Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 12
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Consider the Situation Shown in the Figure. the Two Slits S1 and S2 Placed Symmetrically Around the Central Line Are Illuminated by a Monochromatic Light of Wavelength λ. - Physics


Consider the situation shown in the figure. The two slits S1 and S2 placed symmetrically around the central line are illuminated by a monochromatic light of wavelength λ. The separation between the slits is d. The light transmitted by the slits falls on a screen ∑1placed at a distance D from the slits. The slit S3 is at the central line and the slit S4 is at a distance z from S3. Another screen ∑2 is placed a further distance D away from ∑1.Find the ratio of the maximum to minimum intensity observed on ∑2 if z is equal to

(a) \[z = \frac{\lambda D}{2d}\]

(b) \[\frac{\lambda D}{d}\]

(c) \[\frac{\lambda D}{4d}\]

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Separation between the two slits = d
Wavelength of the light = \[\lambda\]

Distance of the screen = D

The fringe width (β) is given by \[\beta = \frac{\lambda D}{d}\]

At S3, the path difference is zero. So, the maximum intensity occurs at amplitude = 2a.

(a) When \[z = \frac{D\lambda}{2d}\]

The first minima occurs at S4, as shown in figure (a).

With amplitude = 0 on screen ∑2we get

\[\frac{l_{max}}{l_{min}} = \frac{\left( 2a + 0 \right)^2}{\left( 2a - 0 \right)^2} = 1\]

(b) When \[z = \frac{D\lambda}{d}\]

The first maxima occurs at S4, as shown in the figure.

With amplitude = 2a on screen ∑2, we get

\[\frac{l_\max}{l_\min} = \frac{\left( 2a + 2a \right)^2}{\left( 2a - 2a \right)^2} =  \infty \]

(c) When \[z = \frac{D\lambda}{4d}\]

The slit S4 falls at the mid-point of the central maxima and the first minima, as shown in the figure.

Intensity \[= \frac{l_\max}{2}\]

\[ \Rightarrow \text{Amplitude }= \sqrt{2}a\]

\[\therefore \frac{l_\max}{l_\min} = \frac{\left( 2a + \sqrt{2}a \right)^2}{\left( 2a - \sqrt{2}a \right)^2} = 34\]

Concept: Refraction of Monochromatic Light
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HC Verma Class 11, 12 Concepts of Physics 1
Chapter 17 Light Waves
Q 33 | Page 383
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