Why is light with higher frequency more destructive, if it emits the same amount of electrons?

Question

The Photoelectric effect states that if light with high enough frequency hits some matter, that matter will emit electrons. If you increase the intensity of light, more electrons will be emitted, but the kinetic energy of individual electrons will remain the same. However, if you increase its frequency, the same amount of electrons will be emitted, but individual electrons will have higher kinetic energy.

If that is the case, then why are higher frequencies more destructive than lower frequencies? For example, being exposed to UV light has fairly low consequences on the human body, but if we're exposed to x-rays for even a few moments, we have to wear protection. If frequency really has no effect on the number of electrons being emitted, shouldn't those two radiations be equally harmful?

Answer 1

That "high enough" conditional can be a big deal. Bodies are not made of a uniform material with a single response to a single frequency. Ejecting electrons (ionization) is harmful to many materials. Visible light and lower is not ionizing to many biological materials. As you increase the frequency to UV, more material can be ionized, so the damage increases.

As you increase the frequency to X-rays, not only can more materials be damaged, the more penetrating the radiation. So damage occurs not just on the surface, but in deeper tissues as well. Any UV absorbed by the already dead skin cells will not cause any harm.

Answer 2

For a complex mixture of chemical compounds, a lot of things happen.

1. Photochemical reactions may occur without ejecting electrons from the compounds. They are, however, similar to the photoelectric effect in that each reaction has a threshold energy. Thus, higher energy photons tend to induce more photochemical reactions, causing more damage.

2. The threshold energy for the photoelectric effect depends on the specific compound, so higher energy photons can extract electrons from more compounds, causing more damage.

3. Free electrons can themselves induce reactions. Generally, the higher the electron energy, the more damage. Higher energy photons yield higher energy electrons, causing more damage.