Its not that only solvent causes carbocations/carbanions and only light and heat causes radicals. It all depends on the bond in question.

For weak bonds (N-Cl, O-O, etc.) you can achieve homolysis by heating because the bond dissociation energy is relatively low. A C-H bond dissociation energy is higher and so unless you are at fairly high temperatures, you wont see radical reactions (unless you have some initiator).

Therefore, it is all about the bond you are observing that determines what conditions you need to have an ionic reaction or radical reaction.

Thank you sir. Sir Is there any documents/ article related to this ? How can i get it ??

The question is ill formulated and is difficult to answer. Plz, clarify your question.

Yurii V Geletii Sir, In organic reactions what i have found is -If Heat or light energy is used then free radical mechanism is written and if some solvents are used then carbocation or carbanion mechanism is written, Why is this so ??