Scattering: Receiving and Transmitting Light
Particles in materials act as tiny antennas (devices that send and/or receive electromagnetic radiation) by receiving visible light, which is made up of electromagnetic waves, and transmitting the waves in new directions. This process is called Raleigh scattering, after English physicist Lord Raleigh Qohn William Strutt, 1842-1919). In 1871, he solved the question of why the sky looks blue by describing how light is scattered.
In this project, you will determine the effect of small particles on the scattering of light. You will investigate how particle size affects the scattering angle of light. You will also discover how the scattering angle affects the brightness of a substance.
Purpose: To demonstrate the effect of small particles on the scattering of light.
- two 10-ounce (300-ml) colorless, transparent, plastic cups
- distilled water
- permanent marker
- whole milk
- Lay the sheet of paper on a table and lay the flashlight so that its bulb end is in the center of one of the longer edges of the paper.
- Fill the cups with water.
- With the marker, label the cups A and B.
- Add one drop of milk to cup B and stir.
- Bend the poster board in half by placing the short ends together to form a stand-up screen. Set the screen at the edge of the paper opposite the flashlight.
- With the flashlight on, darken the room and note the color of the light from the flashlight on the paper screen.
- Set cup A in the center of the paper in front of the flashlight and do the following:
- Note the color of the light on the screen after it has passed through the water.
- Looking down on the water, note any change in the color of the water.
- Repeat step 7, using cup B.
The color of the flashlight's beam when it is seen on the screen after traveling through the air can vary from white to yellowish. When the flashlight beam passes through plain water, you don't notice a change in the color of the light on the screen or the color of the water. But milky water makes the light on the screen appear more yellow to orange and the milky water appears bluish.
Light, like other electromagnetic radiation, shows both wave and particle properties. Its propagation (motion) is wavelike, but its interaction with matter occurs as if the radiation travels as particles consisting of the unit of energy associated with each frequency of radiation, called a quantum. The particles are called photons (packets of energy consisting of a quantum of electromagnetic radiation that has both a particle and wave behavior). Since photons oscillate like a wave, the size of a photon is considered to be equal to the light's wavelength when acting as a wave.
The flashlight is a source of white light, which is visible light (the part of the electromagnetic spectrum to which the eye is sensitive) made of all colors of light—all wavelengths from red, the longest, to blue and violet, the shortest. When all the wavelengths of light are mixed together, your eye receives them and your brain interprets the combination as white light—no color. Without milk in the water, light travels through the water without being seen as a color. But with the milk, the water has a bluish color and the light transmitted to the screen has a yellowish to orange color. This change is due to scattering (the deflection or spreading out of a beam of electromagnetic radiation as it passes through material) of some of the light waves. Scattering can be a combination of processes, one being elastic scattering (reflection—bouncing off of), in which photons collide with and bounce off particles, much like the collision of two billiard balls and their bouncing off each other. A photon is scattered best when it collides with a particle approximately its own size. The short wavelengths—violet and blue—are scattered most by the small milk particles in the water. Thus as the white light passes through the milky water, a mixture of wavelengths that give the overall sensation of a pale blue color are scattered. Violet—the shortest wavelength—would be in that mix, as would some of the longer wavelengths as well. As the incident light Oight that strikes the surface of a material) hits the milk particles, the mixture of short wavelengths is scattered from the light, and the remaining parts of the light transmitted to the screen have a yellow to orange color (see Figure 25.2). Note that if the particle is bigger than the largest wavelength in visible light, then the scattered light contains all of the wavelengths; thus the light is white.