Question

Five lumen/watt is the luminous efficiency of a lamp and its luminous intensity is 35 candela. The power of the lamp is

• A. 80 W
• B. 176 W
• C. 88 W
• D. 36 W

Answer 1

Answer: (C)

88 W

Answer 2

Efficiency of light source $\eta = \frac{\varphi}{p}$ ..... (i)

and $L = \frac{\varphi}{4\pi}$ ..... (ii)

From equation (i) and (ii)$\Rightarrow p = \frac{4\pi L}{\eta} = \frac{4\pi \times 35}{5} \approx 88W$.