If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

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?

  • aqualine ultimate style avatar for user james.alan.hill
    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?
    (12 votes)
    Default Khan Academy avatar avatar for user
    • leaf red style avatar for user Aadi Bhure
      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!
      (15 votes)
  • male robot donald style avatar for user sanket
    Why doesn't dispersion take place in a glass slab?
    (7 votes)
    Default Khan Academy avatar avatar for user
    • duskpin ultimate style avatar for user Prajjawal Mishra
      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.
      (13 votes)
  • starky tree style avatar for user Hruday.R
    why is the dispersion dependent on wavelength?
    (4 votes)
    Default Khan Academy avatar avatar for user
  • purple pi purple style avatar for user subi chidhamabaram
    cloud is made up of dust particles and water.why not the earth 's gravity pull the water droplets on the cloud?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • female robot ada style avatar for user P Deepthi sree
      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...^.^
      (2 votes)
  • male robot donald style avatar for user Eshanth Mendez
    Can anyone tell me the exact definition of Refractive Index please?
    (1 vote)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user shashank.rai003
    What is the application of tir total internal reflection
    (2 votes)
    Default Khan Academy avatar avatar for user
  • male robot hal style avatar for user shreypatel0101
    Is my assumption right?
    mu = c/u mu = refractive index c= speed of light in vacuum and u = spped of light in medium.
    also u=lambda*nu?(frequency)
    so mu is inversely proportional to u or lambda i.e the wavelength !
    So since blue light have min wavelength therefore it will have the max refractive index i.e mu
    (3 votes)
    Default Khan Academy avatar avatar for user
  • starky seed style avatar for user Dishita
    If the different colors of the spectrum are parallel to each other and are passed through a convex lens, will they focus at a single point? If so why?
    (2 votes)
    Default Khan Academy avatar avatar for user
    • starky sapling style avatar for user ₦Ɇ฿ɄⱠ₳
      Hi,
      Yes, if different colours of the spectrum travel parallel to each other towards a convex lens, they will focus on one point.

      This happens because when parallel rays pass through a convex lens, all the refracted rays converge at a single point called the principal focus.

      Hope this helps!
      (1 vote)
  • orange juice squid orange style avatar for user santhosh prabahar
    Why do we get a different colour if two are more colours are mixed?
    (1 vote)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user Adviti Mishra
    If a person is standing with a blue and white striped jersey under a floodlight, what color would the stripes appear to be?
    (1 vote)
    Default Khan Academy avatar avatar for user

Video transcript

Voiceover: Check out this ray of light. When it enters a new medium, like water, its path will bend, and the larger the index of refraction of the new medium, the more the light will bend from its initial direction that it had in the air. This follows from Snell's Law, since if the index of refraction is larger, the angle of the refracted light must be smaller, and in order to have a smaller angle from the normal line, the light ray has to bend more from its initial direction. But here's the interesting thing, when you send in white light, composed of all visible wave lengths, the colors will disperse and get separated from each other. We call this separation of light, dispersion. So, why does dispersion happen? The reason for dispersion is that the index of refraction for water and most other materials are actually a function of the wavelength of the light. For instance, if you ask a physicist, or look up the index of refraction of water, most sources would say the index of refraction of water is 1.33, but what those sources or physicists really mean is that the index of refraction is pretty much 1.33 for the entire visible range of wavelengths; however, each visible wavelength has a slightly different index of refraction in water. The index of refraction of red light in water is about 1.33, but the index of refraction of blue light is closer to about 1.34. In fact, for most materials the smaller the wavelength of the light, the larger the index of refraction, which means smaller wavelength light will bend more than larger wavelength light will in most materials. This is why in water the violet light would bend the most, since it has the smallest wavelength for visible light. Blue light would bend slightly less, green light a little less, yellow light a little less than that, orange light even less, and red light would bend the least. So, remember, dispersion and the rainbow patterns that emerge result from the fact that most materials have an index of refraction that's a function of the wavelength of the light, and in most materials, the smaller the wavelength, the more the light will bend.