A 20 W light source emits monochromatic light of wavelength 600 nm. The number of photons ejected per second expressed in terms of Avogadro's constant N_Av is approximately
0.N_Av
1.10⁻² N_Av
2.10⁻⁴ N_Av
3.10⁻⁶ N_Av
Explanation
E/photon = hc/λ ≈ 3.33×10⁻¹⁹ J. n = P/E = 20/3.33×10⁻¹⁹ ≈ 6×10¹⁹. N_Av = 6.02×10²³. n/N_Av = 6×10¹⁹/6×10²³ = 10⁻⁴. So n ≈ 10⁻⁴ N_Av.