Light of wavelength $\lambda$ is incident on a single slit of width 'a' and the distance between slit and screen is 'D'. In diffraction pattern, if slit width is equal to the width of the central maximum then 'D' is equal to

When a metallic surface is illuminated with a radiation of wavelength '$\lambda$', the stopping potential is 'V'. If the same surface is illuminated with radiation of wavelength $6\lambda$, the stopping potential is $\left(\dfrac{V}{12}\right)$. The threshold wavelength for the surface is

In photoelectric effect experiment, if the frequency of incident radiation ($v$) is increased, keeping all other factors constant, the stopping potential, ( $v > v_0$) ($v_0$ = threshold frequency)

The electron in hydrogen atom is initially in the second excited state. When it finally moves to ground state, the maximum number of spectral lines emitted are