Magnifying glass

The magnifying glass was a glass that produced a magnified image of an object. Sun rays sent through a magnifying glass could be used to burn things. It was known to humans, Time Lords and Apalapucians. It was usually carried in a handheld tool.

The Doctor
It was carried by the Doctor in several incarnations: second (TV: The Highlanders), third (TV: The Sea Devils), fourth (TV: Genesis of the Daleks) and tenth (TV: The Unicorn and the Wasp).

An oversized magnifying glass was found by the Eleventh Doctor, Amy Pond and Rory Williams in the Two Streams Facility on Apalapucia. Actually it was a time glass. (TV: The Girl Who Waited)

Weapon
Magnifying glass could amplify the heat from sun rays. In one occasion, Donna Noble used the Doctor's glass this way against a giant wasp. (TV: The Unicorn and the Wasp)

As a boy, Archibald Flint would sometimes amuse himself by burning ants with a magnifying glass. (AUDIO: The Spectre of Lanyon Moor)

Behind the scenes
Graeme Harper used magnifying glasses to achieve the effect of a distorted image of a main character, as a directorial "signature", p.e. in Army of Ghosts, The Unicorn and the Wasp, and Utopia.