optical relay?

With a bit more information, we can make better recommendations. When you say 'light source', do you mean when the ambient light in an area goes out, or when an object cuts off a light beam?

For the latter, google 'photointerrupter', 'IR emitter/detector', illumination sensor'.

HTH

It would be when an object cuts off a light beam. Using google I have found components that would do the detecting stage, but none that would switch the 300mA load directly. Sorry if this is a daft question, I only have a limited understanding of electronics, and even that is 30years out of date.

Search for "break beam detector" and look for one with a relay output

- usually good for 5 amps at 120 VAC.

You could probably find something surplus along those lines if you need it inexpensive - they are used in the thousands in manufacturing.

Have tried searching for "break beam detector" and variations, but no luck. Are they known as anything else?

Try " ir beam detector". Or variations.

Where IR is infrared

You could probably easily modify something like this to suit your needs.

Three links for a (partial) solution on a dutch site:

Marlin p Jones had some surplus ones awhile back and I bought two, none in today's catalog but they do have a kit

These are a common enough hobby surplus item, so you might get a list of surplus shops and search

Break beam, detector or sensor, photo or optical controls or sensors, infrared controls, and variations of those terms

Graingers catalog has them in the industrial controls section.

robotics sites are another source - but usually short range "retro reflector" detectors/sensors