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Lesson 3.8
Lesson 3.5 |
Lesson 3.8 Refraction Overview This lesson deals with the measurement of the degree to which light changes diretion when it passes at an angle from one material to another. On completion of this lesson you should be able to define the absolute refractive index and relative refractive index. You should also be able to use Snell’s Law and laws of refraction to estimate refractive indices. MINI LAB
CHOICE OF ACTIVITIES
Wave Fronts When
waves move away from a source, the parts of waves that
are in phase can be joined by lines called wave fronts.
Wave fronts are surfaces of constant phase. Rays Waves usually
radiate outwards from a source. Rays are radial lines
pointing outwards from a source of light or
electromagnetic wave fronts. Refraction The
change in direction of a wave front (or a ray of light)
as it passes from one medium or region to another. Index of Refraction The ratio of the speed of light in a vacuum to the speed of light in a material. This is sometimes called the Index of Refraction. Snell’s Law When light travels from one medium to another, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the refractive index.
Speeds of light The speed of light is constant at roughly 300 000 kilometers per second in the absence of any matter in its path. (i.e. in a vacuum or in space) The letter c is often used to denote the constant that is equal to the speed of light in a complete vacuum. Light travels through glass at roughly 0.66c – depending on the type of glass. The speed of light through water is roughly 0.75c and through diamond is 0.4c. Huygens’ Principle When light passes from one medium into another medium, it either slows down or speeds up, depending on the medium. According to Huygens’ principle, any point on a wave-front is a potential point source of new waves. If part of the wave-front is removed by blockage or is slowed down as a result of its entry into a new medium, part of the remaining wave will move in the direction of the missing or retarded portion of the wave-front. Refraction Refraction is the change in direction of any wave as a result of a change in speed when it moves from one medium to another. The direction in which rays of light are refracted depends on whether they are slowed down or speeded up. If a beam of light traveling through air strikes a flat glass surface at an angle, the beam will change direction as a result of the slower movement of the light in the glass. Because light travels more slowly through glass, the beam will be deflected in the direction of the glass and if it emerges from a parallel surface, it will be deflected away from the glass and travel in a direction parallel to its original path.
Refractive Index The refractive index (or index of refraction) for a pair of transparent substances is the ratio of the speed of light in one substance to the speed of light in the other. The optical density is an indication of its refractive index – usually in relation to air. It is a measure of the relative speed at which light travels through the material. The speed of light is lower in materials with higher optical densities. The ratio of the speed of light in two media is equal to the relative refractive index of the media: h 12 = c1 /c2 Absolute Refractive
Index Relative Refractive Index We commonly use the
relative refractive index when air is not involved. The
relative refractive index is obtained when light passes
from one medium to another and is given by the formula: Apparent depth The bottom of a pond, pool or tank with water in it appears to be less deep than it really is. Because light from the bottom is refracted away from the normal to the surface at the point at which it leaves the water, it appears to come from a depth that is less than the real depth. Similarly objects in the water, such as fish, will appear to be closer to the surface.
If the real and apparent depths can be measured, the refractive index can be measured as the ratio of the real depth to the apparent depth. Laws of Refraction When a ray of light is refracted in going from one medium to another, the following terms are used in describing the result: The incident ray is the ray of light before refraction. The refracted ray is the ray of light after refraction. The normal is a hypothetical line drawn at right angles to the interface between the two media at any point. The angle of incidence is the angle between the incident ray and the normal at the point of incidence The point of incidence is the point at which the incident ray meets the boundary between the two media. This is also the point at which the incident ray becomes the refracted ray. The angle of refraction is the angle between the refracted ray and the normal at the point of incidence. If the speed of light in the second medium is less than in the first, the ray will be refracted towards the normal. The angle of refraction will be smaller than the angle of incidence. If the ray passes into a material with a lower optical density, the ray will be refracted away from the normal. The angle of refraction will be greater than the angle of incidence.
Snell’s Law Snell's law states that
when light travels from one medium to another the ratio
of the sine of the angle of incidence to the sine of the
angle of refraction is equal to the refractive index `h ’.
Review Questions
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