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The absorption of light by blood experiment – an experiment with blood and colored lights.

Red blood molecules illustration

The absorption of electromagnetic radiation happens all around us. In fact, this absorption process is the cornerstone for many modern-day technologies. Absorption of electromagnetic radiation allows the conversion of solar energy into electricity. The absorption of microwave electromagnetic radiation is what makes radars work. And our bodies absorb electromagnetic radiation too. Certain frequencies are absorbed easier by our bodies than others.

Red light shining through fingers

In this experiment, we will demonstrate the body’s absorption of a common frequency range of electromagnetic radiation – visible light.

In a darkened room, we will hold a red LED flashlight to our thumb. The thumb will glow bright red indicating much of the light passed right through our thumb. When we hold a green LED flashlight to our thumbs, however, almost all of the light is absorbed.


  1. Darken the room.
  2. Turn on the red flashlight.
  3. Hold the light behind the thumb.
  4. Notice that the thumb glows as the light passes through it.
  5. Now hold a green flashlight against the thumb.
  6. Almost all of the light is absorbed by the thumb tissue.

Our blood looks red because it absorbs all frequencies of light except for the red spectrum. This allows red light to pass easily through the many blood vessels in our thumb without being absorbed.

One of the frequencies that is easily absorbed by our blood is the green color frequency. When we shine the green light against our thumb, the light cannot pass through the many blood vessels in our thumb. Instead, the green light is absorbed by our blood.

Green light
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