The winter sun caused a house blaze when a concave mirror focussed sunlight onto curtains in Pimlico.
Four fire engines and around 25 firefighters were called to a fire at the property on Ponsonby Place.
A small part of the second floor was damaged by fire. There were no report of any injuries.
London Fire Brigade spokesperson said:
"These sort of fires are not as rare as you would think. I’ve seen everything from sparkly door knobs to crystal balls starting fires.
"Crystal and glass ornaments and items such as mirror tables should be kept out of direct sunlight. This shows these fires are not an urban myth. Also make sure that you have a working smoke alarm on every level of your home."
The Brigade was called at 1320 and the fire was under control at 1417. Fire crews from Lambeth, Clapham, Soho, and Chelsea fire stations attended the scene.