Reflection is etymologically related to Reflect. Reflection is related to Light. 
Radiation is related to Light. Light is a synonym of Weak. Weak is the opposite of Power. Power is related to Transmission. 
Reflect <--EtymologicallyRelatedTo-- Reflection <--RelatedTo--> Light 
Radiation <--RelatedTo--> Light <--Synonym--> Weak <--Antonym--> Power <--RelatedTo--> Transmission 