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Measuring the focal length of a lens for red and green light- Case Study

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Measuring the focal length of a lens for red and green light- Case Study

The case study that I will be studying is how the focal length of a lens for red and green light differs.

Eye defects is not something that is new it has been occurring for many years. Corrective lenses were said to be used by Abbas Ibn Firnas in the 9th century. Around 1284 in Italy, Salvino D'Armate is credited with inventing the first wearable eye glasses.

Focal length and optics both are related to the eyes. The eyes are used to see things without the eyes we would not know where we are going or what anything looks like.  


The diagram to the left shows a diagram of the eye.


The image above is of the human eye. The way the human eye works is that light enters the eye passing thro the iris, pupil and cornea. Once it reaches the lens of the eye the lens refracts the light further and it is made to converge at a single point at the back of the eye where the retina is.

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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. image06.png


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.

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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:









Salters Horners Advanced Physics AS: Student Book (ISBN: 978-1405896023)


Optical Express (Hounslow)

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