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When light enters a material with higher refractive index, the angle of refraction will be smaller than the angle of incidence and the light will be refracted towards the normal of the surface. The higher the refractive index, the closer to the normal direction the light will travel. When passing into a medium with lower refractive index, the light will instead be refracted away from the normal, towards the surface.
For lossy media, Snell's law is invalid. It should be replaced by the general law of refraction which can be expressed as<ref>Y. Chen, "General law of refraction" https://assets-eu.researchsquare.com/files/rs-4783430/v1_covered_eebd8628-fdf9-4366-bfaa-bef42f6128d5.pdf</ref>
: <math>\sqrt{n_1^2+\kappa_1^2}\sin\theta_1=\sqrt{n_2^2+\kappa_2^2}\sin\theta_2</math>
Accordingly, the critical angle of total internal (or external) reflection can be calculated as:<math display="block">\theta_\mathrm{c} =\arcsin\!\left(\frac{|\underline{n_2}|}{|\underline{n_1}|}\right)\!= \arcsin\!\left(\frac{\sqrt{n_2^2+\kappa_2^2}}{\sqrt{n_1^2+\kappa_1^2}}\right)\!.</math>
===Total internal reflection===
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