take a CD or a glass prism . take it to near a source of light like a torch or a natural object that is sun . we will observe that when light falls on CD or the glass prism , it splits into many color. this process is known as dispersal .  you will also observe band of colors . this is called spectrum 
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take a glass prism and pass a ray of white light through it . then inside the glass prism you will see that white light split up into seven color named as '' ROYGBIV''
and this shows that white light split up in seven colors hence it is dipersion of white light.
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