About $20 \%$ of the power of a 100 W bulb is converted to visible radiation. Assuming that the radiation is emitted isotropically and neglecting reflection, the average intensity of visible radiation at a distance of 5 m is $\frac{\alpha}{25 \pi} \mathrm{~W} / \mathrm{m}^2$. The value of $\alpha$ is
Light strikes a metal surface causing photoelectric emission. The wavelength of incident light is 248 nm . If the stopping potential for the ejected electrons is 2.8 eV , then the work function of the metal is (take, $h c=1240 \mathrm{eV}-\mathrm{nm}$ )
The de-Broglie wavelength associated with an electron, accelerated through a potential difference of 121 V is about
(take, Plank's constant $=h=6.6 \times 10^{-34} \mathrm{Js}$, mass of electron $=9 \times 10^{-31} \mathrm{~kg}$ )
The difference in the wavelength between the maximum and minimum of Balmer series (use $R_H=1 \times 10^7 \mathrm{~m}^{-1}$ )
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