See Your Veins with a Flashlight
Have you ever seen an X-ray? This type of photograph uses special wavelengths of radiation, called X-rays, instead of visible light. X-rays can penetrate or pass through most of your body but are absorbed by your bones, so that you can see your skeleton inside your body. Can visible light also penetrate your body? You may have answered "no", but in fact some types of visible light actually can pass completely through most of your body. In this experiment you will use a flashlight to view inside your body, just like in an X-ray image, but instead of your bones you will be able to see the veins deep under your skin. And if that's not creepy enough, your bones will actually disappear!
What you'll need:
- A small flashlight- an LED keychain flashlight works very well, and ones with colored bulbs are ideal (see link below), but about any flashlight should work.
- A dark room.
Basic Experimental Procedure:
- Bring your (white) flashlight to a very dark room (night time is best), and wait about one minute for your eyes to adjust to the darkness.
- Hold one hand in front of your face.
- Press the flashlight into your hand (or finger), on the back side (so you can't see it) and shine the light through your hand (or finger) toward your eyes.
- Your hand will appear to glow red, and if you look closely you should be able to see thin black lines inside your hand or fingers. These are your veins. Move the flashlight around to see different parts of hand.
- Try this with colored lights. Do some colors work even better than white? Are some worse?
First a little background science. Light can appear many colors because each color has a different wavelength, and what we think of as "white" light is actually all of these colors (wavelengths) combined together, but these individual colors can be separated in several ways. One way is to use a specially shaped piece of glass or plastic called a prism (see the PBS video link below), but actually the color in just about any object you see (like a red T-shirt, or green grass, or a blue bicycle) is usually due to separating colors of light. When light from the sun, or a lamp, or in this experiment your flashlight hits an object, any (or all) of three things can happen: some of the light (and more importantly, some of the colors) can bounce off the object; some can pass through the object; and some can be absorbed by the object (kind of like a sponge absorbs water). Normally we see most objects because of the light that bounces off of them and into our eyes, so it's natural to assume that all light always bounces of objects, but grass appears green because it absorbs most of the light which hits it. Only the green wavelengths of light bounce off and into our eyes. Most of the red, blue and other colors are absorbed.
Glass, on the other hand, is usually transparent, which means that most of the light passes through. Imagine that you are standing inside a dark church admiring the beautiful colors of a stained-glass window as the sunlight outside the church passes through each of the colored panels. One panel looks red because only the red wavelengths pass through the glass and into your eyes. The other wavelengths either bounce of the surface (reflection) or are absorbed by the glass. The same is true for any green or blue panels- only green or blue light passes through the glass. The other colors we see are due to mixing these three primary colors (BTW- this is a fascinating subject, but we'll leave that for another experiment. If you're interested, check out the links below.)
Now we're ready to talk about your hand. Just as with stained glass, the various cells in your body (skin, muscle, bone, blood, etc.) can either reflect, absorb or pass (transmit) the various colors of visible light. As you might expect, most of the light (of all colors) bounces off your skin, but if you hold the flashlight very close, some of it will penetrate. Of this light which penetrates your body, most is then absorbed by your cells, but a little bit of it can pass completely through your body as long as it's not too thick. That's why we need to do this is a very dark room, and use a thin part of your body, like your hand and fingers.
What's really cool is that only one color of light passes completely through your hand- and that's why it looks red. All of the other colors are completely absorbed, either by the skin, bones, muscles, blood or other cells in your body. Most of the red light passes right through all of these cells except your blood cells- but not even all of your blood cells. Red light passes through the blood in your arteries, but is absorbed by the blood in your veins. That's why your veins appear black. Did you notice that red light even passes through bones? It's as though your skeleton has vanished!
What's the difference between the blood in your arteries and veins? Oxygen. Arteries carry freshly oxygenated blood away from your lungs to replenish the other cells in your body. Once they have given up all of their oxygen they return to your lungs in the veins to get more and start the process over again. When blood is fully oxygenated it's a very bright red color, but without oxygen it's appears a much darker red color, and this darker blood in your veins absorbs most of what is left of the red light, thus no light passes through (dark = absorption). By the way, it's a well established myth that the blood in your veins is blue because it has lost its oxygen. While the veins which you can see near the surface of your skin do often appear blue, it is not because the blood inside them is blue. Human blood is never blue, although horseshoe crabs and some other animals do have blue blood (see link below). The actual reason that your veins appear blue is rather complicated, but it does have to do with absorption, transmission and reflection of different colors of light (see links below for details).
A white light works in this experiment because white light really contains all of the colors, including some red. It works even better if you use a red flashlight, since all of this light is red, and you will notice how much brighter your hand appears. If you use any other color flashlight, however, you won't see any light pass through your hand. Try it.
Variations and Related Activities:
Do you have a remote control for your television with buttons that light up when you press them? If so, you may have noticed that when you press a button (in the dark) that lights up red, it makes your fingertip glow bright red, but if the button lights up green or blue, your finger doesn't glow at all. Now you know why.
Links to more information and activities
A good place to buy cheap LED flashlights in different colors:
A video from Bill Nye the Science Guy on light & color:
Another PBS video on light & color:
Absorption of light:
More about blood, arteries and veins:
Why do veins look blue?:
Horseshoe Crabs have blue blood: