Question

What is zone plate? Determine the expression for its focal length.

Answer 1

Zone plate i.e. device used for focussing light exhibiting wave character.  

Apart from lenses or curved mirrors zone plates use diffraction instead of reflection.  

Analysis by Augustin-Jean Fresnel they are called Fresnel zone plates in honor of him.  

Its focusing ability is an extension of Arago spot phenomenon occurred by diffraction from an opaque disc.

Expression of its focal length :

Let S is the source, P is the point of observation and XY is the edge wise section zone plate as shown in Figure 5.3. Then

   SMn = (a2 + rn2)1/2 = a+ rn2 /2a

   rn = root n lambda f.

Answer 2

$\bold{ANSWER}$

A zone plate is a device used to focus light or other things exhibiting wave character. Unlike lenses or curved mirrors however, zone plates use diffraction instead of refraction or reflection.

Based on analysis by Augustin-Jean Fresnel, they are sometimes called Fresnel zone platesin his honor.