Topic 7 of 8Cambridge O Levels

Light & Optics

Reflection, refraction, and how mirrors and lenses work

Light is a transverse electromagnetic wave that travels at 3 × 10⁸ m/s in a vacuum (the fastest thing in the universe!).

Reflection:

Angle of incidence = Angle of reflection

Smooth surfaces (mirrors) give regular reflection — clear images

Rough surfaces give diffuse reflection — scattered light, no clear image

Plane Mirrors:

Image is virtual, upright, laterally inverted, same size

Image distance = Object distance (from mirror)

Refraction:

Light changes speed (and direction) when moving between mediums

Light slows down in denser mediums (air → glass → water)

Bending toward the normal = entering a denser medium

Bending away from normal = entering a less dense medium

Total Internal Reflection:

When light hits a boundary at an angle greater than the critical angle (and goes from dense to less dense medium), all light reflects back. Used in optical fibres and binoculars.

Lenses:

Convex (converging) lens: Brings light to a focus point. Used in magnifying glasses, cameras, eyes

Concave (diverging) lens: Spreads light out. Used to correct short-sightedness

Key Points to Remember

1Light speed = 3 × 10⁸ m/s
2Angle of incidence = Angle of reflection
3Light bends toward normal when entering denser medium
4Total internal reflection occurs above the critical angle

Pakistan Example

Truck Side Mirrors on GT Road

Trucks on GT Road use large convex mirrors as side mirrors. Convex mirrors give a wider field of view (you can see more of the road behind), but objects appear smaller and farther than they actually are. That's why you see the warning 'Objects in mirror are closer than they appear' on car mirrors. Flat (plane) mirrors would give accurate sizes but a much narrower view — dangerous on busy Pakistani highways!

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Waves & Sound Electricity