MHT-CET : Physics Entrance Exam

### MHT - CET : Physics - Interference of Light Page 3

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4.

Biprism Experiment:

B : biprism (mounted on a stand, slided along optical bench)
L1, L2 : Convex lenses mounted on stand
E : eyepiece

Interference pattern is obtained when the edge of the biprism becomes exactly parallel to the slit.

Wavelength of monochromatic light l =

 Xd D

X : band width, d : distance between sources
D : Distance between sources and screen.
(Measured from scale on optical bench)

Measurement of Band Width: Micrometer reading when cross wire is coincident with any bright band is noted, say x1. Readings corresponding to consecutive bright bands, x2, x3 … etc. are noted. Average value of x2 - x1, x3 - x2 etc. gives average band width X.

RAY DIAGRAM OF BIPRISM EXPERIMENT

S = Real source (a slit)
S1, S2 = Virtual sources (i.e.,virtual images of the slit),
B = Biprism, R = Region of interference

Measurement of d

• Without disturbing position of biprism and eyepiece, a convex lens is introduced between them.
• Lens L is moved towards slit so that two magnified images of slit are observed through eyepiece.
• Distance d1 between the magnified images noted down on micrometer.
• Lens is moved towards eyepiece so that two diminished images of the slit are seen through the eyepiece.
• Distance d2 between the diminished images is measured.
• Distance between coherent sources is given by
d =

To show that d =

Principle: Property of conjugate foci of the convex lens according to which the positions of object and image can be interchanged.

d = actual distance between S1 and S2
= size of object
d1 = size of magnified image

 Size of object Size of image

=

 Object distance Image distance

 d d1

=

 x y ...(i)

• Diminished image produced when object and image distance are interchanged.
d2= size of diminished image
 d d2

=

 y x ...(ii)

• From (i) and (ii),

 d d1

´

 d d2

=

 x y

´

 y x

=1

• \ d2 = d1d2
\ d =

What happens when
In the biprism experiment, if
(a) the eyepiece is moved towards the slit?
(b) the distance between the virtual sources is decreased?

In the biprism experiment, band width X =

 lD d

(a) If eyepiece is moved towards slit, D will decrease.
Since X
µ D (l, d constant)
\ Band width (X) will decrease.

(b) If d decreases, since X µ

 1 d

(l, D constant)

\ Band width (X) will increase.

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