Monochromatic light of wavelength $ 632.8\; nm$ is produced by a helium-neon laser. The power emitted is $9.42 \;mW$.

$(a)$ Find the energy and momentum of each photon in the light beam,

$(b)$ How many photons per second, on the average, arrive at a target irradiated by this beam? (Assume the beam to have uniform cross-section which is less than the target area), and

$(c)$ How fast does a hydrogen atom have to travel in order to have the same momentum as that of the photon?

The value of Planck's constant is

In photoelectric effect, the K.E. of electrons emitted from the metal surface depends upon

If $c$ is the velocity of light in free space, the correct statements about photon among the following are:

$A$. The energy of a photon is $E=h v$.

$B$. The velocity of a photon is $c$.

$C$. The momentum of a photon, $p=\frac{h v}{c}$.

$D$. In a photon-electron collision, both total energy and total momentum are conserved.

$E$. Photon possesses positive charge.

Choose the correct answer from the options given below:

In a photoemissive cell with executing wavelength $\lambda $, the fastest electron has speed $v.$ If the exciting wavelength is changed to $\frac{{3\lambda }}{4}$, the speed of the fastest emitted electron will be