Q. The relative luminosity of wavelength $600\ nm$ is 0.6. Find the radiant flux of $600\ nm$ needed to produce the same brightness sensation as produced by $120\ W$ of radiant flux at 555 $nm$
A) 50$W$
B) 72$W$
C) $120 \times (0.6)^{2}W$
D) 200$W$
Explanation
(d) If $\eta$ is the luminous efficiency of the bulb then luminous flux by $120\ watt$ at $555\ nm = \eta \times 120$
Let bulb of $P$ watt at $600\ nm$ produces the same luminous flux as by $120\ watt$ at $555\ nm$ then
$\eta \times 120 = \eta P \times 0.6 \Rightarrow P = \frac{120}{0.6} = 200\ watt$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.