How a Delhi school used a DIY spectroscope to study city light pollution

A school science project in Delhi turned a familiar urban problem into a practical investigation: why does the night sky look orange, white or bluish in different parts of the city? Instead of relying on photographs alone, students built simple spectroscopes and examined the colours and emission lines produced by streetlights, shop signs and illuminated buildings.

The project offers a useful model for Australian classrooms too. Students in Sydney, Melbourne, Brisbane or Canberra can investigate local night lighting with inexpensive materials, while learning how scientific evidence differs from impressions, assumptions and online claims.

From bright skies to a testable question

Delhi’s dense traffic, construction, advertising displays and extensive outdoor lighting make light pollution easy to notice. The glow above the city can hide stars and interfere with observations of the Moon and planets. Yet “the sky looks bright” is only an observation. A scientific investigation needs measurable features and a consistent method.

The students therefore asked which types of lamps contributed to the city’s glow. Their DIY spectroscope separated incoming light into a band of colours. The resulting spectrum could reveal whether a source had a relatively smooth spread of colours, as expected from some LEDs, or distinctive bright lines associated with particular gases in discharge lamps.

This distinction matters. A spectroscope does not directly measure the total brightness of the sky. It identifies the composition of light reaching the instrument. To estimate brightness, students would need a calibrated light meter or sky-quality sensor. Used carefully, the spectroscope becomes complementary evidence rather than a magic detector.

Building a low-cost instrument

The classroom instrument used a narrow slit, a dark cardboard enclosure and a diffraction grating. A small piece of an old DVD can act as a crude grating because its closely spaced tracks split light into different wavelengths. A commercial diffraction film is more reliable, but a recycled disc demonstrates the same physical principle at very low cost.

Students cut a slit at one end of a cardboard box and fixed the grating near the other end. The inside was covered or painted dark to reduce stray reflections. A phone camera recorded the spectrum through a viewing opening, allowing the class to compare samples later. The slit had to be narrow enough to produce recognisable features but wide enough to admit useful light.

Safety and consistency were important. Students never pointed the instrument directly at the Sun, and they avoided looking at powerful lamps through an unshielded opening. Each observation was labelled with its location, time, weather, lamp type and approximate distance. These basic records prevented the project from becoming a collection of attractive but incomparable photographs.

What the spectra revealed

The students sampled several urban sources: a roadway lamp, a fluorescent tube, a shop sign and the general glow above a busy neighbourhood. A fluorescent lamp typically produces strong narrow lines from mercury and phosphors, while many modern white LEDs create a broad blue feature combined with a wider yellow-green band. Sodium lamps can show a very strong yellow region.

The exact appearance depends on the lamp design, the camera sensor and the surrounding atmosphere. Dust, humidity and haze scatter shorter wavelengths differently from longer wavelengths, and Delhi’s air quality can change between evenings. That is why students compared several locations rather than treating one spectrum as representative of the entire city.

Light source Likely spectral pattern What students could infer
Low-pressure sodium lamp Very strong narrow yellow emission A distinctive lamp type with limited colour range
Fluorescent tube Several bright lines over a darker background Gas discharge and phosphor coating
White LED streetlight Blue peak with a broad visible band Blue-pump LED technology and phosphor conversion
Shop or billboard display Variable peaks and coloured bands Mixed LEDs, signs or screens
Urban sky glow Weak, blended features Scattered light from many sources and the atmosphere

The comparison helped students separate source identification from environmental impact. A lamp can have a recognisable spectrum without being the brightest contributor at a particular site. The amount of light, its direction, shielding and operating hours all affect light pollution.

Turning observations into evidence

A strong investigation changes one factor at a time where possible. Students could examine the same type of lamp from different distances, compare a main road with a darker school ground, or record spectra before and after nearby businesses switched off their signs. Repeating measurements on several nights would show whether weather and haze altered the results.

Phone cameras introduce another limitation. Automatic exposure and colour processing can reshape the image, making one spectrum appear brighter or more colourful than another. Students should lock exposure and focus if their phone allows it, photograph a reference light source, and treat the images as qualitative evidence unless the system has been calibrated.

Practices that strengthen a student light-pollution study

This approach reflects scientific temper: claims remain proportional to the evidence. Students can say that a source shows a strong yellow emission or a broad LED-like spectrum. They should not claim that one photograph proves a city-wide change in lighting without additional measurements.

What Australian classrooms can investigate

The Delhi project translates well to Australian conditions, although the local sources will differ. A class near Sydney’s Parramatta Road, Melbourne’s inner suburbs or Brisbane’s commercial districts could compare road lighting, sports fields and illuminated signs. In Canberra, students might contrast a well-lit suburban area with a darker open space, while checking local rules about access and safe night-time observation.

Australia also has a strong amateur astronomy culture and recognised dark-sky destinations, including parts of regional New South Wales, Queensland and Western Australia. The Southern Cross and the Milky Way provide familiar reminders of what urban skyglow can hide. Students do not need to travel to a remote observatory: a school oval, safely observed from the grounds, can provide a useful comparison site.

Materials are accessible through Australian hardware and electronics retailers, including cardboard, black tape and simple tools from shops such as Bunnings, while diffraction film and light sensors are available from education suppliers. A classroom can begin with recycled packaging and a DVD, then improve the instrument if the first observations justify greater precision.

The next step is concrete: choose two safe observation points, build identical cardboard spectroscopes, and record one spectrum from each location on the same evening using the same phone settings.