Laws of reflection and refraction of light
A geometric line showing the direction of energy transfer of light is called a light ray. Geometric optics rests on four laws: rectilinear propagation of light (which forms shadows), independence of rays (they do not affect each other when they cross), reflection and refraction. The law of reflection: the incident ray, the reflected ray and the perpendicular at the point of incidence lie in one plane, and the angle of reflection equals the angle of incidence, γ = α. A smooth surface reflects specularly (a mirror), a rough one diffusely. The law of refraction: sin α / sin β = n₂₁ = n₂/n₁ = v₁/v₂. The absolute refractive index is n = c/v (air ≈ 1, water 1.33, glass about 1.5, diamond 2.42). When entering an optically denser medium (greater n) the ray bends toward the perpendicular (β < α), and when entering a rarer one it bends away. A ray falling along the perpendicular is not refracted.
Experiment: in a dark room, direct the beam of a small torch (with an adult; never a powerful laser) onto a plane mirror at different angles and measure the angle between the incident and reflected rays with a protractor. Compare α and γ. Never shine light into anyone’s eyes.