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Measure length and angle · Physics

Protractor

Protractor, with labels: centre point, baseline, scale in degrees.

What it is for

A protractor measures the angle between two lines. Put its centre on the point where the lines meet and its baseline along one of them, then read where the other line crosses the scale.

Range
0° to 180° (a full-circle one, 0° to 360°)
Smallest division
1°
Record to
the nearest 1°, e.g. 35°

Why choose it

For the angles of incidence, reflection and refraction, all measured from the normal. A set square draws the normal at exactly 90° to the surface, but only a protractor measures any angle.

How to read it

0306090120150180centre on the point, 0° on one line:the angle is 35°
  1. Draw the normal at 90° to the surface where the ray meets it.
  2. Put the centre of the protractor exactly on that point.
  3. Lay the baseline along the normal or the surface, whichever the angle starts from.
  4. Read where the ray crosses the scale that starts at 0° on your line: 35°.

Exam-style question

A ray of light strikes a plane mirror. A student measures 55° between the incident ray and the mirror surface.

  1. Calculate the angle of incidence.
  2. State the angle of reflection.
  3. Another student measures the same 55° angle between the ray and the mirror but reads 125°. What went wrong?
Show what a good answer says
  1. Angles are measured from the normal: 90° − 55° = 35°.
  2. 35°: the angle of reflection equals the angle of incidence.
  3. They read the other scale: 180° − 125° = 55°. The angle is clearly less than 90°, so count from the 0° that lies on the mirror line.

Teaching points for checking your own answer, not official Cambridge marks.

Common errors

  • Measuring from the surface instead of the normal: angles of incidence, reflection and refraction are all measured from the normal.
  • Reading the wrong one of the two scales (125° instead of 55°): first decide whether the angle is more or less than 90°.
  • Not putting the centre exactly on the point where the ray meets the surface.
  • Drawing short or thick rays that cannot be lined up well: draw long, thin pencil lines.

Where you use it here

From the Practical Test apparatus lists of the Cambridge IGCSE Biology 0610, Chemistry 0620 and Physics 0625 syllabuses for 2026–2028, and the apparatus the experiments on this site use.

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