Energy of a quanta of frequency ${10^{15}}Hz$ and $h = 6.6 \times {10^{ - 34}}J{\rm{ - }}\sec $ will be

A  radiation of energy $'E'$ falls normally on a perfectly reflecting surface. The momentum transferred to the surface is $( C =$ Velocity of light $)$

If the momentum of a photon is $p$, then its frequency is

Where $m$ is the rest mass of the photon

An important spectral emission line has a wavelength of $21 cm$. The corresponding photon energy is

$(h = 6.62 \times {10^{ - 34}}Js;\;\;c = 3 \times {10^8}m/s)$

A totally reflecting small plane mirror placed horizontally faces a parallel beam of light as hown in figure. The mass of mirror is $20\, gm$. Assume that there is no absorption in the lens and that $30\%$ of the light emitted by the source goes through the lens. Find the power of the source needed to support the weight of the mirror ............... $MW$ (take $g = 10\, m/s^2$) :-

Specific heat of water is $4.2 \,J / g ^{\circ} C$. If light of frequency $3 \times 10^9 \,Hz$ is used to heat $400 \,gm$ of water from $20^{\circ} C$ to $40^{\circ} C$, the number of photons needed will be