How does light start the reaction between methane and chlorine?

The reaction between methane ($C{H}_4$) and chlorine ($C{l}_2$) is a chain reaction initiated by the absorption of light, typically in the form of ultraviolet radiation.12

The process begins with a chain-initiation step where the energy from the light causes the photolysis of a chlorine molecule. This endothermic process breaks the bond between the two chlorine atoms, resulting in the formation of two highly reactive chlorine radicals ($·Cl$).12

Once these chlorine atoms are generated, they collide with methane molecules to begin the propagation steps of the reaction. In the first propagation step, a chlorine radical abstracts a hydrogen atom from methane, producing a methyl radical ($·C{H}_3$) and hydrogen chloride ($HCl$). The methyl radical then reacts with another chlorine molecule to form the final product, chloromethane ($C{H}_{3}Cl$), while simultaneously regenerating a chlorine radical. This regeneration allows the chain reaction to continue through many subsequent steps until a termination step occurs, such as the combination of two radicals.12