Laser beams are used to measure long distances because

$A$ laser beam can be focused on an area equal to the square of its wavelength. $A$ $He-Ne$ laser radiates energy at the rate of $1 \, mW$ and its wavelength is $632.8 \, nm$. The intensity of the focused beam will be:

The figure shows $P$ and $Q$ as two equally intense coherent sources emitting radiations of wavelength $20 \, m$. The separation $PQ$ is $5.0 \, m$ and the phase of $P$ is ahead of the phase of $Q$ by $90^{\circ}$. $A, B,$ and $C$ are three distant points of observation equidistant from the mid-point of $PQ$. The intensity of radiations at $A, B,$ and $C$ will bear the ratio:

Red light differs from blue light as they have :

Write the difference between the interference pattern and diffraction pattern.

Which statement is correct for a zone plate and a lens?