Light of different colours propagates through air:

The radius of the central zone of a circular zone plate is $2.3\,mm$. The wavelength of the incident light is $5893\,\mathring{A}$. The source is at a distance of $6\,m$. What is the distance of the first image in meters?

$A$ monochromatic light source of wavelength $\lambda$ is placed at $S$. Three slits $S_1, S_2$ and $S_3$ are equidistant from the source $S$ and the point $P$ on the screen. $S_1P - S_2P = \lambda / 6$ and $S_1P - S_3P = 2\lambda / 3$. If $I$ is the intensity at $P$ when only one slit is open,the intensity at $P$ when all the three slits are open is

Answer the following questions:
$(a)$ When a low flying aircraft passes overhead,we sometimes notice a slight shaking of the picture on our $TV$ screen. Suggest a possible explanation.
$(b)$ As you have learnt in the text,the principle of linear superposition of wave displacement is basic to understanding intensity distributions in diffraction and interference patterns. What is the justification of this principle?

The intensity of light from a continuously emitting laser source operating at $638 \,nm$ wavelength is modulated at $1 \,GHz$. The modulation is done by momentarily cutting the intensity off with a frequency of $1 \,GHz$. What is the farthest distance apart two detectors can be placed in the line of the laser light,so that they can see the portions of the same pulse simultaneously?
(Consider the speed of light in air $c = 3 \times 10^{8} \,m/s$)

To prepare a print,the time taken is $5\, sec$ due to a lamp of $60\, W$ at a $0.25\, m$ distance. If the distance is increased to $40\, cm$,what is the time taken to prepare the similar print in seconds?