Question

A monochromatic source of light operating at 200 W emits $4\times {10}^{20}$ photons/second. Then the wavelength of light used is

• A. 3000 $A^0$
• B. 5000 $A^0$
• C. 4000 $A^0$
• D. 6000$A^0$

Answer 1

The correct option is C 4000 $A^0$

$P=\frac{nhc}{\lambda }$
$200=\frac{4\times {10}^{20}\times 20\times {10}^{-26}}{\lambda }$
$\lambda =4000A^0$

So, the answer is option (C).