The minimum wavelength of photon is $5000 \mathring A$, its energy will be

A $5$ watt source emits monochromatic light of wavelength $5000\; \mathring A$. When placed $0.5\; m$ away, it liberates photoelectrons from a photosensitive metallic surface. When the source is moved to a distance of $1.0\;m$, the number of photo electrons liberated will

A point source is emitting sound waves of intensity $16 \times 10^{-8} \mathrm{Wm}^{-2}$ at the origin. The difference in intensity (magnitude only) at two points located at a distances of $2 \mathrm{~m}$ and $4 \mathrm{~m}$ from the origin respectively will be______ $\times 10^{-8} \mathrm{Wm}^{-2}$

The photoelectric effect can be understood on the basis of

A beam of electromagnetic radiation of intensity $6.4 \times 10^{-5}\; \mathrm{W} / \mathrm{cm}^{2}$ is comprised of wavelength, $\lambda=310 \;\mathrm{nm} .$ It falls normally on a metal (work function $\varphi=2 \;\mathrm{eV}$ ) of surface area of $1\; \mathrm{cm}^{2} .$ If one in $10^{3}$ photons ejects an electron, total number of electrons ejected in $1 \;s$ is $10^{\mathrm{x}}$.then $\mathrm{x}$ is

$\left(\mathrm{hc}=1240\; \mathrm{eV} \mathrm{nm}, 1\; \mathrm{eV}=1.6 \times 10^{-19} \;\mathrm{J}\right)$

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: