AS/A-level Physics - Refractive index

In this chapter of AS/A-level Physics, you will learn about refractive index, Snell's Law and light refraction.

• Light fastest in a vacuum, slows down in other materials due to interacting with particles in them
• Absolute refractive index of a material n, measure of optical density --> Ratio between speed of light in a vacuum, c, and the speed of light in a material n = c / c_s where c = 3 x 10⁸
• Relative refractive index between to materials, ₁n₂ = c₁ / c₂ which combining equations will be ₁n₂ = n₂ / n₁
• Because we assume n_air= 1 you can say the refractive index for air to glass = absolute refractive index of glass (or others)

• Angle of the incoming light makes to the normal is called the angle of incidence (θ₁), angle of refracted ray makes with the normal and is the angle of refraction (θ₂)
• In AS/A-level Physics, light is crossing a boundary ,going from a medium with the refractive index n₁ to a medium with refractive index n₂
• Light enters an optically denser medium --> reflected TOWARDS the normal n₁ sin₁ = n₂ sin₂
• Light leaving an optically denser medium/material --> reflected AWAY from normal

• Light refracted away from the normal à increase in angle of incidence = angle of refraction getting closer to 90°
• Eventually θ₁ reaches a critical angle θ_c for which θ2 = 90° --> light refracted along the boundary
• In AS/A-level Physics, sin θ = n₂ / n₁ --> At θ, greater than the critical angle refraction is impossible all light is reflected back --> total internal reflected

That's it! You got it!!

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