refraction, in physics, deflection of a wave on passing obliquely from one transparent medium into a second medium in which its speed is different, as the passage of a light ray from air into glass. Other forms of electromagnetic radiation, in addition to light waves, can be refracted, as can sound waves.

The Nature of Refraction

Refraction is commonly explained in terms of the wave theory of light and is based on the fact that light travels with greater velocity in some media than it does in others. When, for example, a ray of light traveling through air strikes the surface of a piece of glass at an oblique angle, one side of the wave front enters the glass before the other and is retarded (since light travels more slowly in glass than in air), while the other side continues to move at its original speed until it too reaches the glass. As a result, the ray bends inside the glass, i.e., the refracted ray lies in a direction closer to the normal (the perpendicular to the boundary of the media) than does the incident ray. A light ray entering a different medium is called the incident ray; after bending, the ray is called the refracted ray. The speed at which a given transparent medium transmits light waves is related to its optical density (not to be confused with mass or weight density). In general, a ray is refracted toward the normal when it passes into a denser medium and away from the normal when it passes into a less dense medium.

The Law of Refraction

The law of refraction relates the angle of incidence (angle between the incident ray and the normal) to the angle of refraction (angle between the refracted ray and the normal). This law, credited to Willebrord Snell, states that the ratio of the sine of the angle of incidence, i, to the sine of the angle of refraction, r, is equal to the ratio of the speed of light in the original medium, v_i, to the speed of light in the refracting medium, v_r, or sin i/sin r=v_i/v_r. Snell's law is often stated in terms of the indexes of refraction of the two media rather than the speeds of light in the media. The index of refraction, n, of a transparent medium is a direct measure of its optical density and is equal to the ratio of the speed of light in a vacuum, c, to the speed of light in the medium: n=c/v.

Indexes of refraction are always equal to or greater than 1; for air, n=1.00029; for water, n=1.33. Using indexes of refraction, Snell's law takes the form sin i/sin r=n_r /n_i, or n_i sin i=n_r sin r. If the original medium is denser than the refracting medium (n_i greater than n_r), sin r will be greater than sin i. Thus, there will be some acute angle less than 90° for the incident ray corresponding to an angle of refraction of 90°. This angle of incidence is known as the critical angle. For angles of incidence greater than the critical angle, refraction cannot take place and the incident ray is instead reflected back into the original medium according to the law of reflection (angle of reflection equals angle of incidence). This phenomenon is known as total internal reflection.

Applications of Refraction

Refraction has many applications in optics and technology. A lens uses refraction to form an image of an object for many different purposes, such as magnification. A prism uses refraction to form a spectrum of colors from an incident beam of light. Refraction also plays an important role in the formation of a mirage and other optical illusions.

Change in direction of a wave as it leaves one medium and enters another. Waves, such as sound and light waves, travel at different speeds in different media. When a wave enters a new medium at an angle of less than 90°, the change in speed occurs sooner on one side of the wave than on the other, causing the wave to bend, or refract. When water waves approach shallower water at an angle, they bend and become parallel to the shore. Refraction explains the apparent bending of a pencil when it is partly immersed in water and viewed from above the surface. It also causes the optical illusion of the mirage.

Learn more about refraction with a free trial on Britannica.com.

or birefringence

Optical property in which a single ray of unpolarized light (see polarization) splits into two components traveling at different velocities and in different directions. One ray is refracted (see refraction) at an angle as it travels through the medium, while the other passes through unchanged. The splitting occurs because the speed of the ray through the medium is determined by the orientation of the light compared with the crystal lattice of the medium. Since unpolarized light consists of waves that vibrate in all directions, some will pass through the lattice without being affected, while others will be refracted and change direction. Materials that exhibit double refraction include ice, quartz, and sugar.

Learn more about double refraction with a free trial on Britannica.com.