Light: Mirrors and Lenses
The first law of reflection
Investigate the first law of reflection with a mirror and a beam of light.
Are there rules behind all this? Activity 10.4 repeats the Grade 7 reflection experiment and goes further. You need a plane mirror on a stand, a torch, a comb, a paper clip to hold the comb up, a white sheet of paper and a strip of black paper.
Cover all the gaps in the comb with black paper except one in the middle, making a thin slit. Spread the white paper on a table and stand the mirror upright on it. Shine the torch through the slit so a thin beam runs along the paper to the mirror. Then move the torch a little so the beam hits at a different angle, and watch the reflected beam move too.
To think about it clearly, draw it. Light is shown by straight lines with arrows called rays: they show the path light takes, and light travels in straight lines.
| Word | Meaning |
|---|---|
| incident ray | the ray going to the mirror |
| reflected ray | the ray coming back from the mirror |
| normal | a line drawn at 90° to the mirror, at the point where the ray hits it (the point of incidence, O) |
| angle of incidence (i) | the angle between the incident ray and the normal |
| angle of reflection (r) | the angle between the reflected ray and the normal |
Measure i and r for several angles and fill in Table 10.1. They always come out (nearly) equal. Done carefully, the experiment shows the first law of reflection: the angle of incidence equals the angle of reflection.
Last, shine the beam straight along the normal, at 90° to the mirror. Both angles are then zero, and the light comes straight back along the same path.
A common trap: both angles are measured from the normal, never from the mirror's surface.
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