Split light into rainbows

This activity is part of our Science Lab series. Check out the ANU Science Lab for more experiments! In this activity, you will construct a DIY spectroscope to split white light into the full spectrum of rainbow colours, unlocking the secrets of light and discovering how astronomical optical telescopes analyse distant stars. Watch the video above to see how it's done.

DIY spectroscope materials & ingredients

For this experiment:

• Spectroscope printable template (or a small cardboard box / cardboard tube): 

  Document

  ASTRO-3D-spectroscope_V2-2023.pdf (PDF, 320.15 KB)
• Small piece of diffraction grating (or a piece of a clean, unused CD/DVD)
• Scissors or craft knife
• Black tape or electrical tape (to prevent light leaks)
• Glue or double-sided tape
• Ruler

Safety Note: When testing your finished spectroscope, never look directly at the sun. Always point your spectroscope at reflected sunlight on a wall or ground, or test it on indoor light sources.

How to make a DIY spectroscope: Step-by-step instructions

Step 1: Prepare the template and cut the openings

1. Print out the spectroscope template onto thick paper or card, or trace the template pattern onto a cardboard box.
2. Carefully cut along the solid outer lines using scissors.
3. Use a craft knife to cut out the narrow slit opening at one end (where light enters) and the viewing window on the opposite end.

Step 2: Attach the diffraction grating

1. Place your piece of diffraction grating over the viewing window on the inside of the template.
2. Check the orientation by looking through the grating at a light source to make sure the rainbow spreads out horizontally across your view.
3. Secure the edges of the diffraction grating firmly in place using tape, taking care not to cover or smudge the clear viewing area in the centre.

Step 3: Assemble and seal the spectroscope

1. Fold along all the dotted crease lines to form the 3D body of the spectroscope box.
2. Apply glue or tape along the assembly flaps to hold the structure together tightly.
3. Wrap black electrical tape over all outer edges, seams, and corners to completely block out external light leakage, leaving only the front slit and viewing window open.

Step 4: Observe different light sources

1. Look through the viewing window while pointing the front slit toward a light source, such as a household LED bulb, a neon lamp, or gas discharge tubes (like helium).
2. Observe the distinct spectrum of lines and colours produced by each unique light source.

How does a spectroscope work? The science explained

A spectroscope works by separating light into its component wavelengths using a process called diffraction.

White light from the sun or a light bulb is made up of all the visible colours of the light spectrum combined. When light enters the narrow slit of your spectroscope:

• The light strikes the diffraction grating, which contains thousands of microscopic parallel lines etched into its surface.
• As light waves pass through these tiny slits, they bend (diffract) and interfere with one another at different angles depending on their wavelength.
• Shorter wavelengths (like blue and violet light) bend at steeper angles, while longer wavelengths (like red light) bend at wider angles, spreading the white light out into a distinct rainbow spectrum.

Astronomers use powerful spectroscopes attached to optical telescopes to analyse light from distant stars and galaxies (spectroscopy). Because every chemical element emits and absorbs unique wavelengths of light, reading these spectral "barcodes" allows scientists to determine what far-away celestial objects are made of, how hot they are, and how fast they are moving through space!

Science fair projects & taking it further: Experimenting with variables

Once you have built your spectroscope, try testing different light sources and setups to see how spectral patterns change:

• Compare light sources: Point your spectroscope at an incandescent bulb, a fluorescent tube, an LED light, and a television screen. Which sources produce a continuous rainbow, and which ones display distinct bright lines (emission spectra)?
• Slit width: What happens if you make the front slit narrower or wider? A narrower slit produces sharper, clearer spectral lines, while a wider slit lets in more light but causes colours to overlap and blur.
• Reflected sunlight: Point your spectroscope toward a white wall illuminated by sunlight outdoors to safely view the solar absorption lines (Fraunhofer lines) created by gases in the sun's atmosphere.

Enjoyed this experiment? Explore more hands-on activities on the Science Lab ANU YouTube channel.