Talk:Refraction of light

Authors: Anika Husen
back to Polarized light microscopy

Introduction

When a ray of light passes from one material into a second one, refraction of light occurs because it has a different speed in each of the two materials.

The process involves a change in wavelength due to the speed change at a given energy level, and since different wavelengths will have different refraction angles, a change in the direction of the light will also occur. This is best exemplified by the case of a prism where white light will be spread out into the spectral colors as it passes consecutively the two interfaces, air-glass and glass-air, that are at an angle:

Refraction of light is used in polarized light microscopy. Knowing the refractive index of one of the materials or phases, it is possible to estimate the refractive index of a second one in contact with it, a useful information for its eventual identification.

Light, waves and particles

Light is an electromagnetic radiation behaving as a wave that propels itself. Its propagation speed is c, the wave length l and the frequency f. Nevertheless, the wave behaves like a particle, especially at interfaces. Like a ball, that is thrown against a wall, the light wave is reflected (at least partially) from the surface. The energy of light is in discrete units, called quanta, and their energy is constant for a given frequency of the wave.

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h is Planck´s constant with the value

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Velocity or speed of light

The speed of light varies within different media. Light travels fastest in a vacuum and its speed differs little from the speed of light in air, therefore both speeds are often equated.

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The slower an electromagnetic wave propagates within a medium, the more optically dense the medium. The optical density (absorption) is described with the value of the refractive index. The refractive index is the ratio between the speed of light inside a vacuum and the material to be considered. The speed of light in the vacuum has the value 1, all other media have the value n>1.

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According to Snell´s law, the angle of the new propagation direction (after refraction), is dependent on n and the ray´s angle of incidence. The angle of incidence, is the angle to the normal, i.e. the line perpendicular to the surface, at the point of incidence. If a ray exits from a medium with a lower optical density and enters a medium with a higher optical density, the angle of incidence becomes smaller, the ray is refracted and vice versa. Therefore, the values of the sines of the angles between the rays and the normal, have a ratio equivalent to both refractive indices or the phase velocities.

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Light refraction

Light refraction occurs due to the variation of the velocity of light within different media (as described above). When imagining a wave that moves forward within a body, lines of the same amplitude, parallel to the wavefront can be expected, if the viewing direction is vertical to the propagation direction. When these parallel lines strike a boundary surface at an angle, every point of incidence produces a new vibration, which propagates within the new material. However, because the velocity has changed, the wavelength and the distance between the parallels of the same amplitude changes, too. Due to the change in velocity, the propagation direction is also newly orientated.

As a means of simplification, only the parallel lines of the same amplitude are examined. If a vibration is triggered in the new material, it spreads spherically from each point. The spheres are superimposed at the tangent that intersects all surfaces of the individual waves. A new wavefront develops. Because the points, where the wave starts have intervals, that depend on the wavelength and the angle of incidence, and because there is a specific time delay between the excitation of each individual point in dependance of the velocity within the first propagation medium, the wavefront in the second medium is differently orientated.

Diagrams missing!!

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SLeithaeuser 12:32, 6 April 2012 (CEST)