Multiple Beam Interference

Optical Reversibility and Phase Changes on Reflection G.G. Stokes used the principle of optical reversibility to investigate the reflection of light at an interface between two media. The reversibility principle states that If there is no absorption of light , a light ray that is reflected or refracted will retrace its original path if its direction is reversed.

© SPK

Thin films: multiple beam interference © SPK

Optical field in reflected beam : the incident wave

Resultant reflected scalar wave

Reflected irradiance

Optical field in transmitted beam

Transmitted irradiance

For Transmitted rays

For Reflected rays

Coefficient of Finesse

Airy function A

I0 d

IR/I IT/I d

Types of localization of fringes

Interference fringes Real Virtual Localized Non-localized

Localized fringe Observed over particular surface Result of extended source

Non-localized fringe Exists everywhere Result of point/line source

POHL’S INTERFEROMETER Real Non-localized Virtual Localized

Newton’s Ring U<<R & U>>d

Interference filter

An interference filter is designed for normal incidence of 488 nm light. The refractive index of the spacer is 1.35. What should be the thickness of the spacer for normal incidence of light. It will pass different wavelength if the angle of incidence is not 90o.

Reference 1. Optics Author: Eugene Hecht Class no. 535 HEC/O Central library IIT KGP