The setup shown is that of Young's Double Slit Experiment.
The fringe width in the experiment is given as $\beta=\frac{\text{D}\lambda}{\text{d}}$
where d is the distance between the slits, $D$ is the distance between screen and slits.
Hence the central fringe also widens when distance between screen and slits increases.
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$(a)$ Maximum $KE$ associated with photoelectrons from the surface of any metal is $0.3\ eV$
$(b)$ No photoelectrons are emitted from the surface of $Ni$
$(c)$ If the frequency of the source of radiation is doubled then $KE$ of photoelectrons is also doubled
$(d)$ Photoelectrons will be emitted from the surfaces of all the three metals if the wavelength of the incident radiation is less than $200\ nm$
Correct statements are