A Newton's disc can be created by painting a disc with the seven different colours: violet, indigo, blue, green, yellow, orange and red. When the disc is rotated, it appears white. This explains that white light consists of seven colours.
The velocity of light in a material medium depends upon this color (wavelength) i.e. the refractive index of a material is different for different colors. If we incident a ray of white light on a prism; on emerging, the different colors are deviated through different angles. Due to this, white light splits into its constituent colors and the phenomenon is called dispersion.
