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# To investigate and demonstrate how the different wavelengths of red and blue light differ by finding their focal lengths using a converging lens.

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Introduction

Physics Coursework Aim To investigate and demonstrate how the different wavelengths of red and blue light differ by finding their focal lengths using a converging lens. Apparatus * Red and Blue LED's (light emitting diodes) * Wires to connect apparatus together * Power supply and mains access to control voltage supplied to the LED's * Ruler in cm and mm * Converging lens * Blocks to adjust height of components Safety This experiment is relatively safe and there are few hazards. However I will be aware throughout the experiment of the electrical components thus minimising any risk of electric shock. Although LED's them selves do not get sufficiently hot enough to burn skin the wires may get quite hot if the current passing through them is high enough. If I conduct the experiment with a high voltage not only may the LED's fuse, the brightness of them may harm my eyes if they are looked at continuously. To reduce this effect I shall have a small voltage but with a high enough brightness within the LED to obtain accurate results. Chromatic Aberration Different wavelengths are refracted by different amounts. The refractive index is different for different colours. This leads to an effect called chromatic aberration. A simple lens has different focal lengths at different wavelengths (Colours). This is because the different colours have been refracted through the glass with different amounts. A well made lens therefore will give a sharp image in any single colour but the image will be blurred by the out-of-focus images of all the other colours combined that will have focused at other points beyond the lens. This experiment should show by how much the two extreme visible colours (red and blue) are refracted. Variables There are many different variables within this particular experiment. For example, it is possible to move the screen or the converging lens or the LED or any combination of the three components. ...read more.

Middle

Any of these ways will result in a change in the lens to object distance (u) and therefore a change in the lens to image distance (v). Thus producing an average result for the focal length of a specific wavelength (colour). I will measure u and v by moving the screen and the converging lens. By using this method I will be able to obtain the largest amount of corresponding pairs without the need to an extensive amount of space to conduct the experiment. As one length increases the other should decrease and my readings should be more consistent then if the area I was working with was large. A larger working area would lead to a larger possibility of greater inaccuracies within my findings. I will also repeat certain values of u to obtain an average v thus a more accurate focal length. The quality of the lens will affect me from comparing my results to that of a similar experiment. This is because lenses have other aberrations to take into consideration, along with the specific quality of the glass (does the quality differ within a lens?) and the fact that a perfect lens will not produce a perfect image because the different wavelengths will focus at different points. Using all these varying factors it is possible to determine that it will be very unlikely for any two lenses to be exactly compatible. Method * Assemble all of the equipment. * Fasten a metre ruler onto a tabletop running horizontally and ensure that when you fasten the LED down to the end of the ruler the filament of the LED is at 0mm. * Adjust height of all the components to ensure that the centre of the lens is level with the LED filament. * Turn on the electricity and move the lens and screen until a focused image is produced upon the screen. ...read more.

Conclusion

* Record the second reading for v and repeat once more to obtain a third reading for the specific u. * Repeat for other u readings. * Tidy away the equipment properly. Data Treatment. All u and v measurements will be taken in mm and only accurate to a mm because that's the smallest possible division on the metre ruler. The lens formula: 1 + 1 = 1 u v f u = Object to lens distance (mm) v = Lens to image distance (mm) f = Focal length (the distance from the centre of the converging lens to the principle focus of the lens i.e. the clearest image distance) When analysing my data I shall draw a graph. 1/u = 1/f - 1/v by rearranging the equation like this I am able to see that y = c - mx. Y = 1/u C = 1/f M = 1/v I shall plot 1/u against 1/v. If my results are accurate I should find that because my scales will be the same and the pairings should correspond to each other. I will produce a graph with the gradient of -1. This will also give my straight line an angle of 45� with the x-axis. The point where the graph cuts the axes should by the same and they should both correspond to 1/f. Prediction Graph: With my graph I will be able to draw both the minimum and maximum lines of best fit to obtain an uncertainty reading. If my two uncertainty readings overlap I can conclude that my experiment was not completely conclusive in demonstrating the aim. It is difficult to obtain precise readings for v because it is everybody's' individual perception of a focused image that is recorded. All of my readings will be consistent because it is only me who is estimating when the image is focused, this ensures that is the images are not all precisely focused they will all be out by the same amount. ?? ?? ?? ?? Abdul Ggafar ...read more.

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