A parallel beam of light of wavelength $900\,nm$ and intensity $100\,Wm ^{-2}$ is incident on a surface perpendicular to the beam. Tire number of photons crossing $1\,cm ^{2}$ area perpendicular to the beam in one second is :

A convex lens of focal length $40 \mathrm{~cm}$ forms an image of an extended source of light on a photoelectric cell. A current I is produced. The lens is replaced by another convex lens having the same diameter but focal length $20 \mathrm{~cm}$. The photoelectric current now is:

If the energy of the photon is increased by a factor of $4$, then its momentum

For photo-electric effect with incident photon wavelength $\lambda$, the stopping potential is $V _0$. Identify the correct variation$(s)$ of $V _0$ with $\lambda$ and $1 / \lambda$. $Image$

A radio transmitter operates at a frequency of $880 \,kHz$ and a power of $10\,kW$. The number of photons emitted per second are

What is velocity of photon ?