Dear student,
The visible  light consists of a collection of  component colors. These colors are often observed as light  passes through a triangular prism. Upon passage through the  prism, the white light is separated into its component  colors - red, orange, yellow, green, blue and violet. The  separation of visible light into its different colors is  known as dispersion. It  was mentioned in the Light and Color unit that each color is  characteristic of a distinct wave frequency; and different  frequencies of light waves will bend varying amounts upon  passage through a prism. In this unit, we will investigate  the dispersion of light in more detail, pondering the  reasons why different frequencies of light bend or refract  different amounts when passing through the prism
						

							
Because of the difference in refractive indices, light of different colors bend through different angles on refraction. If white light passes through a glass prism, the violet rays deviate the most and the red rays deviate the least. thus, white light is separated into its various component colors. This phenomenon of separation of different constituent colors of light while passing through a transparent medium is known as dispersion of light.
When the beam of white light is passed through the prism, light of different wavelengths are deviated by different amounts. Cause of dispersion through glass - due to different wavelength of different colors, thus each each color of light travels with different speeds in given medium. This gives rise to the dispersion of light.
And a glass slab doesnot.