Science 7–10 · Year 9
Total internal reflection: finding the critical angle with a semicircular slab
Physical sciences — Waves and motion: waves, sound and light (NSW Stage 5 focus area)
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The idea
Light leaving glass for air bends away from the normal, and beyond a certain angle, the critical angle, it cannot leave at all and is entirely reflected inside, which is how optical fibres keep light inside them.
What you need
- ray box with single-slit mask and 12 V supply, 1
- semicircular glass or acrylic slab, 1
- A3 white paper, protractor and pencil, 1 set
- optical fibre demonstration length or a clear acrylic rod, 1 (extension)
- darkened room
How to do it
- Draw round the slab and mark the centre of its flat face. Draw the normal at that point.
- Aim a ray through the curved face toward the centre point at 10 degrees to the normal; observe the weak reflected ray inside and the refracted ray leaving the flat face. Mark both.
- Increase the angle in 5 degree steps up to 35 degrees, then in 1 degree steps. Record the angle at which the refracted ray runs along the flat face and disappears; this is the critical angle. Repeat three times.
- Continue to 60 degrees and record that only the reflected ray remains, and that its angle equals the incidence angle.
- Extension: shine the ray into the end of the acrylic rod and bend the rod; observe the light following the curve.
- Use the results to explain how an optical fibre keeps light inside it over long distances, which is how this science is used in communications technology.
What you should see
The refracted ray grazes the flat face at about 42 degrees (41.8 degrees for glass, 42.2 degrees for acrylic), so the 1 degree steps catch it within a degree. Below this angle a faint internal reflection accompanies the bright refracted ray; above it the refracted ray vanishes and the internal reflection becomes bright, at an angle equal to the incidence. Light follows the bent rod out of its far end.
What changes
- What you change
- angle of incidence at the flat face (degrees)
- What you measure
- whether a refracted ray emerges and its angle (degrees)
- What you keep the same
- ray aimed at the centre of the flat face
- same slab
- same normal
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Total internal reflection can happen when light enters glass from air.
- Light always partly escapes at any angle.
- Optical fibres carry light because they are hollow.
Safety card
Hazards
- hot ray box
- dark room
Controls
- 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-06
- Australian Curriculum v9AC9S9U04AC9S9H02AC9S9I03AC9S9I05
Sources
The pages the author read to write this activity.
- curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
- curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/content/stage-5/faafd3c3df
- spark.iop.org/episode-318-total-internal-reflection
- instructional-resources.physics.uiowa.edu/6a4410-blackboard-optics-total-internal-reflection