A modern 200-watt sodium street lamp emits yellow light of wavelength 0.6 µm. Assuming it is 25% efficient in converting electrical energy to light, the number of photons emitted per second is:
Correct answer: C. 1.5 × 10²⁰
- A. 6.2 × 10²⁰
- B. 3 × 10¹⁹
- C. 1.5 × 10²⁰
- D. 6 × 10¹⁸
Explanation
Photon energy E = hc/λ = (6.6 × 10⁻³⁴ × 3 × 10⁸)/(0.6 × 10⁻⁶) ≈ 3.3 × 10⁻¹⁹ J. Effective power = 0.25 × 200 = 50 W. Number of photons N = P/E = 50/(3.3 × 10⁻¹⁹) ≈ 1.5 × 10²⁰.
Last updated
About Dawn of Modern Physics
Classical physics cannot explain blackbody radiation and related observations, leading to Planck’s quantum theory, in which energy is emitted or absorbed in discrete packets. Photons provide the particle model of light, with energy and momentum linked to frequency and wavelength, including the photoelectric effect and its threshold frequency.
Practise Dawn of Modern Physics
739 free Dawn of Modern Physics MCQs from Physics, each with the correct answer and an explanation. Unlimited attempts, no account needed.
Exams that ask Physics questions like this
Physics is on 13 papers prepared for on TestUstad, and all of them draw the same bank, so this question is worth knowing for every one of them.
Related questions
1.5 mW of 400 nm light is directed at a photoelectric cell. If 0.1% of the incident photons produce photoelectrons, the current in the cell is:
1.5 mW of 400 nm light is directed at a photoelectric cell. If 0.1% of the incident photons produce photoelectrons, the current in the cell is:
A 5 watt LED bulb converts 80% of the power into light photons wavelength 660 nm. What is the number of photons emitted from the bulb in one second.
A black body is considered ideal when it:
A body at temperature T radiates heat according to relation: