Refraction is the phenomenon that occurs when light passes from one medium to another medium with a different optical density. It involves a change in the direction and speed of light as it crosses the boundary between the two media. This change in direction is caused by the variation in the speed of light in different materials.
When light travels from a less dense medium to a denser medium, such as from air to glass, it slows down and bends towards the normal, which is an imaginary line perpendicular to the surface of the interface between the two media. This bending is called "refraction towards the normal." On the other hand, when light travels from a denser medium to a less dense medium, such as from glass to air, it speeds up and bends away from the normal, which is known as "refraction away from the normal."
The amount of bending that occurs depends on the angle at which the light hits the interface between the two media. This angle is known as the angle of incidence. The angle between the refracted ray and the normal is called the angle of refraction. The relationship between the angle of incidence and the angle of refraction is described by Snell's law, which states that the ratio of the sines of the angles is equal to the ratio of the velocities of light in the two media:
n₁ * sin(θ₁) = n₂ * sin(θ₂)
where n₁ and n₂ are the refractive indices of the first and second media, respectively, and θ₁ and θ₂ are the angles of incidence and refraction, respectively.
The refractive index of a material is a measure of how much it slows down the speed of light compared to its speed in a vacuum. Different materials have different refractive indices, which determine the amount of bending that occurs during refraction.
The phenomenon of refraction is responsible for various optical phenomena, such as the bending of light in lenses, the formation of rainbows, and the apparent shift in the position of objects when viewed through a transparent medium. It is an essential concept in optics and has numerous practical applications in areas such as lens design, fiber optics, and the study of light propagation in different media.