# Physics Class 12 NCERT Solutions: Chapter 10 Wave Optics Part 6

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Q: 20. Answer the following questions:

(A) When a low flying aircraft passes overhead, we sometimes notice a slight shaking of the picture on our TV screen. Suggest a possible explanation.

(B) As you have learnt in the text, the principle of linear superposition of wave displacement is basic to understanding intensity distributions in diffraction and interference patterns. What is the justification of this principle?

Answer:

(A) Weak radar signals sent by a low flying aircraft can interfere with the TV signals received by the antenna. As a result, the TV signals may get distorted. Hence, when a low flying aircraft passes overhead, we sometimes notice a slight shaking of the picture on our TV screen.

(B) The principle of linear superposition of wave displacement is essential to our understanding of intensity distributions and interference patterns. This is because superposition follows from the linear character of a differential equation that governs wave motion. If and are the solutions of the second order wave equation, then any linear combination of and will also be the solution of the wave equation.

Q: 21. In deriving the single slit diffraction pattern, it was stated that the intensity is zero at angles of . Justify this by suitably dividing the slit to bring out the cancellation.

Answer:

Consider that a single slit of width d is divided into n smaller slits.

Width of each slit,

Angle of diffraction is given by the relation,

Now, each of these infinitesimally small slit sends zero intensity in direction. Hence, the combination of these slits will give zero intensity.