Science 7–10 · Year 9
Refraction: tracing a ray through a rectangular glass slab
Physical sciences — Waves and motion: waves, sound and light (NSW Stage 5 focus area)
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The idea
Light bends toward the normal when it slows on entering glass and away from the normal when it leaves, so a ray that passes through a parallel-sided slab comes out parallel to the ray that went in but shifted sideways.
What you need
- ray box with single-slit mask and 12 V supply, 1
- rectangular glass or acrylic slab about 110 x 65 x 18 mm, 1
- A3 white paper, protractor, ruler and sharp pencil, 1 set
- darkened room
How to do it
- Place the slab on the paper and draw round it. Mark a normal at a point on the long face.
- Aim a single ray at the marked point at 20 degrees to the normal. Mark the incident ray and the point where the ray exits the far face, and mark the emerging ray.
- Remove the slab, join the entry and exit points to show the ray's path inside, and measure the angle of refraction inside the glass and the angle of the emerging ray.
- Repeat for 30, 40, 50 and 60 degrees, then aim one ray along the normal (0 degrees).
- Tabulate the angle of incidence against the angle of refraction and describe the pattern: which angle is larger, and how does the bending change as the angle of incidence grows?
- Check that the emerging ray is parallel to the incident ray and measure the sideways shift for the 60 degree ray.
What you should see
The ray bends toward the normal on entry and away from it on exit, and a ray along the normal passes straight through. For glass the angle inside the slab is 13.2 degrees for 20 degrees of incidence, 25.4 degrees for 40 degrees and 35.3 degrees for 60 degrees: always smaller than the angle of incidence, and the bending grows as the angle of incidence grows. The emerging ray is parallel to the incident ray. The ray enters the long face and crosses the 65 mm width of the slab, so the sideways shift at 60 degrees is about 33 mm.
What changes
- What you change
- angle of incidence (degrees)
- What you measure
- angle of refraction inside the slab (degrees)
- What you keep the same
- same slab and face
- ray aimed at the marked point
- same normal
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Light bends because glass is thicker.
- The ray bends the same way entering and leaving.
- Light speeds up in glass.
Safety card
Hazards
- hot ray box
- chipped glass edges
- dark room
Controls
- inspect slabs for chips
- handle the ray box by its case
- torches for moving about
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-05SC5-WS-06
- Australian Curriculum v9AC9S9U04AC9S9I03AC9S9I04AC9S9I05
Sources
The pages the author read to write this activity.