Question

On introducing a thin film in the path of one of the two interfering beam, the central fringe will shift by one fringe width. If $\mu =1.5$, the thickness of the film is (wavelength of monochromatic light is $\lambda$)

• A. $4\lambda$
• B. $3\lambda$
• C. $2\lambda$
• D. $\lambda$

Answer 1

Answer: (C)

$2\lambda$

Path difference introduced in the path of the interfering beam because of the thin film is given by

$(\mu -1)t=n\lambda$

where, n is the number of fringes shifted, n=1(given).

$\lambda$ is the wavelength of light used.

$\mu$ is the refractive index of the light.

Given $\mu =1.5$

t is the thickness of the film.

$(1.5-1)t=1\lambda$

$t=\lambda /0.5$

$t=2\lambda$