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15090 - Experiment Pack - Diffusion of Light - info
STE 15090 Experiment Pack - Diffusion of Light
Instructions and Teaching Guide
Introduction
With this pack it is possible to perform experiments on the diffusion and polarisation of light as it passes through a vessel containing a solution of diffusing particles. The particle size increases during the course of the experiment and the effect on the transmitted and diffused light can be stiudied. The notes below provide a useful guide for teachers and students alike.
Equipment
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Qty
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Description
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01
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Transparent liquid container
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01
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Dropper with teat
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01
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Sodium sulphite anhydrous 250 g
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01
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Polariser filter
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01
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Aluminium blocks 28 x 10 x 66
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01
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Sulphuric acid 5N 1000 ml
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01
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Mixing rod
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Necessary equipment not included: Dioptric projector, Transformer for projector, Tripod for projector and translucent screen
Operating Instructions
(For best results operate in a low light or dark environment.)
- Pour water into the liquid container up to a few centimetres from the top.
- Place the projector and screen along the long side of the container.
- Switch on the projector and adjust screen position until the light is focused on it.
- Make sure the light from the projector passes through the water in the container. At the other end, on the screen, there will be a clear circular image.
- Mix 5g of salt per litre of water and mix it with the glass rod.
- Pour a few drops of sulphuric acid (about 5 per litre of solution) and wait 2-3 minutes. N.B. The exact quantity of salt and acid is determined by several trials.
Observations
Microscopic sulphur particles will begin to precipitate slowly. When the particles start to former, a diffused blue light will be seen throught the light of the recipient.
Observing the diffused light throught the polariser filter, it will be clear that the light is polarised and then, when a certain number of particles is formed, the polarization is partial.
At the same time, the clear circular image on the screen, changes its colour from white, to yellow, to orange and finally to red.
In the final phase of the experiment, the number of sulphur particles is so high that the light diffused to the side of the container seems to be white while on the screen the light disappears when the it is totally diffused to the side (Fog).
Principles of operation
In the phenomenon of light diffusion an essential role is given by the number per unit of volume and dimensions of the particles.
In the proposed experiment, sodium hyposulphite in the solution and the sulphuric acid interact producing sulphur.
The decrease of the light intensity in the direction of the propagation of the incident beam, affecting the light diffused laterally, can be measured by the extinction factor k = 2.Pi.R/Lambda , in which R is the radius of the diffusing particles and the wavelength is Lambda .
It is obvious the the shorter wavelengths are preferentially extinguished from the incident beam and therefore they are diffused laterally first. For this reason the diffusion of the blue light is firstly observed .
As the chemical reaction proceeds, there will form agglomerates of sulphur particles of increasing dimensions. In the direction of the incident beam, the light, changed by the chromatic components, will appear yellow, orange and finally red.
At the end of the reaction, the dimensions of the agglomerates of sulphur have assumed such dimensions that the wavelengths are equally diffused and for that reason the solution appears whitish.
In the presence of a relatively small number of diffusing particles, the laterally diffused light is totally polarised. It means we are in the Rayleigh condition, because the diffusing particles are smaller than a wavelength ( as observed at the beginning of the experiment).
As the chemical reaction proceeds, the production of a bigger number of agglomerates of sulphur creates the multiple diffusion, and then the diffuse light is only partially polarised (the bigger are the particles the smaller is the degree of polarisation).
In addition to the previous considerations, it is worth noting the fact that the intensity of the radiation diffused by the diffusing particle is inversely proportional to the quarterpower of the wavelenght of the incident radiation (Rayleigh's Law).
It means that the blue component (circa 400nm) of the solar radiation is diffused 9.37 times more than the red component (circa 700nm).
This experiment can be used to explain the blue sky and the red of the sun to the down and to the sunset.
Support and feedback
STE UK Ltd is keen to ensure that you obtain the best performance from your equipment.
If you need advice on this product or any matter relating to practical science, or if you have observations which may be helpful to others we will be very pleased to hear from you by telephone or e-mail.
At STE we believe in providing a first class product with the first class service to back it up. We look forward to your custom and exceeding your expectations.