The radius of fourth half period zone at a point 4 m away from a plane wave front of wavelength 6400 Å is:

Question

Question

Answer 1

CONCEPT:

Fresnel’s Half Period Zone (HPZ):

F1 Prabhu 27.11.20 Pallavi D4

According to Fresnel’s the entire wavefront can be divided into a large number of parts of zones which are known as Fresnel’s half-period zones (HPZ’s).
The resultant effect at any point on the screen is due to the combined effect of all the secondary waves from the various zones.

EXPLANATION:

F1 Prabhu 27.11.20 Pallavi D5

\(⇒ r_n=\sqrt{ndλ}\)

Where n = number of zones, d = distance from point P to the wavefront, and λ = wavelength

Calculation:

Given n = 4, d = 4 m, λ = 6400 A°;

Now the radius of the fourth half-period zone will be

\(⇒ r_4=\sqrt{4×4×6400×10^{-10}}\)

⇒ r₄ = 32 × 10^-4 m;

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