This meeting place of two different media is called the interface between the media. All refraction of light and reflection occurs at the interface. What happens at the interface to make light refract or reflect? When light is incident at a transparent surface, the transmitted component of the light that which goes through the interface changes direction at the interface.
Another component of the light is reflected at the surface. As shown in Figure 1, the refracted beam changes direction at the interface and deviates from a straight continuation of the incident light ray. Figure 1. Light in air incident on glass surface where it is partly reflected at the interface and partly transmitted into the glass.
The angle of refraction r is less than the angle of incidence i. The change of direction of light as it passes from one medium to another is associated with a change in velocity and wavelength. The energy of the light is unchanged as it passes from one media to another. Figure 2 displays in bar chart format the velocity of light in different media.
For air, the velocity is Next, a small amount of liquid is added to the cell and the microscope is refocused on the mark through the liquid and a new measurement is taken. The microscope is finally focused on the surface of the liquid, and a third reading is recorded by measuring the position of the mark on the reticle. The refractive index of the unknown liquid can then be calculated using the following equation:.
Although it is generally true that light must pass from one substance into another to undergo refraction, there are circumstances in which perturbations, such as temperature gradients, can produce enough fluctuation in refractive index within a single medium to generate a refractive effect.
If they have significantly different temperatures, overlapping layers of air in the atmosphere are responsible for producing what are often termed mirages , a phenomenon in which the virtual image of an object is observed to be positioned either above or below the actual object. Layering of warmer and cooler air is especially common over desert areas, the ocean, and hot asphalt pavement such as parking lots and highways. The actual mirage effect that is visualized depends upon whether cooler air overlies warmer air, or vice versa Figure 7 a.
One type of mirage appears as an upside-down virtual image directly beneath the real object, and occurs when a layer of warm air near the ground or water surface is trapped by denser, cooler air lying above. Light from the object traveling downward into the warmer air adjacent to the ground or water is refracted upward toward the horizon. At some point the light reaches a critical angle for the warm air, and is bent upward by total internal reflection , resulting in the virtual image appearing below the object.
Another form of mirage, termed looming , occurs when warm air lies over a layer of cooler air, and is common over large bodies of water that may remain relatively cool when the air above the water is heated during the day see Figure 7 b. Light rays from an object, such as a ship on the water, traveling upward through the cool air into the warmer air are refracted downward toward an observer's line of sight.
The rays then appear to originate from above the object and it appears to "loom" above its actual position. It is common for ships at sea near the horizon to appear to float above the water. Although reference is usually made to a standard and fixed refractive index for a substance, careful measurements indicate that the index of refraction for a particular material varies with the frequency and wavelength of radiation, or the color of visible light.
In other words, a substance has many refractive indices that may differ either marginally, or to a significant degree, as the color or wavelength of light is changed. This variation occurs for nearly all transparent media and has been termed dispersion. The degree of dispersion exhibited by a specific material is dependent upon how much the refractive index changes with wavelength.
For any substance, as the wavelength of light increases, the refractive index or the bending of light decreases. In other words, blue light, which comprises the shortest wavelength region in visible light, is refracted at significantly greater angles than is red light, which has the longest wavelengths.
It is the dispersion of light by ordinary glass that is responsible for the familiar splitting of light into its component colors by a 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 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. 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. Parallel rays of light can be focused in to a focal point. A biconvex lens is called a converging lens. A biconcave lens curves is thinner at the middle than it is at the edges.
Light rays refract outwards spread apart as they enter the lens and again as they leave. Isaac Newton performed a famous experiment using a triangular block of glass called a prism. He used sunlight shining in through his window to create a spectrum of colours on the opposite side of his room. This experiment showed that white light is actually made of all the colours of the rainbow. Newton showed that each of these colours cannot be turned into other colours.
He also showed that they can be recombined to make white light again. The explanation for the colours separating out is that the light is made of waves.
Red light has a longer wavelength than violet light. The refractive index for red light in glass is slightly different than for violet light. Violet light slows down even more than red light, so it is refracted at a slightly greater angle.
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