That verse comes from John's Gospel, which is all about seeing. In the first chapter alone there are 18 references to seeing. John writes about knowing as seeing. John saw in Christ the Word of life, but some did not and crucified Him. Not seeing is no reason for not believing.
Nobody sees radiation.
We wait for its effects. Nobody sees God, but millions find the effects in their lives. Things happen that can only be from Him. Even one prayer answered, one healing, one miracle, one deliverance from addiction is evidence of Him. But it isn't just one.