I've read the definition of anode which is that oxidation happens there and thus electrons are leaving the anode. In photoelectric effect we radiate the cathode which is connected to the negative source. Then electrons travel to the anode through the tube which seems to contradict the definition. However this can be solved by thinking that anode is actually the source of electrons which travel through the circuit to the cathode.

Now let's radiate the anode instead of the cathode, and now electrons travel from the anode to the cathode through the tube. This seems to contradict the previous thought that electrons would travel through the circuit to the cathode. What am I missing here?

Radiating cathode

Radiating anode


2 Answers 2


Anode and cathode are defined according to the direction in which charge-carriers move in the device itself, be it a battery or photocell, or a passive device such as a diode. The anode is the electrode towards which negative carriers move (or away from which positive carriers move); vice versa for the cathode.

So, in your second diagram, the electrode labels are the wrong way round. Applying the rule in the first paragraph to other cases: in a battery (on discharge) the anode is the negative terminal; the cathode the positive, but for a diode the anode is the electrode to which you connect the positive terminal of your power supply if you want the diode to conduct.

Suppose we have a symmetrical device such as a water electrolysis cell with identical platinum electrodes. The same rule applies: the negative ions go to the anode, positive to the cathode. But in this case it is which way round you connect the external battery that determines in which direction the ions travel, and therefore which electrode is which! In the photocell the illuminated electrode is always the cathode, because negative charge carriers can't flow towards the illuminated electrode, whichever way round the battery is connected, and there aren't any positive charge carriers!

The terminology was first used by Faraday (in the 1830s or 40s). I believe he had etymological advice from William Whewell.

  • $\begingroup$ How does this explain the situation above in those pictures? $\endgroup$
    – jte
    Commented Nov 13, 2019 at 10:01
  • $\begingroup$ Anode and cathode are wrongly labelled in the lower picture. Also the battery is the wrong way round if you want to encourage current to flow. If you're trying to measure the maximum KE of the emitted electrons, the battery is the right way round! $\endgroup$ Commented Nov 13, 2019 at 10:03
  • $\begingroup$ So in many physics books they are just wrong? For example in Concepts of Modern Physics, 6th Edition. Seems really weird to me. Edit: yes, the other picture is for measuring the maximum kinetic energy. $\endgroup$
    – jte
    Commented Nov 13, 2019 at 10:07
  • $\begingroup$ Have you really found this in more than one book? $\endgroup$ Commented Nov 13, 2019 at 10:08
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    $\begingroup$ Let us continue this discussion in chat. $\endgroup$ Commented Nov 13, 2019 at 13:15

As far as I know in photoelectric effect we have a photocell which contains two electrodes, anode(negative) and cathode(positive) and when radiation falls on the cathode it ejects the electrons which then travel to the anode and forms the photocurrent.

  • $\begingroup$ See images that I added to the first post. $\endgroup$
    – jte
    Commented Nov 13, 2019 at 8:49

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