When electromagnetic radiation, in the form of visible light, travels from one substance or medium into another, the light waves may undergo a phenomenon known as refraction, which is manifested by a bending or change in direction of the light. Refraction occurs as light passes from one medium to another only when there is a difference in the index of refraction between the two materials. The effects of refraction are responsible for a variety of familiar phenomena, such as the apparent bending of an object that is partially submerged in water and the mirages observed on a dry, sandy desert. The refraction of visible light is also an important characteristic of lenses that enables them to focus a beam of light onto a single point.

Introduction to the Refraction of Light - As light passes from one substance into another, it will travel straight through with no change of direction when crossing the boundary between the two substances head-on (perpendicular, or a 90-degree angle of incidence). However, if the light impacts the boundary at any other angle it will be bent or refracted, with the degree of refraction increasing as the beam is progressively inclined at a greater angle with respect to the boundary. As an example, a beam of light striking water vertically will not be refracted, but if the beam enters the water at a slight angle it will be refracted to a very small degree. If the angle of the beam is increased even further, the light will refract with increasing proportion to the entry angle. Early scientists realized that the ratio between the angle at which the light crosses the media interface and the angle produced after refraction is a very precise characteristic of the material producing the refraction effect.

Friedrich Johann Karl Becke (1855-1931) - Friedrich Johann Karl Becke was an Austrian geologist, mineralogist and petrologist from the University of Prague, who developed a method for determining the relationship between light refraction and refractive index differences observed in microscopic specimens. The phenomenon, which is now referred to as the formation of Becke lines, has been named for him.

Augustin-Jean Fresnel (1788-1827) - Augustin-Jean Fresnel, was a nineteenth century French physicist, who is best known for the invention of unique compound lenses designed to produce parallel beams of light, which are still used widely in lighthouses. In the field of optics, Fresnel derived formulas to explain reflection, diffraction, interference, refraction, double refraction, and the polarization of light reflected from a transparent substance.

Willebrord Snell (1580-1626) - Willebrord Snell was an early seventeenth century Dutch mathematician who is best known for determining that transparent materials have different indices of refraction depending upon the composition. Snell discovered that a beam of light would bend as it enters a block of glass, and that the angle of bending was dependent upon the incident angle of the light beam. Light traveling in a straight line into the glass will not bend but, at an angle, the light is bent to a degree proportional to the angle of inclination. In 1621, Snell found a characteristic ratio between the angle of incidence and the angle of refraction. Snell's law demonstrates that every substance has a specific bending ratio-the "refractive index. The greater the angle of refraction, the higher the refractive index for a substance.

Refraction of Light - Refraction occurs as light passes from one medium to another only when there is a difference in the index of refraction between the two materials. The effects of refraction are responsible for a variety of familiar phenomena, such as the apparent bending of an object that is partially submerged in water and the mirages observed on a dry, sandy desert. The refraction of visible light is also an important characteristic of lenses that enables them to focus a beam of light onto a single point. This interactive tutorial explores how changes to the incident angle and refractive index differential between two dissimilar media affect the refraction angle of light at the interface.

Observing Objects in Water - An object seen in the water will usually appear to be at a different depth than it actually is, due to the refraction of light rays as they travel from the water into the air. This tutorial explores how fish, observed from the bank of a pond or lake, appear to be closer to the surface than they really are.

Refraction by an Equilateral Prism - Visible white light passing through an equilateral prism undergoes a phenomenon known as dispersion, which is manifested by wavelength-dependent refraction of the light waves. This interactive tutorial explores how the incident angle of white light entering the prism affects the degree of dispersion and the angles of light exiting the prism.

The Critical Angle of Reflection - An important concept in optical microscopy is the critical angle of reflection, which is a necessary factor to consider when choosing whether to use dry or oil immersion objectives to view a specimen at high magnification. Upon passing through a medium of higher refractive index into a medium of lower refractive index, the path taken by light waves is determined by the incident angle with respect to the boundary between the two media. This interactive tutorial explores the transition from refraction to total internal reflection as the angle of the incident wave is increased at constant refractive index.

Beam Steering by Wedge Prisms - Circular prisms having plane surfaces positioned at slight angles with respect to each other are termed optical wedges, and deflect light by refraction rather than reflection. Although wedges are prismatic in nature, they can be manipulated to act as beamsplitters or beam steerers. This interactive tutorial explores how two wedge prisms operate together to deflect an incident light beam.

Refraction of Monochromatic Light - Refraction occurs as light passes from one medium to another only when there is a difference in the index of refraction between the two materials. The effects of refraction are responsible for a variety of familiar phenomena, such as the apparent bending of an object that is partially submerged in water and the mirages observed on a dry, sandy desert. The refraction of visible light is also an important characteristic of lenses that enables them to focus a beam of light onto a single point. This interactive tutorial explores how changes to the incident angle and refractive index differential between two dissimilar media affect the refraction angle of monochromatic light at the interface.

Reference Listing - The reference materials listed in this section are an excellent source of additional information on the diverse topic of light refraction and dispersion by isotropic and anisotropic media. Included are references to books, book chapters, and review articles, which discuss the theory and applications of the refraction and refractive index, and how they relate to the physics of light and color.

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