Newton's research in the field of refractive index showed that a prism can split white light into a spectrum of colors. Light rays are refracted by the "thick" side of a prism and thus separated according to the wavelength. In the conventional optical glasses the blue light rays are refracted more strongly the long-wavelength red ones, though with the diffraction grating the red light waves are deflected more. The physical reason for this is dependent on the wavelength of the light refractive index.
In this experiment, using a prism a narrow beam of white light is pulled apart to form a colored band of lights - the spectrum.