I have a 20 amp circuit on which I would like to branch a light bulb. Does code allow me to use 14 gauge wire to the 60 watt light bulb or must I use 12 gauge?

Thank you.

If you are using a 20 amp OCPD (breaker) then you must use #12 AWG cable.

If you use 14 AWG for any part of the circuit, then you must reduce the breaker to 15 Amps.

Running 20 amp through 14 AWG cable is a violation of code and a nasty fire hazard.

The whole circuit must stay #12. There is no provision for going smaller to a fixture.

Sorry, the codes are in place for safety's sake.

Interesting that the light fixture itself uses what looks like 16 gauge wire.
Just for reference, light fixtures, or lumiaires, do not follow the same rules are building wiring. A fixture can have 18ga wire even if attached to a 20A circuit.