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## Class 10 Physics (India)

### Course: Class 10 Physics (India)>Unit 2

Lesson 5: Dispersion of light

# Dispersion

The index of refraction in a material isn't always the same for every wavelength. This is how prisms split white light into so many colors. Created by David SantoPietro.

## Want to join the conversation?

• Why does this occur if the index of refraction is a property of the speed of the light and not the wavelength? Do different wavelengths have different speeds?
• When you are seeing white light, it is made up of lots of independent frequencies and wavelengths. The product of each frequency and its corresponding wavelength is the same as about 3 * 10^8 meters per second. But when this light enters an optically denser medium, the wavelengths are reduced. Hence, smaller wavelengths now travel at lesser speeds than bigger wavelengths. So index of refraction for blue (smaller wavelength) is higher than red (bigger wavelength).
I hope this clears your doubt. If it doesn't, don't hesitate to reply!
• Why doesn't dispersion take place in a glass slab?
• When light passes through glass, it encounters TWO interfaces--one entering and the other leaving. It slows down at the first interface and speeds back up at the second. If the two interface surfaces are parallel to each other, as in a 'slab' of glass, all of the bending (and dispersion) that takes place at the first interfaces is exactly reversed at the second, 'undoing' the effect of the first interface; so although the emerging ray of light is displaced slightly from the entering ray, it travels in the same direction as the incoming ray and all wavelengths that separated at the first interface are re-combined.

If the second interface is NOT parallel to the first, as in a prism, the effects of the first interface are NOT reversed and the colors separated at that interface continue along different paths upon leaving the glass.
• why is the dispersion dependent on wavelength?
• Because different frequencies interact in different ways with the electromagnetic field of the atoms that make up the medium.
• cloud is made up of dust particles and water.why not the earth 's gravity pull the water droplets on the cloud?
• clouds are made up of water vapour which are much lighter than air (as for the dust particles, you can see them already floating in air in a beam of light)... therefore clouds float.
and on top of that, they occupy a large amount of volume (consider taking some cotton with more volume and drop it, it falls slowly..now squeeze the same mass of cotton (occupies less volume) and drop it, it falls faster) hence they float...i guess this is the answer...^.^
(1 vote)
• Is a rainbow created when it passes through a plane of water?
(1 vote)
• yes,you are absolutely right rainbows are created when they pass through a water droplet.

when light passes from the droplet it refracts and disperses into seven colors after wards it reflects internally inside the droplet and the seven colors exit in the same direction through which the white light has entered.
(1 vote)
• At , the video says that the index of refraction is uselly a function of the wave length of light. What is the function?
• it means that it depends on the wavelength.

or; it is related to the wavelength
• when a light of specific wavelength enter a denser medium,does the wavelength itself get changed with the change in its speed?
(1 vote)
• the frequency of the light remains the same but the wavelength may change (if speed changes)
• What are the reasons for dispersion?
(1 vote)
• Refractive index is different for different light colours and different the refractive index different is the bending.