$A$ 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

Light of different colours propagates through air:

Choose the incorrect statement.

Statement $(S)$: Using Huygen's eye-piece,measurements can be taken but are not correct.
Reason $(R)$: The cross wires,scale,and final image are not magnified proportionately because the image of the object is magnified by two lenses,whereas the cross-wire scale is magnified by one lens only.
Identify the correct option from the following:

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 $c = 3 \times 10^{8} \,m/s$)

Statement $(S)$: Using Huygen's eye-piece,measurements can be taken,but they are not correct.
Reason $(R)$: The cross wires,scale,and final image are not magnified proportionately because the image of the object is magnified by two lenses,whereas the cross-wire scale is magnified by one lens only.
Identify the correct one of the following: