skin cells, in place of your skin. Then they release the energy as a tiny amount of harmless heat, which keeps your skin safe.
Solar panels use light absorption too. When a semiconductor absorbs light, electrons inside get excited and can move. The panel draws off that flow of electrons as electricity, which gives us clean energy.
Common misconceptions
A red object looks red because it has red light inside it.
A red object doesn't hold red light. It absorbs most of the other colors and mainly reflects red, so it looks red.
Where you meet it
In the summer sun, a black shirt feels much hotter than a white one.
Sunglass lenses absorb glaring visible light and harmful ultraviolet rays to protect your eyes.
Chlorophyll in a plant's leaves absorbs red and blue light for photosynthesis.
In one sentence
Light absorption is when a material takes in light energy of certain wavelengths and turns it into heat or another form of energy.