Turn on the flashlight and put your hand over it so that the light shines through the skin between your thumb and first finger. What colour is the light that you see through your hand?
Why it works: Our bodies are mostly transparent to red light. That means red light can pass through our body’s tissues. The red laser light at 690 nanometre wavelength that Levy uses to activate photodynamic drugs passes right through skin and flesh. Even hemoglobin, the protein that makes blood red, does not absorb red light at that wavelength.
Why do you think red light passes through our bodies? This turns out to be a handy quality because it means that many diseases can be treated by taking an injection of photodynamic drug, waiting a few hours, then sitting in a room with strong red light.