Crews were called to Old Hollings Hill in Guiseley shortly before 12.30pm on Monday after smoke was seen coming from the semi-detached property.
They had to break in after discovering that the property had been locked up for several days as the occupants were away.
Once the fire was extinguished, investigators found a small fire had started between a bed and a bookcase before increasing and leaving the room severely damaged.
They ruled out an electrical fault, smoking or candles as causes, but found a small butterfly mirror with bevelled edges among the debris.
Fire investigator Jamie Lister said the sun would have been at the perfect angle to reflect off the mirror at the time the fire started.
He said: “Although rare these types of fires do tend to occur at this time of year when the sun is low in the sky.
“It is important, especially in predominantly south facing windows, to remove any object that could refract light.
“These include magnified shaving and make up mirrors, any mirrors with bevelled edges, crystal ornaments or handles, any ornaments or fixtures that contain diamantes and the domed bottom of aerosol cans can also refract light.”
For a free home fire safety visit go to www.westyorksfire.gov.uk or call 0800 5874536.