Even with an old school filament light bulb, this probably wouldn’t have been enough to “warm” anything inside the refrigerator even if the light didn’t turn off.
For real, how long did he leave the refrigerator open for the bulb to noticably warm up? Dude's habits are wasting more energy than all the lightbulbs in his house.
The heat from those bulbs was used to keep the content over 0 degrees Celsius when the fridge got colder than the set temperature (e.g. very cold room in winter) to prevent the goods in the fridge from freezing.
That's what I know about this topic. Valid for Germany, don't know how this is handled in other parts of the world.
No. There is a thermostat for this purpose. You can open a fridge 0 or 50 times per day and it will keep the set temperature (on average). If things freeze in your fridge, it does not mean you don't open it enough – it’s misadjusted or faulty. And the evaporator (cooling) tubes are in the back wall, on which water condenses and does get cold enough to freeze sometimes. Anyway, the droplets fall into a drain and a narrow tube carries them out, usually onto a little tray on top of the compressor, whose heat helps the water evaporate. Replacing the bulb with an LED has no effect besides decreasing energy usage.
In fact, if an E14 LED burns out, you can often remove the plastic cover, find the burnt LED chip (if that’s what failed; look for a black spot), pry off all its plastic parts and short the contacts by scraping the aluminum/copper pads together with a knife. A soldering iron works better but I cannot bother waiting for it to heat up. Another LED chip will likely burn out sooner or later if it runs hot again but it will not get over 50 °C in usual fridge operation. Check if it fits into the fridge and add an aluminum foil reflector to redirect light if it helps. I did this in some Liebherr, Zanussi and Electrolux fridges, which I believe are also available in neighboring Germany. I use 2~4W “candle” bulbs for this because they are tiny without the plastic diffuser and not great for general lighting in most fixtures. If you don't have failed LED bulbs, there is a free selection at dm or Lidl near the entrance. Be quick and sneaky but don't take any CFLs (center of mass or shining a phone flashlight through the milky glass helps reveal a CFL tube if coiled inside). If you accidentally take a CFL or fail to fix an LED, return it on your next visit with an obvious hand movement so that your activity appears legitimate overall.
With the way bulbs are designed, all the heat is in the filament, and a bit gets transferred to the glass. The filament cools very quickly, hence it goes dark, and the glass is so thin that there isn't a meaningful change in temperature of the surrounding air once it's shut off.
Add that into the massive heat sink that is the contents of your fridge, not to mention the sheer volume of air that tiny amount of heat has to warm up..... Anyone legitimately concerned about it heating the inside of the fridge needs some refresher courses in science.
The smaller the fridge, the more it'll affect the inside, but it's still negligible.
I have no data beyond anecdotal experience, so if someone wants to set up a scientific study, I'll happily skim the abstract and make sweeping judgements about the content!
In a large fridge a small incandescent lightbulb won’t make much difference since it turns off when the door closes and would have a relatively small thermal mass, which I think is what you’re saying. That said I don’t think it’s accurate to say that “all of the heat is in the filament.” Heat spreads, and it will leave the light bulb. If you have ever touched an incandescent light bulb that is on you would have no doubts about this! Easy bake ovens used to use incandescent light bulbs to cook things, people leave their oven lights on to keep their oven warmer to let bread rise. Larger incandescent light bulbs can absolutely warm up a full sized room, 60W or 100W is a fair amount of heat. There are stories about extremely temperature controlled rooms where they would turn on a 100W bulb when a person leaves it because a person produces about 100W of heat.
Yeah, I can't touch a 100w incadescent bulb that's been on for an hour, but an equivalent LED bulb could be on for days and I still can touch it without issues.
40w is a lot of energy going into such a small space though! Granted, its off most of the time anyways but that's still a lot of energy (and therefore heat)