Scattering of Light

Sorry, folks, before we get to the experiment we have to explain scattering briefly.

Although we don't realize it, scattering of light is very important for our daily life. Almost all objects scatter light, that means they reflect the light that illuminates them in all directions. If the objects around us would reflect light perfectly without scattering - like polished metal - it would be like in a mirror cabinet.

There are several ways how scattering occurs, we pick two important examples:

• Random reflection on a rough surface.

  
  If you don't polish your car or jewelry for a while you'll have a good example for this.

• Reflection (or refraction or diffraction) on impurities in the volume.

  This is the type of scattering which plays the major role in medical applications. In tissues the light is scattered at cells or their components. In milk it is scattered mostly at the tiny fat droplets.

Experiment with Light Scattering

It is quite easy to demonstrate the effect of scattering with simple equipment. If you have a laser pointer you can do it by yourself like this:

A simple glass instead of our fancy tank will be just fine. Because the camera we used was to insensitive we actually used a stronger laser for the pictures below. However, if you dim the light you should see the same effects using your laser pointer, only better.

1. Fill the glass with clear water and place a white sheet of paper behind the glass.
2. Shine with the laser pointer through the water (see picture a) below). Depending on how many small air bubbles (or other particles like dust) you have in the water you might see a dim red line where the laser beam goes through the water. The bright spot on the paper is actually just red, the white spot in our picture is caused by overexposure.
3. Add a small amount of milk and stirr untill it is mixed. If you hold the laser so that the beam passes right below the surface you should be able to see where the beam goes.
   We had 600 cc water (little more than 1/2 Gallon), and added 1 cc of vitamin D milk (see picture b)). You will probably have less water, so be very carefull and start by adding only a few drops! If you add too much milk you will not see the dim spot (see arrow in picture b)) on the screen anymore.
4. Add more milk and see how the light pattern changes. In pictures c) to f) you can see how the light beam eventually becomes a glowing ball. (In the last picture the level is so low because I ran out of milk.)

OMLC Home | Science © SAP 13 Dec 1996