Can a 240V lightbulb 25W work in a 120V socket and would it just be dimmer?

== == == == Assuming this inquiry is a request for information rather than related to a practical application, here is the answer: It will work but you probably won't like the results. You are not going to get the amount of light you should or probably want. I would hook up the proper size light, or the proper juice for the light. A 240 Volt rated tungsten filament lamp will glow dimly if 120 Volts are applied.

The resistance of the filament will limit the current to a level below that which will raise the filament temperature to high enough a level to glow fully bright.

The same thing would hold true if one applied 60 Volts to a 120 Volt lamp.

If you are contemplating installing a 240 Volt European fixture in the U.S. [the idea that may have motivated the question], there are manufacturers who make a 120 Volt lamp with a base for the 240 Volt fixture sockets. Yes, a 240 Volt incandescent lamp will work in a 120 Volt application, but you won't get half the light. You'll get a lot less than half, even though the applied voltage is half the rated voltage. There are hardly any sensible reasons to run a 240 Volt lamp in a 120 Volt application. <><><>

As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,

on electrical circuits, equipment or appliances,

always use a test meter to ensure the circuit is, in fact, de-energized.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB

SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS.