According to Snell's law, when an EM radiation, in this case infrared, passes through a denser material, its speed changes, but when it exits the dense material and returns to the air, does its speed return to the previous one, or does it continue to be slowed down?
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From Snells Law you can answer this as the light speeds back up since the angle of refraction exiting is the same as the angle during entrance.
DWin
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It returns to the previous velocity. If electromagnetic wave phase velocity would stay the same while exiting denser material, then according to the Snell law, $${\frac {\sin \theta _{1}}{\sin \theta _{2}}}={\frac {n_{2}}{n_{1}}}={\frac {v_{1}}{v_{2}}} \tag 1$$
it would hold $\theta_1 = \theta_2$ (since $v_2=v_1$) i.e. beam would not deflect from the surface normal, which is not the case and so speed changes back.
Also according to the Maxwell equation, $$ v_{\text{p}}={\frac {1}{\sqrt {\mu~\varepsilon}}} \tag 2$$
phase velocity of electromagnetic radiation in the air can't stay the same as in denser material since permittivity and permeability of air is different than that of denser material.
Agnius Vasiliauskas
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