Question 14

  Suppose that a laser produces green light instead of the red light produced by a helium/neon laser. Are the photons emitted by this laser more energetic, less energetic, or of equal energy as compared to those emitted by the helium/neon laser? Justify your answer.

SOLUTION:
Green light has a shorter wavelength than red light (c.f., Table 26.2). So what? Well, we can express the energy of a photon in terms of its wavelength. First, we know by the Planck quantization relation that

E = hf .

We also know that

$$c = \lambda f$$

if you recall equation 16.1. (Oh, you forgot?!? Well, speed is distance per time, and wavelength is a distance and frequency is inverse period or inverse time.) Combining, we get

$$E = \frac{hc}{\lambda}$$

so we see that, since green light has a shorter wavelength than red light, its energy is larger.