why different colours of white light travell with different speed in a glass prism,while they travell with same speed in air?

Dear Abhishek,

Let''s start with refraction. In a way, refraction simply means that light travels at a different speed in different materials. It''s fast in air, and slower in water or glass. This has the effect of changing the direction of light, or bending it, when it moves from one material to the other. Here''s one way to think about why it changes direction. Let''s say you''re driving a Segway along a road next to a thick, grassy field. And say you can drive faster on the road than on the grass. Now if one wheel drops off the road into the grass, what happens? That wheel will suddenly slow down, which will turn you toward the field (assuming you don''t steer to correct it). Once both wheels are on the grass, you''ll go straight again--but away from the road. The transition from the road to the grass made you take a turn. The same is true of light moving from air (the road) to glass (the grass).

But why do different colors go in different directions? This is because different colors of light travel at *different speeds* in the glass. So from the analogy above, slower speeds in the glass make the light bend more. We call this effect dispersion, since it disperses the colors in different directions. Blue light travels slightly slower in glass than red light, so it bends a sharper angle when it enters the glass from air. In physics, we say the glass has a higher refractive index for blue light than red light. There is a formula (Snell''s Law) that you can use to calculate angles from refractive index, or the other way around.

Let''s start with refraction. In a way, refraction simply means that light travels at a different speed in different materials. It''s fast in air, and slower in water or glass. This has the effect of changing the direction of light, or bending it, when it moves from one material to the other. Here''s one way to think about why it changes direction. Let''s say you''re driving a Segway along a road next to a thick, grassy field. And say you can drive faster on the road than on the grass. Now if one wheel drops off the road into the grass, what happens? That wheel will suddenly slow down, which will turn you toward the field (assuming you don''t steer to correct it). Once both wheels are on the grass, you''ll go straight again--but away from the road. The transition from the road to the grass made you take a turn. The same is true of light moving from air (the road) to glass (the grass).

But why do different colors go in different directions? This is because different colors of light travel at *different speeds* in the glass. So from the analogy above, slower speeds in the glass make the light bend more. We call this effect dispersion, since it disperses the colors in different directions. Blue light travels slightly slower in glass than red light, so it bends a sharper angle when it enters the glass from air. In physics, we say the glass has a higher refractive index for blue light than red light. There is a formula (Snell''s Law) that you can use to calculate angles from refractive index, or the other way around.

Since in diffeernt medium frequecy remains constant but differnt colors have different wavelengths so velocity changes.