Science 7–10 · Year 9
Law of reflection: ray box and plane mirror
Physical sciences — Waves and motion: waves, sound and light (NSW Stage 5 focus area)
This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.
The idea
Light reflects from a flat mirror so that the angle of reflection equals the angle of incidence, both measured from the normal.
What you need
- ray box with single-slit mask and 12 V supply, 1
- plane mirror on a stand, 1
- A3 white paper, protractor, ruler and sharp pencil, 1 set
- darkened room
How to do it
- Draw a straight line on the paper and stand the mirror on it. Draw the normal at the mirror's centre point.
- Aim a single ray at the centre point at about 20 degrees to the normal. Mark two points along the incident ray and two along the reflected ray.
- Remove the mirror, join the points, and measure the angles of incidence and reflection with the protractor.
- Repeat for 30, 40, 50, 60 and 70 degrees.
- Tabulate the angle pairs and plot reflection against incidence.
- Aim the ray along the normal and record what happens.
What you should see
The two angles are equal within the reading precision of the protractor and the pencil lines at every setting, so the graph is a straight line of gradient 1 through the origin. A ray along the normal returns along its own path. The learner knows it worked when each pair of angles agrees within that precision.
What changes
- What you change
- angle of incidence (degrees)
- What you measure
- angle of reflection (degrees)
- What you keep the same
- ray aimed at the same centre point
- mirror on its line
- angles measured from the same normal
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Angles are measured from the mirror surface rather than the normal.
- Light bounces off a mirror at any angle.
- A mirror image is on the mirror's surface.
Safety card
Hazards
- hot ray box
- trailing 12 V leads
- moving about in a dark room
Controls
- handle the ray box by its case
- leads taped down
- torches for moving between benches
Note
Heat or electrical energy is involved. Complete the school's risk assessment for the activity before the lesson, using CSIS 1.7 (Risk assessment – a pre-requisite for risk control) from the department's Chemical Safety in Schools package (2021 Technical Update), which the NSW Department of Education Science safety and compliance page names for risk assessment advice.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC5-WAM-01SC5-WS-01SC5-WS-05
- Australian Curriculum v9AC9S9U04AC9S9I03AC9S9I04
Sources
The pages the author read to write this activity.