# Refraction

This article refers to refraction in waves. For refraction in metals, see refraction (metallurgy).

File:Refraction.jpg
Refraction in a Perspex (acrylic) block.
File:Refraction in a ripple tank.png
refraction of waves in a ripple tank
File:Pencil in a bowl of water.png
Refraction of light waves in water. The dark rectangle represents the actual position of a pencil sitting in a bowl of water. The light rectangle represents the apparent position of the pencil. Notice that the end (X) looks like it is at (Y), a position that is considerably shallower than (X).

Refraction in geometric optics is the change in direction of a wave due to a change in velocity. It happens when waves travel from a medium with a given refractive index to a medium with another. At the boundary between the media the wave changes direction; its wavelength increases or decreases but frequency remains constant. For example, a light ray will refract as it enters and leaves glass; understanding of this concept led to the invention of the refracting telescope.

In the diagram on the right, ripples travel from the left and pass over a shallower region inclined at an angle to the wavefront. The waves travel more slowly in the shallower water, so the wavelength decreases and the wave bends at the boundary. The dotted line represents the normal to the boundary. The dashed line represents the original direction of the waves. The phenomenon explains why waves on a shoreline never hit the shoreline at an angle. Whichever direction the waves travel in deep water, they always refract towards the normal as they enter the shallower water near the beach.

An example of this is looking into a bowl of water. Air has a refractive index of about 1.0003, and water has a refractive index of about 1.33. If a person looks at a straight object, such as a pencil, which is placed at a slant, partially in the water, the object appears to bend at the water's surface. This is due to the light rays from the object being bent as they move from the water to the air. This causes water to appear shallower than it really is. The depth that the water appears to be when viewed from above is known as the apparent depth.

Refraction is also responsible for rainbows and for splitting up of white light into a rainbow-spectrum as it passes through a glass prism. Glass has a higher refractive index than air and the different frequencies of light travel at different speeds (dispersion), causing them to be refracted at different angles. The different frequencies correspond to different colours observed.

Snell's law is used to calculate the degree to which light is refracted when traveling from one medium to another.

Recently some metamaterials have been created which have a negative refractive index. Whith metamaterials, we can also obtain the total refraction phenomena when the wave impedances of the two media are matched. There is no reflected wave.

In medicine, particularly ophthalmology and optometry, refraction (also known as refractometry) is a clinical test for the determination of an eye's refractive error and the best corrective lenses to be prescribed. A series of test lenses in graded powers are presented to determine which provide the sharpest, clearest vision. [1]