Two prisms (I) and (II) of same material are used with same refracting angle (A). They are arranged as shown in the above figure. Sunlight from a narrow slit 'S' falls on the first prism and gets dispersed into constituent colours (VIBGYOR). Now this dispersed light falls on the second prism so that it deviates the light upwards. It is found that the light coming out from the second prism is almost white and is in direction parallel to the direction of light incident on the first prism. Thus prism (I) and prism (II) combined together effectively act like a parallel sided glass slab. This shows that prism (I) disperses the white light into its constituent colours so it is called dispersing prism and prism (II) recombines these colours to form white light so it is called recombination prism.