In the case of Fresnel zones
a) the phase difference between the waves spreading from consecutive zones and reaching a point on the screen is 1800
b) the path difference between the waves spreading from alternate zones and reaching a point on the screen is $λ$
c) the area of the zone increases with the increase of obliquity
d) the area of each zone is proportional to the product of the wavelength and the distance of the given point from the zone

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only a,b and c are true

only a,b and d are true

only a and b are true

only b and d are true

answer is B.

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Detailed Solution

The half period zone is provided by an optical device known as zone plate. It is simply a plane parallel gloss plate having concentric circles of radii accurately proportional to the square roots of the consecutive natural numbers 1,2,3 ... etc. The area is given by π $r^{2}$ . Hence the areas are π,2π,3π,4π,---
The phase difference is thus π between each successive half period zones of strips.

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