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# Thought of the day_12_Light!

Why the speed of light decreases when the light moves from vacuum into a medium??

Note by Rohit Gupta
2 years, 1 month ago

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Light is an electromagnetic wave. According to Maxwell's equation, speed of an electromagnetic wave is given by $$v = \dfrac{1}{\sqrt{\epsilon \mu}} = \dfrac{1}{\sqrt{\epsilon_r \epsilon_0 \mu_r \mu_0}} = \dfrac{c}{\sqrt{ \epsilon_r \mu_r}}$$, where $$c$$, $$\epsilon_0$$ and $$\mu_0$$ are speed of light, electric constant or permittivity and magnetic constant or permeability in free space (vacuum) respectively, and $$\epsilon_r$$ and $$\mu_r$$ are relative permittivity and relative permeability of a medium respectively. $$\epsilon_r$$ and $$\mu_r$$ are properties of a medium. For free space $$\epsilon_r = \mu_r = 1$$ and for a medium denser than free space, it is always that $$\epsilon_r > 1$$ and $$\mu_r > 1$$. Therefore, $$v < c$$. · 2 years, 1 month ago

I wish to know the reason of slow down of light..!! yups I agree that these formulas show that the speed decreases.. but my question is physically what is happening there which is reducing the speed of light..!! · 2 years, 1 month ago

As mentioned $$\epsilon$$ and $$\mu$$ are properties of a medium. These properties are affected by the matter in the medium with density. Another way of seeing this is that light always travels at the speed of light. But in a medium of matter it travels at the speed of light until it hits an atom. It is then absorbed and re-emitted in the same direction. The process takes a small amount of time and hence slows it down. The denser the medium, the more atoms per volumn and hence the more this happens and the lower the average speed and hence the higher the refractive index. · 2 years, 1 month ago

When light passes thru denser mediums , it coliides with the particles of the medium which decreases its speed . I guess. · 1 year, 2 months ago

Light gets obstructed in the medium.So its speed decreases,i guess · 1 year, 7 months ago