The minimum wavelength of X-rays produced by 20 kV electrons is nearly

The de-Broglie wavelength associated with an electron accelerated through a potential difference of $\frac{200}{3} \mathrm{~V}$ is nearly

The ratio of de-Broglie wavelengths associated with thermal neutrons at temperatures $127^{\circ} \mathrm{C}$ and $352^{\circ} \mathrm{C}$ is

A laser produces a beam of light of frequency $5 \times 10^{14}$ Hz with an output power of 33 mW . The average number of photons emitted by the laser per second is (Planck's constant $=6.6 \times 10^{-34} \mathrm{Js}$ )