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### The number of photons per second on an average ....

The number of photons per second on an average emitted by the source of monochromatic light of wavelength 600 nm, when it delivers the power of $3.3 \times 10^{-3}$ watt will be: ($h=6.6\times 10^{-34}$Js)

(1) $10^{18}$
(2) $10^{17}$
(3) $10^{16}$
(4) $10^{15}$

Solution

Let there be n number of photons per second.

Total energy per second of n number of photons = $n. \frac {hc}{\lambda} = 3.3 \times 10^{-3}$

$\therefore n=\frac {3.3 \times 10^{-3} \times 600 \times 10^{-9}}{6.6 \times 10^{-34} \times 3 \times 10^8} =10^{16}$

### Sum of the coefficients in the expansion of $(x+y)^n$ ....
If the sum of the coefficients in the expansion of $(x+y)^n$ is 4096, then the greatest coefficient in the expansion is _ _ _ _ . Solution $C_0 + C_1 + C_2 + C_3 + ......................... + C_n =4096$ $\therefore 2^n = 4096 =2^{12}$ $\Rightarrow n = 12$ Greatest coefficient = ${}^{12}{C_6} = 924$