Question

The number of photons falling per second on a completely darkened plate to produce a force of $6.62\times 10^{-5}N$ is $n$ . If the wavelength of the light falling is $5\times 10^{-7}m,$ then $n=\dots \times 10^{22}\cdot \left(h=6.62\times 10^{-34}J-s\right)$

• A. $1$
• B. $5$
• C. $0.2$
• D. $3.3$

Answer 1

Answer: (B)

Energy carried by $n$ photons per second
$P=n.\frac{hc}{\lambda }$
As photons get absorbed by dark plate, force experienced by the plate
$F=\frac{1}{C}.n\frac{he}{\lambda }$
Or $F\times c=n.\frac{hc}{\lambda }$ $\left[aspower\left(P\right)=force\times speed\right]$
$=F\times c$
Or $n=\frac{F\lambda }{h}=\frac{6.62\times 10^{-5}\times 5\times 10^{-7}}{6.62\times 10^{-34}}=5\times 10^{22}$