Science 7–10 · Year 9
White light: dispersion by a prism and absorption by colour filters
Physical sciences — Waves and motion: waves, sound and light (NSW Stage 5 focus area)
The idea
White light is a mix of colours that a prism spreads out by refracting each wavelength a different amount, and a colour filter transmits its own colour while absorbing the rest.
What you need
- ray box with single-slit mask and 12 V supply, 1
- equilateral glass prism (60 degree), 1
- white card screen, 1
- colour filters: red, green, blue, cyan, magenta, yellow, 1 set
- coloured card squares (red, green, blue, white, black), 1 set
- darkened room
How to do it
- Aim a narrow beam into one face of the prism at about 50 degrees to the normal and catch the light leaving the opposite face on the screen 30 cm away. Rotate the prism slowly one way and watch the spectrum: it moves along the screen, stops and then moves back. Set the prism where the spectrum stops (for glass this is close to 50 degrees of incidence) and record the order of colours.
- Measure the width of the spectrum on the screen at 30 cm and at 60 cm.
- Before each filter and card test, predict what will be seen and write down the reason.
- Place the red filter in the white beam before the prism and record which part of the spectrum remains; repeat with green and blue.
- Place the red filter and then the blue filter together in the beam and record what passes.
- Shine red-filtered light onto each coloured card in the dark room and record the colour each card appears; repeat with green-filtered light.
- Explain each observation in terms of colours transmitted, absorbed and reflected.
What you should see
The prism spreads the beam into a band with red deviated least and violet most, in the order red, orange, yellow, green, blue, violet; the band is about twice as wide at 60 cm as at 30 cm. A red filter passes only the red end of the spectrum, green the middle, blue the violet end; red and blue filters together pass almost nothing. Under red light the red card looks red, the white card looks red, and the green and blue cards look dark; under green light the green and white cards look green and the red card looks dark. The learner knows it worked when the spectrum order is repeatable and the two crossed filters give darkness.
What changes
This activity lists no variables to change, measure and keep the same.
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- The prism adds colour to white light.
- A filter changes the colour of light passing through it rather than removing colours.
- A red object under green light looks red.
Safety card
Hazards
- hot ray box
- prism edges
- dark room
Controls
- handle the ray box by its case
- prism returned to its box
- 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-02SC5-WS-08
- Australian Curriculum v9AC9S9U04AC9S9I01AC9S9I08
Sources
The pages the author read to write this activity.