MediumChemistryHigh School (11–12)Structure of the Atom › Light, Atomic Spectra and the Bohr ModelConceptual

Light above the threshold frequency falls on a metal, and electrons come out.

Try it on paper first. A hint, the key idea and the worked solution are here when you need them.

Light above the threshold frequency falls on a metal, and electrons come out. Say what happens to (a) the number of electrons each second and (b) their greatest kinetic energy in two cases.

The wavelength is halved, with the same number of photons.

(ii) A filter removes half the photons, and the colour stays the same.

Need a reminder? The key ideaShow

A photon carries $E = h\nu = \dfrac{hc}{\lambda}$ , with $h = 6.626 \times 10^{-34}\ \text{J s}$ ; in eV, $E = \dfrac{1240}{\lambda\ (\text{nm})}$ . Photoelectric effect: $h\nu = W + KE_{\max}$ , and $eV_0 = KE_{\max}$ . Brightness sets the number of electrons; frequency sets their energy.

The worked solution

Have you tried it?

Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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← The threshold wavelength of a metal is 540 nm.Light of wavelength 250 nm and power 2.0 mW falls on potassium, with a work function of 2.30 eV. →

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