An envelope with the girlfriend's address-Riddle

Riddle 
It was Kushal's birthday. He lay dead in the recreation room of his house. Next to his body was a note written in pencil. The note read Happy Birthday, Friend. Kushal had a girlfriend; the police suspected her ex-boyfriend Jagga. They could find no obvious evidence. 
While searching the ex-boyfriend Jagga's car, the police saw an envelope with the girlfriend?s address written on it. 
They thought they would get the handwriting on the envelope checked against the note. The scientist in charge. Bhool Bhulaiya came in early to work the next day; it was 7am.
He looked out his window which faced east and stared at the rising sun and it was then that he realized how to prove Jagga killed the man, even though the girlfriend's address was not written in the same handwriting. How did Mr. Bhool Bhulaiya do it?

    Bhool Bhulaiya office faced east, and the sun was coming in through the window at a very low angle. He saw some very faint shadows on the surface of the envelope. He looked closer. There were the words embossed on the paper. They read ?Happy Birthday, Fri

Mindyourlogic Riddle: Answer:
An envelope with the girlfriend?? address-Riddle