Science 7–10 · Year 9

Beyond the visible: Herschel's infrared experiment and a remote seen on a phone camera

Physical sciences — Waves and motion: waves, sound and light (NSW Stage 5 focus area)

Practical, model not builtLow risk

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

Sunlight carries energy in radiation beyond the red end of the visible spectrum, so a blackened thermometer just past the red edge still warms, and a phone camera shows the invisible infrared flash of a remote control.

What you need

glass prism, 60 degree equilateral, crown glass (the Cool Cosmos lesson advises that plastic prisms do not work well for this experiment), 1
cardboard box with an open top, such as a photocopy-paper box, with a notch cut to hold the prism, 1
spirit-filled (alcohol) thermometers or digital thermometers with thin probes, bulbs or tips blackened with permanent marker, 3 (never mercury-in-glass)
sheet of plain white A4 paper, 1
stopwatch, 1
infrared television remote control, 1
phone with front and rear cameras, 1
a clear sunny day and an outdoor bench in the sun

How to do it

Place the three blackened thermometers side by side in shade for 5 minutes and record each reading; this checks they agree and gives the shade temperature. Note any offset between them and correct later readings by it.
Put the white paper in the bottom of the box. Outdoors, set the box with the prism notch facing the Sun and turn the prism slowly one way: the spectrum moves across the box, stops and moves back. Leave the prism where the spectrum stops, the minimum-deviation setting the model below uses. If the band does not fall on a shaded part of the paper, or is too narrow, move the paper or prop it at a slant on a folded card so the band lands there and spreads out, rather than turning the prism away from that setting. Never look at the Sun or at its image through the prism.
Lay one thermometer bulb in the blue, one in the yellow and one in the red part of the spectrum. Read each thermometer every minute for 5 minutes without shading the spectrum, and record the final temperature rise above the shade reading.
Move the thermometer from the blue into the dark region just past the red edge. The infrared region is narrow, so first slant the paper until the violet-to-red band measures at least 30 mm along the paper, about three and a half times its width on paper square to the beam 30 cm from the prism; on a band that long the 700 to 1000 nm region is about 9 mm wide and the model's maximum near 835 nm sits about 5 mm past the red edge. Set the bulb with its near edge touching the last visible red, so its centre sits about 3 mm past the edge, near 780 nm on the model, and repeat the 5-minute run. Keep a fourth reading in the shade beside the box as a control if a spare thermometer is available.
Repeat the whole set three times, swapping which thermometer sits in which region each time so that a faulty thermometer cannot explain the pattern. Average the temperature rise for each region and draw a column graph of rise against region.
Indoors, point the remote control at your eye and press a button: nothing is seen. Then point it at the phone camera with the screen showing, press the buttons and watch the screen; try both the front and the rear camera and record which one shows the flashes.

What you should see

The temperature rise increases from blue to yellow to red, and the thermometer just beyond the red edge, where no light is seen, still reads above the shade control; the Cool Cosmos lesson reports that the highest temperature should be just beyond the red. The computed model below explains why: through a 60 degree N-BK7 prism the energy arriving per degree of spectrum, relative to blue at 470 nm, is 1.80 at 580 nm, 2.22 at 650 nm and 2.68 at 800 nm, peaking near 835 nm, beyond the visible, because the prism squeezes 700 to 1000 nm into only 0.48 degrees while the whole visible band spreads over 1.58 degrees. On a screen square to the beam the visible band is only about 8 mm wide at 30 cm and the whole 700 to 1000 nm region only about 2.5 mm, and each colour is smeared by the Sun's 0.53 degree width, which is why the band has to be stretched before a bulb is placed in it: a 6 mm bulb set a full width past the red edge on a screen square to the beam at 30 cm sits 1.15 degrees out, near 1600 nm on the same model, past the 835 nm maximum and outside the region, while on a band stretched to 30 mm the same 6 mm is only 0.32 degrees, near 870 nm. Turning the prism away from the minimum-deviation setting widens the band (the 400 to 700 nm span grows from 1.58 degrees to 2.00 degrees at 40 degrees of incidence and 3.95 degrees at 32 degrees) but changes the figures above, so the prism is left at that setting. The remote's flashes, invisible to the eye, appear on at least one of the phone's cameras as a flickering light; NASA gives about 940 nm for a typical television remote (frequency 3.19 x 10^14 Hz, below red light at 700 nm, 4.28 x 10^14 Hz), radiation that the prism would send 0.41 degrees beyond the red edge. The learner knows it worked when the beyond-red reading is above the shade control in every repeat, whichever thermometer is placed there.

What changes

What you change: position of the thermometer bulb in the spectrum (blue, yellow, red, just beyond red)
What you measure: temperature rise above the shade reading after 5 minutes (degrees C)
What you keep the same: same thermometers, corrected by their shade offsets
same amount of blackening on each bulb
same 5-minute exposure
same prism, box angle and time of day
runs abandoned if cloud crosses the Sun

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Light is only what the eye can see, so there is nothing beyond the red end of a rainbow.
The dark region past the red receives no energy because it looks dark.
A remote control works by sound or by a signal unrelated to light.
Infrared is the same thing as heat rather than one kind of radiation that warms what absorbs it.

Safety card

Low riskLearners carry it out

Hazards

eye damage from looking at the Sun or at its bright image through the prism
broken glass from a dropped prism or thermometer
sunburn during outdoor work

Controls

learners told before going outside never to look at the Sun or into the prism
spirit-filled or digital thermometers only, no mercury-in-glass
prism held in the box notch, not in the hand, and returned to its case
hats and sunscreen, runs kept short

Note

No chemicals and no heat source beyond sunlight: the hazards are sunlight on the eyes and broken glass. Complete the class risk assessment before the lesson, using CSIS 1.7 (Risk assessment – a pre-requisite for risk control), 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-01 SC5-WS-04 SC5-WS-06
Australian Curriculum v9AC9S9U04AC9S9H02AC9S9I02AC9S9I05

Sources

The pages the author read to write this activity.

All Concept Studio activities