What is Snell's Law?
Snell's Law, also known as Descartes' Law of Refraction, is a formula used to describe the relationship between the angles of incidence and refraction for light or other waves passing through a boundary between two different isotropic media. It quantifies how much a wave changes direction (bends) when it enters a new medium where its speed is different, such as light going from air to water.
Key Principles and Formula
The law states that the ratio of the sines of the angles of incidence and refraction is equivalent to the ratio of the phase velocities in the two media, or equivalently, the reciprocal of the ratio of their indices of refraction. Mathematically, it's expressed as: n₁sin(θ₁) = n₂sin(θ₂), where n₁ and n₂ are the refractive indices of the first and second media, and θ₁ and θ₂ are the angles of incidence and refraction, measured from the normal (a line perpendicular to the surface).
A Practical Example
Consider a common example: a straw placed in a glass of water. When viewed from the side, the straw appears bent or broken at the water's surface. This visual distortion occurs because light rays traveling from the water (a denser medium) into the air (a less dense medium) bend according to Snell's Law, causing the submerged part of the straw to appear shifted from its actual position due to the difference in refractive indices.
Importance and Applications
Snell's Law is a foundational principle in optics, essential for the design and function of countless optical instruments, including lenses in eyeglasses, cameras, telescopes, and microscopes. It is also critical for understanding how light behaves in fiber optics, enabling high-speed data transmission. Furthermore, it explains natural phenomena such as the formation of rainbows (light refracting and reflecting in water droplets) and the appearance of mirages.