What Is Interference of Light?

The wave properties of light, the way light travels, mean that light can experience interference from other types of waves. This phenomenon is called optical interference.The color of a wave determines the type of interference and the effect on the interfered wave.

History

When scientists in the 17th century began to experiment with the nature of light, their first observations of light's properties came from watching light passing through a slit. The phenomenon,coined as "diffraction," means that the light broke up. Other experiments dealt with the phenomenon of colors and the nature of white light. Sir Isaac Newton performed experiments in the latter category, although he did not accept the theory that light has wave properties. At the beginning of the 19th century, Thomas Young developed the double-slit experiment, and demonstrated light's wave properties. Since then, science has measured the wavelength of light and the speed of light.

Function

Optical interference occurs when two or more waves come together. The result of two or more waves coming together is a dark or light band called a fringe. In Thomas Young's Double-Slit Experiment, he made sunlight pass through long parallel slits. The result was "interference fringes," or bands of color. When artificial, monochromatic light is used, the bands are dark and white. One way to see how interference of light functions, is with soap bubbles. When monochromatic (not sunlight) light passes through the thin film of a soap bubble, the wave reflects from the top and bottom surfaces of the bubble. This creates constructive and destructive interference, which gives the prismatic colors one sees in bubbles.

Types

There are two basic types of optical interference. Constructive Interference occurs when two or more waves come together to form a larger and stronger wave, matching their crests (tops of waves) and troughs (lowest points of waves). The two waves' crests and troughs coincide. With constructive interference, wave amplitude is amplified because there are two coinciding crests and troughs, which increase amplitude. When each waves continues on, they return to their original amplitudes. Destructive Interference occurs when two or more wave's crests coincide with the waves' troughs. The result is that the waves cancel each other out. Their amplitudes weaken until they continue and resume normal amplitude.

Effects

With a simple bubble experiment, one can see fringes, dark bands, the effects of constructive and destructive interference (see "Using Bubbles" in Additional Resources). Thin film interference as displayed in the thin film (bubble) experiment has practical uses. The color of the fringes (the light and dark bands), is related to the thickness of the bubble's walls. This correlation is used to measure thin films that cannot be measured by conventional means. Scientists also can use thin film properties to develop filters that allow only specific colors to pass.

Considerations

Optical interference is being studied for futuristic considerations. The science of holography, producing three-dimensional images with laser beams and mirrors, creates interference fringes that becomes a hologram. When laser light waves pass through a hologram, they produce a a three-dimensional image. As the study of light interference relates to careers, optical engineers have futures in aerospace mission systems, astronomy, and satellite optical sub-systems.

Resources

• What is Interference?
• Interference
• Using Bubbles to Learn about Light Interference