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When a ray of light falls obliquely on any polished surface, as that of a mirror, a piece of water, a plate of burnished metal, or any other reflecting substance, the ray, like an elastic ball, is immediately projected in a contrary direction to that in which it fell. Moreover, the direction in which it is reflected is at right angles to the surface, and in the same plane as that of the ray in the first instance. This experiment may be tried very easily, and will show the reason for the two following laws.

1. The angle of incidence is equal to the angle of reflection, and vice versâ.

2. Reflection can only take place in one direction—in that of the incident rays, both of which are always in a plane perpendicular to the reflecting surface.

The following figure will assist the student in performing experiments on the reflection of light from flat surfaces.

The ray falling obliquely on the horizontal mirror, is reflected upwards at the same angle in the direction. This may be proved geometrically by placing a graduated circle in a vertical position in the plane, when we shall find that the angle formed by (the incident ray) with the perpendicular is equal to the angle formed by this perpendicular line and the reflecting ray. You may also prove