Refraction of Light: Laws, Lenses, Prisms & Total Internal Reflection: 20 flashcards
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1. What is refraction of light?
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The bending of light when it passes obliquely from one transparent medium to another, caused by a change in its speed.
2. Why does light refract when it enters a new medium?
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Because the speed of light is different in different media; the change in speed at the boundary causes the ray to change direction.
3. State Snell's law.
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$\dfrac{\sin i}{\sin r} = \mu$, a constant for a given pair of media, where $i$ is the angle of incidence and $r$ is the angle of refraction.
4. What is the absolute refractive index of a medium?
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$\mu = \dfrac{c}{V}$, the ratio of the speed of light in vacuum (or air) to its speed in the medium.
5. Why does refractive index have no unit?
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Because it is a ratio of two speeds (or two sines), so the units cancel out.
6. When light goes from a rarer to a denser medium obliquely, how does it bend?
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It bends towards the normal, $i > r$, and its speed decreases.
7. When light goes from a denser to a rarer medium obliquely, how does it bend?
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It bends away from the normal, $i < r$, and its speed increases.
8. What happens to a ray that strikes a surface normally ($i = 0$)?
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It passes undeviated (straight through) even though its speed changes.
9. Which quantity does NOT change during refraction?
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Frequency remains unchanged; speed and wavelength change.
10. What is the relation between real depth and apparent depth?
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$\mu = \dfrac{\text{real depth}}{\text{apparent depth}}$.
11. What is the critical angle?
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The angle of incidence in the denser medium for which the angle of refraction is $90^\circ$.
12. State the relation between refractive index and critical angle.
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$\mu = \dfrac{1}{\sin C}$ or $\sin C = \dfrac{1}{\mu}$.
13. What are the two conditions for total internal reflection?
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Light must travel from a denser to a rarer medium, and the angle of incidence must exceed the critical angle.
14. Why do diamonds sparkle?
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Diamond's small critical angle ($\approx 24^\circ$) traps light in repeated total internal reflections before it escapes.
15. What is lateral displacement?
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The sideways shift of a ray emerging from a glass slab; it emerges parallel to the incident ray but displaced.
16. Why do stars twinkle but planets do not?
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Stars are point sources whose apparent position wobbles due to atmospheric refraction; planets appear as discs, so the wobble rarely removes the whole disc.
17. What is optical density?
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A measure of how much a medium slows light (related to refractive index), not the same as mass density.
18. Give the speed of light in vacuum, water and glass.
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Vacuum/air: $3 \times 10^8$ m/s; water: $2.25 \times 10^8$ m/s; glass: $2.0 \times 10^8$ m/s.
19. What happens to light at every boundary between two media?
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Part of the light is reflected and the rest is refracted.
20. How is a lens like a set of prisms?
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A lens can be thought of as a stack of tiny prisms, which explains why a convex lens converges and a concave lens diverges light.
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