The Law of Reflection states that when an object hits a flat surface, the angle at which it approaches the surface will be equal to the angle at which it travels away from the surface. The angles are described as the angle of incidence (or the angle of entry) and the angle of reflection (or exit angle). An angle is made up of two rays with a common endpoint, called the vertex. The two rays that make up the angle of incidence are the ray along the object's path and the ray perpendicular to the surface, which is called the normal. The vertex of the angle is the point of impact (Note: this is not the angle to which Inez refers in the video. The angle she refers to as the "angle in" is the angle formed by the ray along the object's path and the ray formed by the wall).

It is important to mention that some particular conditions must exist in order for the Law of Reflection to be applied. The ball must be perfectly round, and not weighted in any part. And there can be no spin placed on the ball—that is, it cannot be spinning along an axis other than the horizontal one about which it moves toward the wall. The surface that the ball hits must be completely flat and not able to be moved upon impact. Also, there can be no other external force applied to the ball, as that may change its direction. For example, if a person pushes the ball or a wind gust affects it or the ball hits some bumps on the surface, then its path will change. If any of the conditions are not met, the Law of Reflection cannot be applied to the situation.

The Law of Reflection may come in handy when playing a game of pool, but the proper conditions must be met (i.e. smooth surfaces on the pool table and no spin on the ball). The Law of Reflection is also applied to the path light takes when it is reflected off a mirror. The light reflection angle will equal the measure of its angle of incidence. A periscope makes use of this property in allowing a person to see an object that is not in their direct line of sight. A periscope works by using two mirrors positioned parallel to each other and at a 45° angle to the object and the viewer's eye. The light from the object reaches one mirror, bounces to the other, and then reaches the viewer's eye.

To learn more about angles, check out Introduction to Angle Measure and Directions to Hen Rock.

To learn more about the properties of light, check out Light and the Law of Reflection.

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