Question

Two monochromatic light beams, one red and one green, have the same intensity and the same cross sectional area. How does the energy of each photon and the number of photons crossing a unit area per second in the red beam compare with those of the green beam?

| | Energy of Photon | Number of Photons Crossing Unit Area per Second |
| (A) | Same | Same |
| (B) | Greater for red | Less for red |
| (C) | Greater for red | Greater for red |
| (D) | Less for red | Less for red |
| (E) | Less for red | Greater for red |

Answer 1

The correct option is: (E)

Explanation:

Energy of a photon is related to frequency. The red light has a lower frequency and thus less energy per photon. Intensity is the total energy of the beam. To have the same intensity, there would need to be more of the lower energy red photons.