# Refraction

Refraction refers to the way that electromagnetic radiation (light) changes speed and direction as it travels across the interface between one transparent medium and another.

Refraction refers to the change in speed and direction of light as it passes from one transparent medium to another.

• When light passes from a fast medium like air to a slower medium like water, it bends towards the normal and slows down.
• When light passes from a slower medium like diamond to a faster medium like glass, it bends away from the normal and speeds up.
• In a diagram illustrating optical phenomena like refraction or reflection, the normal is a line drawn at right angles to the boundary between two media.
• A fast (optically rare) medium is one that slows down light less than a slow medium.
• A slow (optically dense) medium is one that slows down light more than a fast medium.
• The refractive index (also known as the index of refraction) of a medium indicates how much the speed and direction of light are altered when it passes through the medium due to refraction.
• The refractive index of a material can be calculated by dividing the speed of light in a vacuum by the speed of light in the material. This ratio is always greater than or equal to one.
• In practice, resources such as refractiveindex.info contain databases of most media through which light propagates.
• To determine the direction of light bending at the boundary between transparent media, we need to know:
• Which medium is faster and less optically dense, with a smaller refractive index?
• Which medium is slower and more optically dense, with a higher refractive index?
• Snell’s law, (also known as the law of refraction), describes the relationship between the angle of incidence and the angle of refraction of light as it passes through the boundary of two media with different refractive indices.
• Snell’s law states that the ratio of the sine of the angle of incidence (the angle between the incoming light ray and the normal, a line perpendicular to the boundary) to the sine of the angle of refraction (the angle between the outgoing light ray and the normal) is equal to the ratio of the refractive indices of the two media.

Refraction refers to the way that electromagnetic radiation (light) changes speed and direction as it travels across the interface between one transparent medium and another.

• As light travels from a fast medium such as air to a slow medium such as water it bends toward the normal and slows down.
• As light passes from a slow medium such as diamond to a faster medium such as glass it bends away from the normal and speeds up.
• In a diagram illustrating optical phenomena like refraction or reflection, the normal is a line drawn at right angles to the boundary between two media.
• A fast (optically rare) medium is one that obstructs light less than a slow medium.
• A slow (optically dense) medium is one that obstructs light more than a fast medium.
• The speed at which light travels through a given medium is expressed by its index of refraction.
• If we want to know in which direction light will bend at the boundary between transparent media we need to know:
• Which is the faster, less optically dense (rare) medium with a smaller refractive index?
• Which is the slower, more optically dense medium with the higher refractive index?
• The amount that refraction causes light to change direction, and its path to bend, is dealt with by Snell’s law.
• Snell’s law considers the relationship between the angle of incidence, the angle of refraction and the refractive indices (plural of index) of the media on both sides of the boundary. If three of the four variables are known, then Snell’s law can calculate the fourth.