The Law of Reflection is one of the simplest rules in science.
It states that when an object bounces off a flat surface, the angle it at which it hits the surface will be equal to the angle at which it bounces away.

The angles are named this way:
  • The angle at which the object hits the flat surface is called the angle of incidence (I).
  • The angle at which it rebounds from the flat surface is called the angle of reflection (R).
Both angles are measured from the path of the object to the normal line, which is a line perpendicular to the surface at the point of impact. It is shown in blue here.

The Law of Reflection can be restated this way:

"Under ideal conditions, the angle of incidence always equals the angle of reflection."

What does 'ideal conditions' mean?
Well, the Law of Reflection is only true if we make certain assumptions:

1. The collision is simple. The ball is not spinning, and the surface it hits is immobile
2. The ball is perfectly round, and its mass is symmetrically distributed

3. The flat surface is truly flat, and if it is compressed by the object hitting it, it compresses and uncompresses uniformly and in the direction of the normal line
4. No other unbalanced force is applied to the ball to change its direction

In real life, these assumptions may or may not be true. For example:
  • If you put a spin on the ball when you hit it, it will rebound at an angle different than the angle of incidence
  • If the ball's shape and mass distribution are not perfectly symmetric, the ball's path won't be a straight line, and the angles won't be equal
  • If the surface it hits isn't perfectly flat, or not uniform in composition, the ball may rebound at an unpredictable angle
  • Bits of fluff on the table, or bumps, may cause the path of the ball to change unpredictably

The Law of Reflection works only under ideal conditions.

When you are playing pool, you can assume that, if no spin is put on the ball, (which will likely be a perfect sphere), and if the table is expensive and clean, nothing will cause the ball to deviate from its ideal path, and the angles will be equal.

Of course, being able to predict where the ball will go when some of these conditions aren't ideal is what makes a good pool player!

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