[LHM]

Which element will exhibit the photoelectric effect with light of the longest wavelength?
A) K
B) Rb
C) Mg
D) Ca

How do you know the relative wavelengths of the elements?

Thanks.

[opti384]

First do you understand the photoelectric effect?

[LHM]

Isn't it something along the lines of: light contains packets of energy called photons and when the light has enough energy and a metal surface is exposed to it, some electrons will be emitted?

[opti384]

The enough energy you said will be the key to answering your problem. Rather than referring to energy, the light requires a threshold frequency.

[LHM]

Then how do you know how much energy each element has?

[opti384]

Then how do you know how much energy each element has?

Technically, it's know how much energy an element has. Rather it's finding the threshold frequency of the elements. You'll find it if you google it up.

[Borek]

For a photoelectric effect you need to bounce an electron from the atom, that is - ionize it. Think or check which of the elements listed has the lowest first ionization energy.