Physics 11–12 · Year 11
Dispersion of white light with a prism, and Herschel's infrared thermometer (syllabus practical)
Module 3: Waves and Thermodynamics (Ray Model of Light)
The idea
The refractive index of glass depends on wavelength, so a prism deviates violet more than red and spreads white light into a spectrum.
What you need
- Equilateral glass prism (60 degrees), ray box with a narrow slit and white lamp, white screen
- Darkened room, protractor, coloured filters (red and blue)
- For Herschel's experiment (NSW Department of Education Module 3 guide): direct sunlight through a window, a prism, three blackened-bulb thermometers on white card
How to do it
- Aim a narrow white ray at one face of the prism near minimum deviation and catch the emerging light on the screen 1 m away; sketch the order of colours.
- Place the red filter over the slit and mark the red spot; repeat with blue and measure the separation of the two spots on the screen.
- Rotate the prism and note that the spectrum moves as a whole while its order stays the same.
- Herschel: form a solar spectrum on the card, place one thermometer in the blue, one in the red and one just beyond the red; read all three every minute for five minutes.
What you should see
The spectrum runs red (least deviated) to violet (most deviated); for a 60 degree prism of BK7 crown glass at minimum deviation, 656 nm red light (n = 1.5143) is deviated 38.43 degrees and 486 nm blue light (n = 1.5224) 39.14 degrees, so the two bands land about 12 mm apart on a screen 1 m away (indices from the SCHOTT Sellmeier formula). In Herschel's arrangement the thermometer just beyond the red end warms although no visible light reaches it (Herschel found it warmed most), showing an invisible band of radiation beyond red.
What changes
- What you change
- wavelength (colour) of the light
- What you measure
- angle of deviation
- What you keep the same
- prism angle
- angle of incidence
- screen distance
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- The prism adds colour to white light; white light already contains every colour and the prism separates them.
- Red bends most because it is strongest; violet bends most because glass has the higher index at shorter wavelength.
Safety card
Hazards
- looking at the Sun
- hot lamp
- glass prism edges
Controls
- never look towards the Sun through the prism
- handle the prism by its ends
Note
Record the activity in RiskAssess (https://www.riskassess.com.au/) and follow the Science ASSIST risk management information sheet (https://asta.edu.au/resource/ais-risk-management-and-risk-assessment/).
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
Sources
The pages the author read to write this activity.