Q 11 :

A point source of 100 W emits light with 5% efficiency. At a distance of 5 m from the source, the intensity produced by the electric field component is            [2023]

• $\frac{1}{2\pi }\frac{W}{{\mathrm{m}}^2}$

   

• $\frac{1}{40\pi }\frac{\mathrm{W}}{{\mathrm{m}}^2}$

   

• $\frac{1}{10\pi }\frac{\mathrm{W}}{{\mathrm{m}}^2}$

   

• $\frac{1}{20\pi }\frac{\mathrm{W}}{{\mathrm{m}}^2}$

   

Q 12 :

A point source of light is placed at the centre of curvature of a hemispherical surface. The source emits a power of 24 W. The radius of curvature of hemisphere is 10 cm and the inner surface is completely reflecting. The force on the hemisphere due to the light falling on it is ____ $\times {10}^{-8}$ N.                   [2023]

Q 13 :

A photoemissive substance is illuminated with a radiation of wavelength ${\lambda }_i$ so that it releases electrons with de-Broglie wavelength ${\lambda }_e$. The longest wavelength of radiation that can emit photoelectron is ${\lambda }_0$. Expression for de-Broglie wavelength is given by ($m$ = mass of the electron, $h$ = Planck's constant and $c$ = speed of light)         [2025]

• ${\lambda }_e=\sqrt{\frac{h{\lambda }_0}{2mc}}$

   

• ${\lambda }_e=\sqrt{\frac{h{\lambda }_i}{2mc}}$

   

• ${\lambda }_e=\frac{h}{\sqrt{ 2mc(\frac{1}{{\lambda }_i}-\frac{1}{{\lambda }_0})}}$

   

• ${\lambda }_e=\sqrt{\frac{h}{2mc(\frac{1}{{\lambda }_i}-\frac{1}{{\lambda }_0})}}$