Psalm 97:11: Light has sprung up for the righteous and joy for the true of heart.
One thing I used to get confused over was Christians talking about this 'light' and 'joy' they had - they didn't look any different to me!
But as I got to know Jesus and as I grew as a Christian, I because to recognise this light and joy I had - which nobody else could physically see!
Our relationship with Christ allows us to exude this joy we find in the relationship with him. That joy may well be a light to others who are living in darness in their lives.