Law of reflection
Practical Activity for 14-16
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.
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.
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
- 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 interferencebut rather a Moire Pattern analogue.
This experiment was safety-tested in January 2007.
Download the support sheet / student worksheet for this practical.