In the narrow part of the instrument there is a diffraction grating. Direct it toward the Sun and observe the image on the focusing screen in the wide part.
How does it work
The diffraction grating is a densely ruled plane, where there can be 1000 parallel lines per millimetre. In this experiment, the sunlight exhibits a wave nature – going through the slits, it bends and propagates as semicircles. This phenomenon is called diffraction (bending) and takes place at every slit of the grating. The waves that propagate behind the slits overlap and amplify at specific points (different for light of different length, therefore of different colour), which we call wave interference. As a result, we observe the colours conforming to opposite order than in prism.
Diffraction gratings are used to build spectrometers – instruments allowing to decompose light into its basic colours. Such analysis of light emitted by various substances (spectral analysis) allows to determine which chemical elements the light source is composed of. Due to spectral analysis, the scientists can determine the chemical composition of even very distant stars.
Similarly, rainbow pattern can be observed on a compact disc when light is reflected from its surface. A CD is a reflective diffraction grating.