Light is a form of energy that produces the sensation of sight. It travels in straight lines at a speed of about 3 x 10 to power 8 metre per second in vacuum. The behaviour of light through reflection and refraction is a key science topic for exams.
Reflection of Light
- Reflection is the bouncing back of light from a surface.
- Laws of reflection: angle of incidence equals angle of reflection.
- Plane mirrors form virtual, erect and same-size images.
- Concave mirrors are used in headlights and shaving mirrors.
- Convex mirrors are used as rear-view mirrors in vehicles.
Refraction of Light
- Refraction is the bending of light when it passes from one medium to another.
- Light bends because its speed changes in different media.
- A pencil in a glass of water looks bent due to refraction.
- The ratio of speeds gives the refractive index.
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Lenses
- Convex lens is converging; used in magnifying glasses and to correct hypermetropia (long sight).
- Concave lens is diverging; used to correct myopia (short sight).
- Power of a lens is measured in dioptres (D).
Total Internal Reflection
- It happens when light travels from a denser to a rarer medium beyond the critical angle.
- It causes the sparkle of diamonds and the formation of mirages.
- Used in optical fibres.
Quick Revision Points
- Speed of light in vacuum = 3 x 10 to power 8 m/s.
- Angle of incidence = angle of reflection.
- Convex mirror used as a rear-view mirror.
- Concave mirror used in headlights.
- Refraction = bending of light.
- Convex lens corrects hypermetropia; concave lens corrects myopia.
- Lens power unit is dioptre.