The tutorial also explores how changes in refractive index affect dispersion of light passing through the prism. In the late seventeenth century, Sir Isaac Newton performed a series of experiments that led to his discovery of the visible light spectrum, and demonstrated that white light is composed of an ordered array of colors starting with blue at one end and progressing through green, yellow, and orange, finally ending with red at the other end.
Working in a darkened room, Newton placed a glass prism in front of a narrow beam of sunlight emerging through a hole drilled into a window shutter. When the sunlight passed through the prism, an ordered spectrum of color was projected onto a screen placed behind the prism.
From this experiment, Newton concluded that white light is produced from a mixture of many colors, and that the prism spread or "dispersed" white light by refracting each color at a different angle so they could be easily separated Figure 8.
Newton was unable to further subdivide the individual colors, which he attempted by passing a single color of dispersed light through a second prism. However, when he placed a second prism very close to the first, so that all of the dispersed colors entered the second prism, Newton found that the colors were recombined to produce white light again.
This finding produced conclusive evidence that white light is composed of a spectrum of colors that can easily be separated and reunited. The phenomenon of dispersion plays a critical role in a wide variety of common observations. Rainbows result when sunlight is refracted by raindrops falling through the atmosphere, producing a spectacular display of spectral color that closely mimics that demonstrated with a prism.
In addition, the sparkling colors produced by exquisitely cut gems, such as a diamond, result from white light that is refracted and dispersed by precisely angled facets. When measuring the refractive index of a transparent substance, the particular wavelength used in the measurement must be identified. This is because dispersion is a wavelength-dependent phenomenon, and the measured refractive index will depend on the wavelength of light used for the determination.
Table 2 categorizes the dispersion of visible light in various media as shown by the variation of refractive index for three different wavelengths or colors of light.
The most commonly used wavelength to measure refractive index values is emitted by a sodium lamp, which features a strong and closely spaced doublet having an average wavelength of This light is termed the D line spectrum, and represents the yellow light listed in Table 2. Likewise, F line and C line spectra correspond to blue and red light of specific wavelengths also presented in Table 2 emitted by hydrogen.
From the values given in the table, it is apparent that increasing the wavelength of light from Dispersion can be quantitatively defined, using the three specific wavelengths for yellow, blue, and red light, as :. Many factors play a key role in the dispersion values of various materials, including the elemental and molecular composition, and the crystalline lattice morphology. Several inorganic solids have unusually high dispersions, including the chromates, dichromates, cyanides, vanadates, and halide complexes.
Organic substituents can also contribute to high dispersion values when incorporated into certain materials. Dispersion is also responsible for chromatic aberration, a lens artifact resulting from refractive index variation with wavelength.
When white light is passed through a simple convex lens, several focal points arise in close proximity, which correspond to the minor refractive index differences of the component wavelengths. This effect tends to produce colored either red or blue, depending upon focus halos surrounding the images of objects.
Correction of this aberration is accomplished by the use of combinations of two or more lens elements composed of materials having different dispersive properties. A good example is an achromatic doublet lens system constructed of two individual elements using both crown and flint glasses.
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. If the incident angle increases past a specific value dependent upon the refractive index of the two media , it reaches a point at which the angle is so large that no light is refracted into the medium of lower refractive index, as illustrated in Figure 9.
In this figure, individual light rays are represented by either red or yellow colored arrows moving from a medium of higher refractive index n 2 to one of lower refractive index n 1. The angle of incidence for each individual light ray is denoted by the value, i , and the angle of refraction by the variable, r.
The four yellow light rays all have an angle of incidence i low enough to allow them to pass through the interface between the two media. However, the two red light rays have incident angles that exceed the critical angle of reflection approximately 41 degrees for the water and air examples and are reflected either into the boundary between the media or back into the higher refractive index medium. The critical angle phenomenon takes place when the angle of refraction angle r in Figure 9 becomes equal to 90 degrees and Snell's law reduces to :.
When the critical angle is exceeded for a particular light wave, it exhibits total internal reflection back into the medium. Usually the higher index medium is considered the internal medium, because air having a refractive index of 1. This concept is especially critical in optical microscopy when attempting to image specimens with a medium other than air between the cover glass and the objective front lens.
The most common immersion medium other than air is specialized oil having a refractive index equal to that of the glass used for the objective front lens element and the coverslip. Optical devices ranging from microscopes and telescopes to cameras, charge-coupled devices CCDs , video projectors, and even the human eye, rely in a fundamental way on the fact that light can be focused, refracted, and reflected.
The refraction of light produces a wide variety of phenomena, including mirages, rainbows, and curious optical illusions such as making fish appear to be swimming in more shallow water than they really are. Refraction also causes a thick-walled beer mug to appear fuller than it really is, and deceives us into thinking the sun is setting several minutes later than it really does. Millions of people use the power of refraction to correct faulty vision with eyeglasses and contact lenses, which enable them to see the world more clearly.
By understanding these properties of light, and how to control them, we are able to view details that are invisible to the unaided human eye, regardless of whether they are located on a microscope slide or in a distant galaxy. Thomas J. Fellers and Michael W. Introduction to the Refraction of Light. Refraction of Light Explore how changes to the incident angle and refractive index differential between two dissimilar media affect the refraction angle of both white and monochromatic light at the interface.
Start Tutorial. Observing Objects in Water Explore how a fish observed in a body of water is actually swimming much deeper than it appears to be. Start Tutorial ». Refraction by an Equilateral Prism Discover how the incident angle of white light entering the prism affects the degree of dispersion and the angles of individual light rays exiting the prism. The centre of the circle of the rainbow will always be the shadow of your head on the ground.
The secondary rainbow that can sometimes be seen is caused by each ray of light reflecting twice on the inside of each droplet before it leaves. This second reflection causes the colours on the secondary rainbow to be reversed.
Red is at the top for the primary rainbow, but in the secondary rainbow, red is at the bottom. Learn more about the many different kinds of rainbows and how they are formed from the Atoptics website — Rainbows reflect and Rainbow orders.
Learn more about human lenses, optics, photoreceptors and neural pathways that enable vision through this tutorial from Biology Online.
Add to collection. Activity ideas Use these activities with your students to explore refration further: Investigating refraction and spearfishing — students aim spears at a model of a fish in a container of water. When they move their spears towards the fish, they miss! Angle of refraction calculator challenge — students choose two types of transparent substance. They then enter the angle of the incident ray in the spreadsheet calculator, and the angle of the refracted ray is calculated for them.
Light and sight: true or false? This activity can be done individually, in pairs or as a whole class. Useful links Learn more about the many different kinds of rainbows and how they are formed from the Atoptics website — Rainbows reflect and Rainbow orders.
Go to full glossary Add 0 items to collection. Download 0 items. Twitter Pinterest Facebook Instagram. In all ray diagrams, all angles of incidence and refraction are measured between the ray and the normal. Light Refraction When a wave or light ray moves from one medium to another its speed changes. Example: Light rays passing through a glass block Step 1 Step 2 Step 3 As can be seen in the diagram the light ray changes direction as it enters and leaves the block.
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