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Find the hidden rainbows

Created: 2024-06-08
Niall Deacon (OAE - IAU Office of Astronomy for Education)

This activity is about discovering the spectra around us and understanding the physical processes that make light. Using diffraction grating glasses, students will be able to view the spectra of everyday light sources. This will introduce line and continuum emission sources and can spark discussions on the nature of white light and how different light sources work.

  • Diffraction grating glasses (see image 1). The images shown in this activity use glasses with 500 lines/mm. If you do not have these a diffraction grating with approximately 500 lines/mm will do. If you do not have the glasses or a diffraction grating then you might want to try the Hunting Spectra or Light in a Matchbox activities
  • Light sources of different types. At least one of these should be continuum emission (halogen, incandescent or white light LED) and at least one should be line emission (fluorescent, compact fluorescent)
  • Power for the light sources
  • Optional, coloured optical filters
  • Optional, some sort of black background to make spectra easier to see

Image 1: the diffraction grating glasses


This activity will introduce students to the many different spectra around them. It will allow them to learn that different light sources emit light from different physical processes and thus have different spectra.

Learning Objectives

Students will learn that:

  • common “white” light sources are made up of light of many colours;
  • different artificial light sources have different spectra;
  • basic concepts of line and continuum emission.

Light which appears to the human eye as white is actually made up of lots of different colours. Every lightbulb, every star that you see is a hidden rainbow waiting to be discovered. However not every white light source is made up of all of the colours of the rainbow. Some light sources, such as the Sun, shine in all of the colours of the visible spectrum. These are called continuum emitters (see Image 2, first line). Others, like fluorescent bulbs, only emit in narrow specific bands of colour. These are called line emitters (see Image 2, second line).


Image 2: Spectra of the Sun and a fluorescent light bulb. Credit: NASA, ESA, Leah Hustak (STScI) License: Public Domain

Whether a light source is a line emitter or a continuum emitter depends on the physics that lead to it emitting light:

Incandescent and halogen-incandescent bulbs shine due to an electrical current heating a tungsten filament causing it to emit light. Generally warm, dense objects such as filaments emit in continuum emission. The Sun is also warm and dense so it emits light as a continuum.

Fluorescent light sources typically contain low pressure mercury vapour. An electrical current flows through this vapour. The electrons in this current will sometimes collide with a mercury atom. If the electron has the correct energy then it can kick an electron in the mercury atom up to a higher energy level. Eventually the electron in the higher energy level will drop down to a lower energy level. In doing so they emit a photon (a parcel of light), with that photon’s energy being equal to the difference between the energies of the higher and lower energy levels.

LEDs emit in one relatively small colour range. White LEDs can be created either by combining red, green and blue LEDs, or taking one colour of LED (normally a blue LED) and surrounding it with a phosphor material. When the blue LED shines on the phosphor, the phosphor absorbs that light and re-emits it over a wider spectrum, usually a continuous spectrum covering green-red colours.


Image 3: The spectrum of a white LED made of a blue LED and phosphor material Credit: Wikicommons/Deglr6328 Licence: CC-BY-SA-3.0

Full Description

Set up a number of different light sources in the classroom or laboratory. Preferably the main lights in the room should be dimmed to make it easier to see the spectra.

Ask the students to look at the light sources without the diffraction grating glasses and describe the colour of the light. Then give them the glasses and ask them to look at a continuum source such as the halogen incandescent light bulb through the diffraction grating glasses. Ask them to describe the colours of light they see from this source: they should describe that the white light is turned into a rainbow.

Next, ask them to look at the other white light sources and describe what they see. For continuum sources like a fluorescent bulb, they will see individual copies of the light source in different colours above and below the light source. Each of these copies will be at one of the colours the line emitter emits light in. Ask them what they think that means for these white light sources.

Ask the students if they see a difference between the different types of light source. You may need to guide them by asking which look like a rainbow and which look like copies of the light sources.


Image 4: spectra of different lights: a - an image of a halogen incandescent bulb shown through the diffraction grating glasses; b- a fluorescent tube light shown through the diffraction grating glasses; c- a compact fluorescent tube light shown through the diffraction grating glasses; d- a white LED light shown through the diffraction grating glasses. This light consists of a blue LED and a phosphor material. Credit: Niall Deacon/IAU OAE.

If you have some coloured filters to hand, you could ask the students to look through both the diffraction grating glasses and a coloured filter (the filter may only cover one of their eyes). Ask the students to describe how the spectra they see change with and without the filters.

You can also demonstrate to the students how a diffraction grating can turn a smartphone camera into a spectroscope. Do this by covering the phone’s camera lens with the grating of the glasses. While you should never look directly at the Sun through the diffraction grating glasses, you can point a smartphone camera with the lens covered with the grating at the Sun. This will allow students to see the spectrum of the Sun safely. It may also be possible to split the light of bright astronomical objects such as the Moon or Jupiter into spectra using the diffraction grating glasses.


Image 5: The Sun photographed through the diffraction grating glasses. Credit: Niall Deacon/IAU OAE

  • Ask students to identify if a mystery light source has line or continuum emission
  • Ask the students what they think a colour screen on a computer, TV or smartphone would look like through the glasses. You could discuss the difference between the way white light is made on an RGB LED screen vs a “white” LED
  • Ask students to draw the spectra of the different light sources with wavelength/colour vs brightness