Question

Suppose 10 J of light energy is needed by the interior of the human eye to see an object. The photons of green light($\lambda =550nm$) needed to see the object:

• A. $27$
• B. $28$
• C. $29$
• D. $30$

Answer 1

Answer: (A)

$27$

Given : wavelength $\left(\lambda \right)=550nm$

$=550\times {10}^{-9}m$

we know that,

$\therefore E=\frac{hc}{\lambda }$

Energy of a single photon,

$E=\frac{hc}{\lambda }$

$h=6.63\times {10}^{-34}JS$

$c=3\times {10}^{8}m/s$

$E=\frac{6.63\times {10}^{-34}\times 3\times {10}^8}{550\times {10}^{-9}}$

$=3.61\times {10}^{-19}J$

Energy $3.61\times {10}^{-19}J=1$photon

Energy $10J=\frac{10\times {10}^{-17}}{3.61\times {10}^{-19}}$

$=27.7$

$\approx 27$ photons