Light, reflection and seeing — Science, 11–13 years
We see objects when light from a source reaches them and then enters our eyes. Reflection explains mirrors, shiny surfaces and many everyday images. This topic follows the path of light and shows why the angle of reflection matches the angle of incidence.
Idea
Light travels from a source, such as the Sun or a lamp, in straight lines through the same material. When it hits a surface, it may be absorbed, pass through or reflect. We see a non-luminous object when reflected light travels from it into our eyes; the eye detects the light, not the object itself.
Why reflection rules help
People needed to predict where light would go, rather than treating mirrors as mysterious surfaces. The reflection rule solves this problem: measure both angles from the normal, an imaginary line at right angles to the surface, and they are equal. This lets us place mirrors and explain how an image appears behind one.
Worked example
A ray hits a flat mirror at an angle of incidence of 35°. Step 1: draw the normal at 90° to the mirror. Step 2: measure the incoming ray from that normal, not from the mirror surface. Step 3: use the rule: angle of reflection = angle of incidence = 35°. The reflected ray leaves on the other side of the normal at 35°.
A common trap
A frequent mistake is measuring the angle between the ray and the mirror surface. That feels natural because the ray seems to skim along the mirror. Physics measures the angle from the normal instead, so an angle of 35° to the surface means 55° to the normal. Naming the reference line prevents the answer being flipped.
Outside school
Reflection is used in car mirrors, bicycle reflectors, periscopes and solar cookers. Road reflectors send car headlights back towards drivers, helping them notice the road at night. Smooth mirrors form clear images, while rough walls reflect light in many directions, letting us see them from different places.
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