optical rotation(redirected from Optically active)
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polarization of light
Characteristics of Polarization
Polarization is a phenomenon peculiar to transverse waves, i.e., waves that vibrate in a direction perpendicular to their direction of propagation. Light is a transverse electromagnetic wave (see electromagnetic radiation). Thus a light wave traveling forward can vibrate up and down (in the vertical plane), from side to side (in the horizontal plane), or in an intermediate direction. Ordinarily a ray of light consists of a mixture of waves vibrating in all the directions perpendicular to its line of propagation. If for some reason the vibration remains constant in direction, the light is said to be polarized.
It is found, for example, that reflected light is always polarized to some extent. Light can also be polarized by double refraction. Any transparent substance has the property of refracting or bending a ray of light that enters it from outside. Certain crystals, however, such as calcite (Iceland spar), have the property of refracting unpolarized incident light in two different directions, thus splitting an incident ray into two rays. It is found that the two refracted rays (the ordinary ray and the extraordinary ray) are both polarized and that their directions of polarization are perpendicular to each other. This occurs because the speed of the light in the crystal—hence the angle at which the light is refracted—varies with the direction of polarization. Unpolarized incident light can be regarded as a mixture of two different polarization states separated into two components by the crystal. (In most substances the speed of light is the same for all directions of polarization, and no separation occurs.)
Unpolarized light can be converted into a single polarized beam by means of the Nicol prism, a device that separates incident light into two rays by double refraction; the unwanted ray is removed from the beam by reflection. Polarized light can also be produced by using a tourmaline crystal. Tourmaline (a double-refracting substance) removes one of the polarized rays by absorption. Another commonly used polarizer consists of a sheet of transparent material in which are embedded many tiny polarizing crystals.
Any system by which light is polarized in a particular direction is transparent only to light polarized in that direction. Thus, when originally unpolarized light passes successively through two polarizers whose directions of polarization are mutually perpendicular the light is completely blocked; light transmitted by the first polarizer is polarized and is stopped by the second. If the second polarizer is rotated so that the directions of polarization are no longer perpendicular, the amount of light transmitted gradually increases, becoming brightest when the polarizers are exactly aligned. This property is used in various light filter combinations.
A number of substances can polarize light in other ways than in one plane, causing what are called circular polarization or elliptical polarization, for example. Organic substances that affect polarized light that passes through their solution are called optically active. In certain acids and other solutions the plane of polarized light is rotated to either the right or the left; their activity is usually indicated by the prefix dextro- or d- if the rotation is to the right and by levo-, laevo-, or l- if the rotation is to the left.
The instrument used to determine in which direction this optical rotation occurs is called a polariscope. A very simple form consists essentially of two crystals of some polarizing substance such as tourmaline. The solution to be tested is placed between them. Light is then directed through the first crystal, or polarizer, and is plane-polarized. After passing through the solution its plane is rotated; the direction and the degree of rotation are indicated by the position in which the second crystal must be placed to permit passage of the light that has gone through the solution. The polarimeter is a polariscope that measures the amount of rotation; when used for sugar solutions it is commonly called a saccharimeter.