Final answer:
The theorem is proven by applying the law of reflection and utilizing the properties of right angles to show that an outgoing ray from two mirrors at a right angle is parallel to the incoming ray.
Step-by-step explanation:
To prove the theorem that light reflects from two mirrors at a right angle and the outgoing ray is parallel to the incoming ray, we can use the law of reflection and the properties of right angles. According to the law of reflection, the angle of incidence is equal to the angle of reflection. This implies that when light reflects off the first mirror, the path of the reflected ray relative to the normal will create a right angle with the second mirror, since the mirrors meet at a right angle. When the light reflects off the second mirror, it will again obey the law of reflection, causing the outgoing ray to be parallel to the incoming ray, because reflection over a right angle preserves the direction of the path.