Your friend says that impulse equals momentum. your friend's statement is not correct, and the missing word is: change. does anyone know why the word is change ?

A moving object has some momentum. In order for its momentum to change, a force must act on it. The product of (force applied) x (length of time the force continues) is called the "impulse" delivered to the object. After the impulse, the object's momentum is different.

The impulse is equal to the CHANGE in the object's momentum.

54 degrees incident angle = reflected angle

angle is from normal turned to line of ray