The wire that carries the current from the power source to the outlet or receptacle is the hot wire. Whenever you see a black wire, you know it is hot. There are other wires that carry current in more complicated circuits, but black are always hot.
The minimum angle at which total internal reflection occurs is known as the critical angle (for Perspex this is about 43Ë). At any angle greater than the critical angle, total internal reflection occurs and the light ray obeys the normal rules for reflection (i.e. angle of incidence = angle of reflection). 1  Page. 2  1. 3  2.
When these rays strike a concave mirror, they are reflected so they go through a common point and then go on their way; this point is called the focal point. The distance from the mirror to the focal point F is called the focal length and is labeled f. The focal length f is positive for a concave mirror.