Photon

Photons were particles of light. They could be produced by the recombination of an electron and a positron. (TV: Time and the Rani)

Application
Weapons based on these particles included photon beams, (TV: The Invasion of Time et al.) photonic bombs (COMIC: Pureblood) and photonics missiles. (TV: Horror of Fang Rock et al.)

Artemis the Chronovore could pause time enough that the individual photons of sunlight could be observed. (PROSE: No Future)

TARDIS
The Time-Space Visualiser allowed one to see any event in "past" history by examining photons, since light "recorded" an image and thus could be displayed. (TV: The Chase)

The photon accelerator coils were a TARDIS component. (PROSE: The Eye of the Giant)