The work function of a substance is $$3.0 \mathrm{~eV}$$. The longest wavelength of light that can cause the emission of photoelectrons from this substance is approximately;

Two sources of light emit with a power of $$200 \mathrm{~W}$$. The ratio of number of photons of visible light emitted by each source having wavelengths $$300 \mathrm{~nm}$$ and $$500 \mathrm{~nm}$$ respectively, will be :

The de-Broglie wavelength of an electron is the same as that of a photon. If velocity of electron is $$25 \%$$ of the velocity of light, then the ratio of K.E. of electron and K.E. of photon will be:

The threshold frequency of a metal with work function $$6.63 \mathrm{~eV}$$ is :