Question

A modern 200 watt sodium street lamp emits yellow light of wavelength 0.6 µm. Assuming it to be 25% efficient in converting electrical energy to light, number of photons of yellow light it emits per second is –

• A. 6.2 × 10²⁰
• B. 3 × 10¹⁹
• C. 1.5 × 10²⁰
• D. 6 × 10¹⁸

Answer 1

Answer: (C)

1.5 × 10²⁰

Answer 2

= $\frac { \mathrm { IA } } { \mathrm { h } v }$ = = $\frac { \mathrm { W } \lambda } { \mathrm { hc } }$

= $\frac { 50 \times 6 \times 10 ^ { - 7 } } { 6.6 \times 10 ^ { - 34 } \times 3 \times 10 ^ { 8 } }$

= $\frac { 300 } { 20 } \times 10 ^ { 19 }$

= 1.5 × 10²⁰