When a wave strikes a surface and transitions from one medium to another, such as from air to water or from air to a denser medium like glass, it experiences a change in speed. This change in speed causes the wave fronts to bend, or undergo a process called refraction. The bending of wave fronts toward the normal is governed by Snell's law of refraction.
Snell's law states that the ratio of the sine of the angle of incidence (θ₁) to the sine of the angle of refraction (θ₂) is equal to the ratio of the speeds of the wave in the two different media. Mathematically, Snell's law can be expressed as:
n₁ * sin(θ₁) = n₂ * sin(θ₂)
Where:
- n₁ is the refractive index of the first medium (incident medium)
- n₂ is the refractive index of the second medium (transmitting medium)
- θ₁ is the angle of incidence, measured between the incident wave direction and the normal to the surface
- θ₂ is the angle of refraction, measured between the refracted wave direction and the normal to the surface
The refractive index of a medium is a measure of how much the speed of light (or any electromagnetic wave) is reduced when it passes through that medium compared to its speed in a vacuum. Different media have different refractive indices, which determine the degree of bending or refraction that occurs when a wave passes from one medium to another.
The reason wave fronts bend toward the normal during refraction is that the speed of the wave changes as it transitions between media with different refractive indices. If the wave enters a medium where its speed is reduced, it tends to bend toward the normal. Conversely, if the wave enters a medium where its speed increases, it bends away from the normal.
This bending of wave fronts during refraction is a consequence of the change in wave velocity caused by the change in the optical properties of the medium. The phenomenon of refraction is observed in various wave types, such as light waves, sound waves, and water waves, when they encounter interfaces between different media.