GCSE Physics · Edexcel 1PH0 · Topic 5A
Reflection, refraction and lenses
Key points
- Law of reflection: angle of incidence = angle of reflection, both measured from the normal.
- Specular reflection from smooth surfaces gives clear images; diffuse reflection from rough surfaces scatters light.
- Refraction: light bends towards the normal when it slows down entering a denser medium (e.g. air to glass).
- Total internal reflection happens when light in a denser medium hits the boundary at more than the critical angle; used in optical fibres.
- A converging (convex) lens brings parallel rays to a real principal focus; a diverging (concave) lens spreads them out from a virtual focus.
- The shorter the focal length, the more powerful the lens. Ray diagrams show whether an image is real or virtual, upright or inverted, magnified or diminished.
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Ray diagrams let you predict exactly where light will go when it meets a mirror, a block of glass or a lens. This sub-topic covers the rules of reflection and refraction and how lenses form images.
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