Question

Two plane glass plates are placed on top of one another and on one side a paper is introduced to form a thin wedge of air. Assuming that a beam of wavelength 600 nm is incident normally, and that there are 100 interference fringes per cm, calculate the wedge angle

Answer 1

Given:

wavelength λ = 600 nm

no of fringes n = 100 per cm

To find:

wedge angle θ =?

Explanation:

• A thin film having zero thickness at one end and continuously increasing to a particular thickness at the other end is called a wedge.

i.e. t = 0

• The wedge angle is defined as the angle by which an isodose curve at a certain depth is tilted along the central beam is called wedge angle.

• The place where the two glass plates come in contact produces a dark fringe and that leaves 99 along with the 1cm plate of glass.

• So The separation distance is calculated as

$\displaystyle d =99 \frac{\lambda}{2}$

Put  λ = 600 nm in above equation, we get,

d= 2.97x$10^{-5$ m.

The wedge angle can be calculated as

$\displaystyle tan\theta =\frac{d}{10^{-2}}$

tanθ = $\displaystyle \frac{ 2.97\times 10^{-5}}{10^{-2}}$

θ = 0.17°

The required wedge angle is 0.17°.