The sun started a fire in a bedroom after its rays reflected off a mirror and onto some curtains.
Firefighters were called to the house on Altofts Lodge Drive, Altofts, on Tuesday.
A mirror had been left on a bedroom windowsill.
Watch commander Darren Hague, from Normanton Station, said the sun had deflected off the mirror and set the curtains alight. They fell to the floor and damaged the flooring, but had not set fire to any other property.
He said the sun had also left scorch marks on the wooden sill.
He added: “This is unusual, but we have seen it happen a few times before.
“People should be warned not to leave mirrors or glass crystals hanging in their windows.”
Crews from Normanton were called at 7.15pm after the fire had been smouldering all day.