Interference

Interference of light waves

1. Interference of light waves By Mrs.P.Kanmani M.Sc., M.Phil. , Assistant Professor of Physics, Department of Physics (self), V.V.Vanniaperumal College for Women, Virudhunagar.

2. What is Interference ? To grasp interference of light waves we need to understand the terms interference, light, wave and wave front. Interference is defined as the combination of two or more waveforms to form a resultant wave in which the displacement is either reinforced or cancelled.

3. What is light? Light is defined as electromagnetic wave with wavelength lies between 400 and 700 nm which is visible to the human eye.

4. What is a wave? A wave is characterized by its amplitude ,wavelength and phase. Amplitude tells about its intensity. Wavelength ( λ ) tells about its energy – inversely proportional. Phase of the wave is defined as the distance between the first zero-crossing and the point in space that is defined as the origin.

5. What is a wave front? A wave front is a line or plane on which vibrations at every points on it are in phase and are at the same distance from the source of the wave.

6. Wave front

7. Interference of light waves With this basic knowledge lets get into interference of light waves. In order to observe interference of light waves, the following conditions must be met: The sources must be coherent- that is, they must maintain a constant phase with respect to each other. The sources should be monochromatic - that is, of a single wavelength.

8. Young’s double slit experiment Interference of light waves from two sources was first demonstrated by Thomas Young in 1801. The schematic of young’s demonstration is shown in figure.

9. Interference of light waves Wave fronts from source arrive at a barrier that contains two parallel slits. These two Slits serve as a pair of coherent light sources because waves emerging from them originate from the same wave front and therefore maintain a constant phase relationship. The light from two slits produces on a viewing screen a visible pattern of bright and dark parallel bands called fringes.

10. Constructive and destructive Interference The bright bands appearing on the screen are due to constructive interference. Constructive interference occurs when the phase difference between the waves is an even multiple of π (180°). The dark bands appearing on the screen are due to destructive interference. Destructive interference occurs when the difference is an odd multiple of π. 

11. Practical applications of interference of light waves The phenomenon of light-wave interference with oily or filmy surfaces has the effect of filtering light. Thus, has a number of applications in areas relating to optics: sunglasses, lenses for binoculars or cameras, and even visors for astronauts. In each case, unfiltered light could be harmful or, at least, inconvenient for the user, and the destructive interference eliminates certain colours and unwanted reflections.