A simple harmonic progressive wave is given by equation $y=\operatorname{asin} 2 \pi\left(n t-\frac{x}{\lambda}\right)$. If the wave velocity is equal to $\frac{1}{4} \times$ (maximum particle velocity), then the wavelength ' $\lambda$ ' is (Given $\rightarrow \mathrm{a}=$ amplitude, $\mathrm{n}=$ frequency, $\mathrm{t}=$ time, $\mathrm{y}=$ displacement, $\mathrm{x}=$ distance )

$$ \text { In the following figure, the current } \mathrm{I} \text { is equal to } $$

The work function of a photosensitive metallic surface is $h v_0$. If photons of energy (2.5) $h v_0$ fall on this surface, the electrons come out with maximum velocity ' v '. When the photon energy is increased to $7 \mathrm{~h} v_0$, the maximum velocity of photoelectrons will be

A magnet having a magnetic dipole moment ' M is placed in two magnetic fields ' $\mathrm{B}_1$ ' and ' $\mathrm{B}_2$ ' respectively. If it is displaced slightly from the equilibrium position, it oscillates 60 times in 20 second in field ' $\mathrm{B}_1$ ' and 60 times in 30 second in field ' $B_2$ '. The ratio of field ' $B_1$ ' to that of ' $\mathrm{B}_2{ }^{\prime}$ is