Refraction Light

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Refraction light

Optical refraction of light – Law of refraction

Refraction refers to the discontinuous change in direction of energy transport at the interface (see: phase boundary surface) between two media into the adjacent medium. Energy transport can occur in the form of electromagnetic waves (e.g. light, heat) and sound waves. In optically transparent or translucent (partial light transmission) materials, an obliquely incident light wave is partially reflected at an interface between medium 1 and 2 and simultaneously refracted depending on the optical properties. The deflection of light rays in the second medium is also referred to as light refraction or refraction [1].

While in the case of reflection, the angle of incidence α₀ of the light beam LW is identical to the angle of exit or reflection α_R, the angle of refraction β_B differs from the angle of incidence. The decisive factor here is the ratio of the index of refraction (refractive index) n₁ and n₂ of the two media, whereby the behaviour of refraction is described by SNELLIUS's law of refraction or the generalised law of refraction (Fig. 1) and (Eq. 1) [2, 3].

Fig. 1:

Optical refraction and reflection at flat interfaces

${\frac {\sin \alpha _{0}}{\sin \alpha _{R}}}={\frac {n_{1}}{n_{2}}}$

(1)

Determination of the index of refraction

If the index of refraction transition at the interface is sharp, a kink occurs in the beam path, which typically occurs in optical prisms, for example (Fig. 1). If the index of refraction changes continuously from medium 1 to medium 2, the light is also continuously curved, whereby the incident, reflected and refracted rays also lie in one plane. During refraction, the product n_i sin α_i remains constant, and this product is then referred to as the invariant of refraction.

Test methods for determining the index of refraction

For the optical testing of plastics, the determination of the index of refraction or refractive index is of practical importance. For the examination of plastic powders and test specimens, a number of physical measurement methods for determining the index of refraction have been found to be particularly suitable [1].

The most commonly used test methods in practice are:

• determination of the index of refraction by measuring the angle of total reflection with a refractometer on liquid or compact solid media,
• determination of the index of refraction of powdered plastics using the immersion method by changing the embedding liquid,
• index of refraction determination using the temperature and/or wavelength variation method on plastic powders with an embedding agent, and
• index of refraction determination on plane-parallel test specimens (films, plates, thin slices, thin sections) with assumed or measured knowledge of the thickness

See also

• Reflection light
• Light absorption
• Light remission
• Light transmission
• Refraction index

References

[1]	Trempler, J.: Optical properties. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022), 3rd Edition, pp. 299–330 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-806-5; see AMK-Library under A 22)
[2]	Eichler, J., Eichler, H. J.: Laser – Bauformen, Strahlführung, Anwendungen. Springer, Berlin (2003) 5th Edition, p. 258 (ISBN 3-540-00376-2)
[3]	Recknagel, A.: Physik – Optik. Technik Publishing, Berlin (1977) 7th Edition