# Interference Phenomena

Interference phenomena can be observed throughout nature and often in everyday events. The myriad of colors on the surface of an oil slick, the brilliant patterns on peacock feathers-- these both result from two or more light waves interfering to form a total wave that is different from both of the original waves. In these particular cases, the interference is called thin-film interference.

Consider a beam of light travelling through a piece of glass: As the beam strikes a thin layer of air within the glass (like an air bubble), some of the light will be reflected when the beam enters the layer of air and some will be reflected as it leaves the layer of air and passes into the glass on the other side. This makes two reflected beams travelling in the same direction that might interfere with each other.

When light strikes a surface, some light may be reflected and some may be transmitted, depending on the index of refraction of the surface that the light strikes. If the surface has a higher index of refraction than the original medium, reflected light will undergo a phase change of 180 degrees. On the other hand, when light strikes a surface with lower index of refraction than that of the original medium the light is reflected with no phase change. Two waves with the same amplitude and a phase difference of 180 degrees will completely cancel each other.

#### Index of Refraction:        Width:

Wavelength in vacuum (n = 1) is 8. Change the width and index of refraction of the middle layer to answer the following questions. You can drag the right surface to change its width but the text boxes usually give better control.

• Why does the total reflection depend on the film's width?

• Under what circumstances would the reflected beams from the two surfaces cancel each other?

• When no light is reflected, how is the width related to the wavelength? Does the index of refraction of the glass play a role?

• When maximum light is reflected, how is the width related to the wavelength?