An equilateral triangle is not the optimum cross-section for a glass prism intended for making good "rainbows" or spectrums. While an equilateral triangle can produce some dispersion of light, it is not the ideal shape for maximizing the separation of colors in a spectrum.
The most commonly used shape for a prism to create a rainbow spectrum is a "right-angle prism" or a "dispersion prism." This type of prism is typically made from a right-angled triangular piece of glass. When light enters the prism at a specific angle, it undergoes multiple internal reflections and refractions, which cause it to disperse into its component colors.
The main reason for using a right-angle prism instead of an equilateral triangle is that the angle of deviation (the angle by which different colors are spread out) is greater for a right-angle prism, resulting in better spectral separation. Equilateral triangles have a smaller angle of deviation, which leads to less dispersion and a less pronounced rainbow effect.
For even better dispersion and larger rainbows, prisms with higher angles of deviation can be used, such as the "60-30-90 degree prism" or "30-60-90 degree prism." These prisms are also made from right-angled triangles, but with angles of 60 degrees and 90 degrees, or 30 degrees and 60 degrees, respectively.
In summary, while an equilateral triangle can produce some dispersion and create a rainbow spectrum, a right-angle prism or other prisms with higher angles of deviation are more commonly used for this purpose, as they provide better spectral separation and more vivid rainbows.