Refraction
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Speed of Light and Refractive Index
- Light travels at 300 000 000 m/s in a vacuum – nothing faster.
- In a medium (e.g., glass) light slows down; in glass ≈ 200 000 000 m/s.
- Refractive index measures how much a material slows light: higher index → slower speed.
- When light enters a medium with higher refractive index, its wavelength decreases but frequency stays the same.
Refraction at a Boundary
- Refraction is the change of direction of light as it passes from one medium to another.
- The normal is an imaginary line perpendicular to the surface at the point of incidence.
- When light goes from a lower to a higher refractive index (e.g., air → glass), it bends towards the normal.
- The angle of refraction (r) is smaller than the angle of incidence (i).
- If the angle of incidence is 0° (along the normal), light continues straight but slows down.
Emergent Ray
- When light exits a denser medium (e.g., glass → air), it speeds up and bends away from the normal.
- The ray that leaves the block is called the emergent ray.
- The emergent ray is parallel to the incident ray (same direction) but displaced sideways.
The 'Bent Pencil' Illusion
- A pencil in water appears bent because light from the underwater part refracts at the water surface.
- Light travels slower in water than in air, so it bends away from the normal when leaving water.
- Our brain assumes light travels in straight lines, so it traces the rays back to a virtual image that appears higher, making the pencil look bent.
Refraction of light from air into glass: the ray bends towards the normal.
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Câu hỏi luyện tập
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1.What is the speed of light in a vacuum?
Easy- A300 000 000 m/s
- B200 000 000 m/s
- C150 000 000 m/s
- D100 000 000 m/s
2.True or false: Light travels at the same speed in a vacuum as it does in glass.
Easy- AFalse
- BTrue
- CCannot say
- DOnly if the glass is clear
3.When light passes from air into glass, what happens to its wavelength?
Easy- AIt decreases
- BIt increases
- CIt stays the same
- DIt becomes zero
4.When light passes from air into glass, what happens to its frequency?
Easy- AIt remains unchanged
- BIt decreases
- CIt increases
- DIt becomes zero
5.When light travels from air into a glass block at an angle (not 0°), which way does it bend?
Medium- ATowards the normal
- BAway from the normal
- CIn the same direction as the incident ray
- DIt does not bend
6.In the diagram of refraction from air into glass, which angle is the angle of refraction?
Easy- AThe angle between the refracted ray and the normal
- BThe angle between the incident ray and the normal
- CThe angle between the incident ray and the surface
- DThe angle between the refracted ray and the surface
7.What is the name given to the ray that leaves a glass block?
Easy- AEmergent ray
- BIncident ray
- CRefracted ray
- DNormal ray
8.If the angle of incidence of a ray of light entering a glass block is 0°, what happens to the ray?
Easy- AIt continues in the same direction (but slower)
- BIt bends towards the normal
- CIt bends away from the normal
- DIt reflects back into air
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