Educator

Thin Film Interference

• Change of phase upon reflection: When light, traveling in a medium of index of refraction n_1, is reflected from a medium of index of refraction n_2 > n_1, a phase change of 180 degrees takes place.
• The light waves reflected from a thin film of index of refraction n and thickness t, interfere destructively (i.e., no reflection takes place, the film looks dark) if

  2 n t = m * lambda

  Where lambda is the wavelength of the light and m = 0, 1, 2,...

  Constructive interference occurs when 2 n t = (m + 1/2)*lambda.