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Physics library
Course: Physics library > Unit 15
Lesson 1: Reflection and refractionDispersion
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.
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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)
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)
Why doesn't dispersion take place in a glass slab?(7 votes)
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)
why is the dispersion dependent on wavelength?(4 votes)
Because different frequencies interact in different ways with the electromagnetic field of the atoms that make up the medium.(6 votes)
cloud is made up of dust particles and water.why not the earth 's gravity pull the water droplets on the cloud?(3 votes)
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)
- Can anyone tell me the exact definition of Refractive Index please?(1 vote)
n = (Speed of light in vacuum) / (speed of light in material)(5 votes)
What is the application of tir total internal reflection(2 votes)- TIR is useful in optical fibers and also prisms which are used in binoculars (
I think they are in porro prism binoculars and roof prism binoculars .For more information look it up in wikipedia)(2 votes)
- 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) 
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)
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)
Why do we get a different colour if two are more colours are mixed?(1 vote)- Because that's how your eyes happen to work. Color is a subjective interpretation of the signals produced by your eyes photo receptors in response to light that falls on them.(2 votes)
- 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)
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.