In standard K9 models, a metal tube came out of a hole at the front of the robot's head and it fired the beam. However, in the case of K9 Mark I's later incarnations, such as K9 Mark 2, it was simply fired out of the hole. Several smaller holes inside also lit when the weapon was charged. (TV: Mind Snap) For Mark II and early instances of Mark III, the beam was red. (TV: The Invasion of Time, The Ribos Operation, TV: A Girl's Best Friend) Although in K9 Mark I and III's later life, along with K9 Mark IV and K9 Mark I's later incarnations, the beam was a more orange colour. (TV: School Reunion, TV: The Gift, TV: Liberation)
Abilities and Weaknesses
Note that abilites demonstrated in more advanced models may not be present in earlier versions, as such, the specific model is stated.
K9 Mark I's photon beam was ineffective against Sontaran armour. (TV: The Invasion of Time) K9 Mark II's photon beam was able to destroy Daleks. (DWM: The Dogs of Doom) K9 Mark IV's photon beam could disintergrate a swarm of Rakweed spores. (TV: The Gift)
K9 Mark 2's photon beam was able to heat and cook food, such as a tin of baked beans, although it resulted in the beans bursting out of the tin. When K9 applied this technique to a Korven, white light seem to emerge from its eyes and mouth and its body cracked with white light before finally exploding, leaving its clothes behind. (TV: The Korven) It could deactivate a device such as a Space-Time Manipulator without damaging it. (TV: Oroborus) It could also be turned into a photon spread to dispersef gas from a vapour bomb. (TV: The Last Oak Tree)
The Jixen were able to reflect and enhance photon beams. K9 was merely able to halt them with a shot. This ability was later incorperated into Trojan, who was so strong it was able to knock K9 back. (TV: The Eclipse of the Korven)