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Monochromatic light of frequency 6.0×10⁸ MHz is produced by a laser. The power emitted is 2×10⁻³ W. What is the energy of a photon in the light beam?

Asked in GSEB Board August 2020 · Photon energy, momentum and mass

• (1) 3.98×10⁻¹⁹ J
• (2) 3.98×10⁻¹⁹ eV
• (3) 3.98 eV
• (4) 3.98×10⁻¹⁹ kJ

Step-by-step solution

Why the other options are wrong

• (2) Right number, wrong unit: hf with h in J s comes out in joules. In electron-volts it is (3.98×10⁻¹⁹)/(1.6×10⁻¹⁹) ≈ 2.49 eV.
• (3) The factor 10⁻¹⁹ has been dropped and the unit relabelled. Converted properly, the photon energy is about 2.49 eV, not 3.98 eV.
• (4) 3.98×10⁻¹⁹ kJ is 3.98×10⁻¹⁶ J, a thousand times the photon's energy; hf with SI values comes out in joules, not kilojoules.

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