Question

A light of wavelength $\lambda$ is incident on an object of size b. If a screen is at a distance D from the object. Identify the correct condition for the observation of different phenomenon :
a) if $b^2=D\lambda$ , Fresnel diffraction is observed
b) if $b^2>>D\lambda$, Fraunhoffer diffraction is observed
c) if $b^2<<D\lambda$, Fraunhoffer diffraction is observed
d) if $b^2=D\lambda$, Fraunhoffer diffraction is observed

• A. a, b and d are true
• B. a,c and d are true
• C. a and c are true
• D. a and d are true

Answer 1

Answer: (C)

a and c are true

Fresnel diffraction equation for the near - field diffraction , is an approximation of the kirchoff.
Fresnel diffraction that can be applied to the propagation of waves in the near field.
$b^2=D\lambda$
Fraunhofer diffraction equation is used to model the diffraction of waves when the diffraction pattern is viewed at a long distance from the diffracting object.
$\frac{b^2}{D\lambda }<<1$