Investigation of the chromatic aberration of a converging lens Essays

Investigation of the chromatic aberration of a converging lens Essays Investigation of the chromatic aberration of a converging lens Essay Investigation of the chromatic aberration of a converging lens Essay Chromatic aberration arises from dispersion- the property that the refractive index of glass differs with wavelength of light. The focal length of a lens is determined by a combination of its geometry and the refractive index of the material from which it is made. The refractive index varies slightly with the wavelength of the light that is being transmitted. This means that the focal length of a lens will vary for different colours of light. For blue light (short wavelengths), the focal length is larger than that of red light (long wavelengths). Prediction It was predicted that the values of the focal length of the lens obtained from red and blue light are different, given that the standard focal length is 10cm. Proposed Method: The method is to use the lens formula. 1/f = 1/u +1/v To use this formula both the object distance (u) and the image distance (v) need to be measured. This method is not as simple as method 1 above, but the result obtained will be more accurate. Therefore, the focal length of a converging lens in this experiment was determined by method 2. Detailed Procedures: All the lights were switched off to ensure the laboratory was dark. 1.The apparatus were set up as shown in the diagram below. Some black cupboards were put surrounded the apparatus to provide light insulation. 2. The light bulb, object grill, appropriate light filter, lens, image screen were positioned along a straight line 3. The light bulb was fixed at the end of the fixed metre rule (exactly 100cm). The object grill was placed in front of the light bulb in order to give a clear image. 4.The lens and screen were positioned against the metre rule in a straight line with the object grill so that all their positions can be measured. The lens was started approximately 60cm from the grill. The experiment was firstly started with white light (no light filters) 3.The 12V power supply was switched on. The screen was moved until a sharpest (brightest) image was formed on the screen. 4.The position of the grill (a), lens (b) and screen (c) against the metre rule was recorded. 5.Values for u and v were calculated by finding the differences between the positions. 6. 1/u and 1/v were calculated. 7. A graph of 1/u against 1/v was plotted. 8. The above procedures were repeated with red and blue light filters respectively. 9. The above method was repeated three times for each measurement of three different colours of lights to ensure that the results were accurate List of Apparatus: Light bulb (light source) Red and blue light filters (turn white light into red and blue respectively.) Object grids (to help to make the image clear) Converging lens (focal length=10cm) Lens holder (to hold the lens firmly) Image screen (so image can be cleared seen and observed) Metre rule (to measure u and v) 12V power supply (supplies power to the light bulb.) Variables to be considered (fixed and changing) The positions of light bulb and object grid are fixed during the experiment. The positions of the lens and image screen vary in order to obtain different sets of values of distance (u) and the image distance (v). Intended readings (number and range) The object distance (u) and the image distance (v) need to be measured Design justification Explanation of principles The lens formula (1/f=1/u+1/v) was used to determine the focal length of the lens in order to find the difference in focal length of a converging lens when used to produce images with red and blue light. A lens will not focus different colour in exactly the same place because the focal length depends on its geometry and the refractive index of the material from which it is made. for blue light (short wavelengths) is larger than that of red light (long wavelengths). The amount of chromatic aberration depends on the dispersion of the glass Design justification calculations. Lens formula 1/f = 1/u +1/v A graph of 1/u against 1/v was plotted. when 1/v = 0 then 1/f = 1/u and when 1/u = 0 then 1/f=1/v Consideration of Errors The values were suitable for producing a good line. Possible sources of errors The range of data was not big enough. The image was not clear enough The apparatus were not positioned in a straight line. Implementing Place the light and screen at opposite ends of the optical bench, and move a converging lens until a sharp image is formed on the screen. The distance between lens and screen is (nearly) the focal length of the lens. Short (blue appearing) wavelengths are refracted more than long (red appearing) wavelengths. Consequently, lenses like the one shown above will not image light all in one place. Chromatic aberration arises from dispersion- the property that the refractive index of glass differs with wavelength. positioned along a straight line Data Evaluation Identification of Sources of Error Possible sources of errors The range of data was not big enough. The image was not clear enough The apparatus were not positioned in a straight line. Variations in readings Each set of readings was taken twice to ensure the accuracy. As seen from the graph, there is no anomalies The actual outcome was very similar to the expected result. Summary The chromatic aberration can be corrected by making the lens by different glass.