Phase Change AP Physics B Phase Change on

Phase Change AP Physics B

Phase Change on Reflection l When light is reflected off a more dense medium with higher index of refraction, crests get reflected as troughs and troughs get reflected as crests. ¡ The wave is said undergo a 180° change of phase on reflection. ¡ The net effect of the phase change is that the reflected ray ‘jumps ahead’ by half a wave length. Light in air, reflecting off just about anything (glass, water, oil, etc. ) will undergo a 180° shift. There is no phase change as light goes through a substance.

Thin Films l Phase changes can be seen in interference patterns of thin films. • Constructive and destructive interference of light waves is also the reason why thin films, such as soap bubbles, show colorful patterns.

Thin Film Interference l Consider the case of a thin film of oil floating on water. Thin-film interference can take place if the following two light waves interfere constructively: 1. Light from the air reflecting off the top surface. 2. Light traveling from the air, through the oil, reflecting off the bottom surface, traveling back through the oil and out into the air again. 180° Phase Shift

Constructive/Destructive Interference l Thus, if one of the rays undergoes an odd number of phase changes, the conditions for constructive and destructive interference are as follows: • These formulas are valid whenever m can also be zero an odd number of phase changes occur. • If an even number of phase changes occur then the formulas are flipped for different types of interference. • The phase change does not happen when light is reflected off a less dense medium, i. e. for total internal reflection.

180° Phase Shift Between Reflected Rays l To get constructive interference between the reflected rays, the two reflected waves have to be shifted by a half wavelength multiples. l The thickness of the film must be half multiples of wavelengths.

180° Phase Shift Between Reflected Rays l To get destructive interference between the reflected rays, the two reflected waves have to be shifted by an integer multiple of wavelengths. l The thickness of the film must also be multiples of wavelengths.

In or Out of Phase l Constructive and Destructive interference path difference flip-flops when two light rays are in phase or 180° out of phase. • The wavelength in these equations is the wavelength of the light through the film. • What is the P. D. of the interfering waves?

In or Out of Phase: Thin Films l In Phase: no phase shift; path difference similar to double slit ¡ Constructive Interference l Out of Phase: 180° phase shift; path difference similar to single slit ¡ Constructive Interference ¡ Destructive Interference

Light Sources Monochromatic Source Broadband Source l In the case where incident light is monochromatic in nature, interference patterns will appear as light and dark bands. l As the thickness of the film varies from one location to another, the interference may change from constructive to destructive. l If the incident light is broadband, or white, such as light from the sun, interference patters will appear as colorful bands. l Different regions of the film will appear to be different colors depending on the local film thickness.

Thin Film Applications l Thin films are used commercially in antireflection coatings, mirrors, and optical filters. l They can be engineered to control the amount of light reflected or transmitted at a surface for a given wavelength.