A LASER emits a power of 10−310^{-3} W. Calculate the number of photons emitted per second if the wavelength is 692.8 nm.
Solution:
We use the formula:
Number of photons per second = P / E = P / (hc / λ)
Where:
- P=10−3P = 10^{-3} W
- λ=692.8\lambda = 692.8 nm = 692.8×10−9692.8 \times 10^{-9} m
- h=6.626×10−34h = 6.626 \times 10^{-34} J·s (Planck’s constant)
- c=3×108c = 3 \times 10^8 m/s (speed of light)
Step 1: Energy per photon
E = (h × c) / λ = (6.626 × 10⁻³⁴ × 3 × 10⁸) / (692.8 × 10⁻⁹) ≈ 2.869 × 10⁻¹⁹ J
Step 2: Number of photons per second
N = P / E = (10⁻³) / (2.869 × 10⁻¹⁹) ≈ 3.49 × 10¹⁵ photons/second
Final Answer:
N ≈ 3.49 × 10¹⁵ photons per second