Question

Explain this observation, sodium is irradiated for 2 seconds with infrared light and no electrons are ejected. the same metal is irradiated for 2 seconds with ultraviolet light and thousands of electrons are emitted.

Answer 1

Each piece of metal, depending on its composition (i.e. what element it is) and preparation (i.e. the surface), will have a specific work function. This is the minimum amount of energy required promote an electron(s) to a state with a principal quantum number n = infinity (i.e. an electron will be removed from the surface of a metal. The electrons in the metal can absorb/gain energy from the light irradiated onto it, and if this light has an energy above the work function, then the leftover energy appears as the kinetic energy of the ejected electron(s).

Back to the question...the energy (E) and the frequency (f) of the light sources can be related through the formula:

\[E = hf\]
(where 'h' is a constant known as Planck's constant).

So, this equation tells us that the higher the frequency, the higher the energy of the light. If you were to look at the electromagnetic spectrum, you would see that the frequency of ultraviolet light is higher than that of infrared light. So, clearly, the value of the work function for the metal is high enough such that only the ultraviolet light is effective in emitting electrons, whilst the infrared does not have sufficient energy for emission to occur. Hope that helps! :)