Eye defects:
The eye above shows an eye that has no problems with it. However you can have eyes that have certain defects. There are two main problems with eyes.
Firstly myopia (more commonly known as short sight) this is when a human can see objects that are closer to them better than objects which are further away. Myopia is caused when the cornea is too curved, or your eyeball too long this means that when light passes through the lens it converges to a point that is ahead of the retina, this is because the distance between the retina and the lens has increased.
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The second defect is known as hypermetropia (more commonly known as being long sighted) if a human suffers from hypermetropia a person cannot see objects that are closer to them as well as they can see objects that are a lot further away. Hypermetropia is caused when the eyeball is a lot smaller in length than usual, this means that when light enters the lens in the eye it converges to a point that’s beyond the retina meaning that the person will not be able to see properly.
How to correct eye defects with lenses:
There are many ways in correcting eye defects, for example eye laser surgery but the most common way of correcting them is by using lenses (glasses)
A lens is an object that refracts light in order to adapt the focal point so that the person wearing them can now see properly. There are many different types of lenses that can be used to correct eye defects. Below I will show how to correct myopia and hypermetropia.
Myopia- to fix myopia a concave lens can be used. A concave lens moves the focal length backwards by moving the lens apart to increase the distance. The diagram on the right shows how this is done:
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as you can see the concave lens has spread the light rays so that the focal point moves where the retina is rather than being in front or behind it.
Hypermetropia-
To fix hypermetropia, a convex lens (a converging) is used. A converging lens refracts the light entering it at a single point. The diagram on the right shows a convex lens can be used to correct people who are long sighted.
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The diagram above shows a lens that is placed in front of the eye. The way a converging lens works is that it moves the rays closer together bringing the focal point near the retina. This helps to fix hypermetropia.
Limitation of lenses:
As you can see using lenses has many benefits when correcting certain eye defects, these are that the strength of the lenses can be adapted to how bad the persons eyes are. However there are social drawbacks for eye defects and this is that a person may not actually find out that they have an eye defect straight away meaning that they go through a period in there life where they do not know, another social drawback may be that some people may feel uncomfortable wearing glasses to help them in helping their eye defects. There are also limitations to how much they can magnify something so if a human’s eye is too bad lenses may not be able to fully help them.
Chromatic aberration:
Chromatic aberration is a defect whereby they lens cannot converge all the different colours in the spectrum at the same point. The problem with this is that all the different colours of light have different wavelengths. These different wavelengths of the different coloured light affect the convergence.
The refractive index of a lens is a way to measure how much the light slows down when it enters the lens. The refractive index is highly dependent upon the wavelength of the different coloured lights. The lower the wavelength the higher the refractive index, the higher the wavelength the lower the refractive index.
The refraction that occurs also depends on the strength of the lens that is being used. The higher the power of the lens the stronger the refraction of the light. This means that the focal length will be smaller for a more powerful lens.
Higher refractive index between lens and light wave = light being refracted more
The stronger the refraction of the light = the shorter the focal length
This means that as different colours of light have different wavelengths the refractive indexes between the lens and the light will be different, this means that the focal point will be different for each colour.
For red and green light:
Red light has a wavelength of 650 nm and green light has a wavelength of 510 nm, this means that the red light has a higher wavelength thus meaning it has a lower refractive index meaning that its focal length should be longer than that of green light.
What I will do in my practical:
I will be testing chromatic aberration to see whether my theory is true, I will do this by doing an experiment using two different lights, I will then measure the focal length of the different coloured lights to wee whether they actually are different for the different colour lights.
Sources of information:
Websites:
Books:
Salters Horners Advanced Physics AS: Student Book (ISBN: 978-1405896023)
Shops:
Optical Express (Hounslow)