Law of reflection

Class practical

Showing angle of incidence equal to angle of reflection.

Apparatus and Materials

• For each group of students
• Ray box or lamp (12 V 24 - 48 W)
• Low voltage power supply for lamp
• Single slit
• Plane mirror
• Holder for mirror
• Paper protractor (see below)
• White paper

Health & Safety and Technical Notes

Many ray boxes of traditional design become very hot after a lesson of use. Warn the class and provide them with heat-proof gloves or cloths if they need to handle the ray box when still hot.

Read our standard health & safety guidance

A suitable protractor template is provided (see below).

A cylindrical lens can be fitted within the ray box to give a clear, long ray streak.

Procedure

Set up the apparatus to produce a single ray streak on the paper. Stand the mirror on the paper protractor with the two bases aligned. Students will quickly see the equal angles.

Teaching Notes

• Students should see rays of light being reflected at a plane mirror. They should extract some kind of rule about equal angles. It is possible to sketch in a series of rays in order to keep a record of the experiment. But it is just as easy to read off the angles of incidence and reflection from the protractor, if it is aligned with the mirror.
• An alternative is to use a Hartl disc or some other arrangement, which has a protractor and a scheme for showing the behaviour of a single ray.
• The template (see below) with two sets of parallel lines can be used with an Over Head Transparency (OHT} to simulate reflection and interference of plane waves, at a straight barrier.
• Photocopy or print the lines onto an OHT. Cut into two sets. Use the lines as wave fronts towards the mirror at an angle.
• This is not real interference but rather a Moire Pattern analogue.

This experiment was safety-tested in January 2007.