cathode vs photocathode

cathode and photocathode both are nouns.

Word	Noun	Adjective	Verb	Adverb
cathode	Yes	No	No	No
photocathode	Yes	No	No	No

As nouns, photocathode is a hyponym of cathode; that is, photocathode is a word with a more specific, narrower meaning than cathode:

cathode: a negatively charged electrode that is the source of electrons entering an electrical device
photocathode: a cathode that emits electrons when illuminated

Other hyponyms of cathode include cold cathode.

cathode (noun)	photocathode (noun)
a negatively charged electrode that is the source of electrons entering an electrical device	a cathode that emits electrons when illuminated
the positively charged terminal of a voltaic cell or storage battery that supplies current