Dispersive prism
In optics, a dispersive prism is a type of optical prism, usually having the shape of a geometrical triangular prism. It is the most widely known type of optical prism, although perhaps not the most common in actual use. Triangular prisms are used to disperse light, that is, to break light up into its spectral components (the colors of the rainbow). This dispersion occurs because the angle of refraction is dependent on the refractive index of a certain material which in turn is slightly dependent on the wavelength of light that is travelling through it. This means that different wavelengths of light will travel at different speeds, and so the light will disperse into the colours of the visible spectrum, with longer wavelengths (red, yellow) being refracted less than shorter wavelengths (violet, blue).
A good mathematical description of single-prism dispersion is given by Born and Wolf.[1] The case of multiple-prism dispersion is treated by Duarte.[2]
Prism dispersion played an important role in understanding the nature of light, through experiments by Sir Isaac Newton and others.
Overview
Although the refractive index is dependent on the wavelength in every material, some materials have a much more powerful wavelength dependence (are much more dispersive) than others. Crown glasses such as BK7 have a relatively small dispersion, while flint glasses have a much stronger dispersion (for visible light) and hence are more suitable for use in dispersive prisms. Fused quartz is used in the ultraviolet as normal glasses lose their transparency there.
The top angle of the prism (the upper corner in the accompanying picture) can be chosen to influence the exact dispersion characteristics. However, it is typically chosen such that both the incoming and outgoing light rays hit the surface approximately at the Brewster's angle, so that reflection losses are minimized. An example is the use of this type of prisms in prism compressors for generation of ultrafast laser pulses.
Types
Types of dispersive prism include:
- Triangular prism
- Abbe prism
- Pellin–Broca prism
- Amici prism
- Compound prism
Grisms (grating prisms)
A diffraction grating may be ruled onto one face of a prism to form an element called a "grism". Spectrographs are extensively used in astronomy to observe the spectra of stars and other astronomical objects. Insertion of a grism in the collimated beam of an astronomical imager transforms that camera into a spectrometer, since the beam still continues in approximately the same direction when passing through it. The deflection of the prism is constrained to exactly cancel the deflection due to the diffraction grating at the spectrometer's central wavelength.
A different sort of spectrometer component called an immersed grating also consists of a prism with a diffraction grating ruled on one surface. However in this case the grating is used in reflection, with light hitting the grating from inside the prism before being totally internally reflected back into the prism (and leaving from a different face). The reduction of the light's wavelength inside the prism results in an increase of the resulting spectral resolution by the ratio of the prism's refractive index to that of air.
Grating and prism mountings
There are six grating/prism configurations which are considered to be "classics":[3]
In popular culture
An artist's rendition of a dispersive prism is seen on the cover of Pink Floyd's The Dark Side of the Moon, one of the best-selling albums of all time. The iconic graphic shows a coherent ray of white light entering the prism and beginning to disperse, and shows the spectrum leaving the prism.
See also
References
- ^ M. Born and E. Wolf, Principles of Optics, 7 ed. (Cambridge University, Cambridge, 1999), pp. 190–193.
- ^ F. J. Duarte, Tunable Laser Optics (Elsevier Academic, New York, 2003).
- ^ George J . Zissis (1995). "Dispersive prisms and gratings" (pdf) in Michael Bass et al. (eds.) Handbook of Optics. Vol. 2, Ch. 5. McGraw Hill.