| Aim | To determine the angle of minimum deviation for a given prism by plotting a graph between the angle of incidence and the angle of deviation. |
| Apparatus Required | A prism A spectrometer A light source (such as a sodium lamp or a laser) A screen or a piece of paper to capture the image A ruler or measuring tape A protractor or an angle measuring device |
| Theory | A prism is a transparent optical element that refracts light. When light enters a prism, it is bent towards the base of the prism due to the difference in refractive index of the prism material. The angle of minimum deviation is the angle at which the deviation of the light passing through the prism is minimum. To find the angle of minimum deviation, we can plot a graph between the angle of incidence and the angle of deviation. |
| Procedure | Set up the spectrometer by placing the prism at the center of the instrument, and the light source on one side of the prism. Adjust the prism so that the light enters the prism at the center of the face, and exits the prism from the opposite face. Place the screen or piece of paper on the opposite side of the prism, so that it can capture the image formed by the prism. Adjust the spectrometer until the prism is aligned with the light source and the screen. Measure the angle of incidence by using the protractor or angle measuring device to measure the angle between the incident ray and the normal to the surface of the prism. Measure the angle of deviation by using the protractor or angle measuring device to measure the angle between the emergent ray and the normal to the surface of the prism. Repeat steps 5 and 6 for different angles of incidence, ranging from small to large angles. Record the values of the angle of incidence and the corresponding angle of deviation for each measurement. Plot a graph between the angle of incidence and the angle of deviation. Draw a line of best fit through the points on the graph. Determine the angle of minimum deviation by finding the point on the graph where the slope of the line of best fit is zero. |
| Observation and Result | Observation: As the angle of incidence increases, the angle of deviation also increases. At a certain angle of incidence, the angle of deviation becomes minimum. Beyond the angle of minimum deviation, the angle of deviation starts increasing again. The graph between the angle of incidence and the angle of deviation is a curve that has a minimum value at the angle of minimum deviation. The line of best fit on the graph represents the trend in the data, and can be used to determine the angle of minimum deviation. Result: The angle of minimum deviation can be determined by finding the point on the graph where the slope of the line of best fit is zero. The accuracy of the measurement can be improved by taking multiple measurements and calculating the mean and standard deviation. The refractive index of the prism can be calculated using the formula n = sin[(A+D)/2]/sin(A/2), where n is the refractive index, A is the angle of incidence, and D is the angle of deviation at the angle of minimum deviation. |
