Lab report:
Introduction
For this experiment, the focal length is taken to be the image distance that is in correspondent to an infinite object distance; in which case, the incoming light rays are taken to be parallel. By performing this experiment, it presents a platform for determining the focal length by using the focal length equation:
1f= 1p+1q
Whereby, f is the focal length, q is the object distance and p is the image distance. To get these two variables (object distance and image distance) involves focusing the image of an object under illumination on a screen and the measuring the distance for the object and for the image.
Since the refraction index is dependent on the light wavelength, there is a difference between the light colors for the focal length. In which case, with the increase in n there comes a decrease in wavelength, and thereby the focal length associated with lens of a blue light is shorter than for red light. This is what brings about chromatic aberration, C. in this case, the formula to be used for chromatic aberration is as follows:
There is also a difference in terms of the focal point for the rays that move across the outer lens’ part and those that move through the center of the lens. This leads to spherical aberration, S. The effect of spherical aberration can be achieved by carrying out a comparison between the lens’ focal length with the central part that is blocked to the length of the focal lens with the blocked outer section. The formula to be used or spherical aberration is as follows:
s= fcenter - fperimeterfuncovered ×100%
Objectives
Methodology
Apparatus
Lightsource(incandescent bulb), Rail, Red filter, Blue filter, Screen, Reference grid, Stands, Light blockers
Procedure
The setup was checked to localize all the components, the object was made close to the source of the light and properly fixed on rail. The objet position measurement was taken after the turning of the source of light. The lens was then properly positioned at some distance from the object and the position measurement of the lens taken. The image was then positioned to make sure that the sharpest image of the given object was formed on the screen. The measurement of the position of the image was thereafter taken. The procedures above were then done repetitively 10 times with each time there was change in the distance of the object and the image. In the determination if the chromatic abbreviation, the red filter was placed on an empty stand and the stand placed directly in front of stand that held the object. The procedures were then done repetitively 10 times with each time there was change in the distance of the object and the image. The red filter paper was then removed from stand and stored. A blue paper was then placed on the stand that was empty and the procedures were then done repetitively 10 times with each time there was change in the distance of the object and the image. The stand was thereafter removed from the rail and the filter paper removed. The fitter and the stand was then stored. In the determination of spherical the light blocker was applied to the stand that held the lens. The blockers functional part was then centered with the reference to lens. The procedures were then done repetitively 10 times with each time there was change in the distance of the object and the image. The light blocker was then removed and stored. The hole that was reinforced with the tape was applied to pin that faced the object. The light blocker was then removed and stored; lastly the light was turned off.
Results
The following results were calculated in excel using the formula for finding focal length, spherical aberration and chromatic aberration. From this calculation, different colors of the lens were considered, blue, white and red to give chromatic aberration.
Discussion
As shown from the above, it comes out that red light have the longest focal average as compared to blue and white lenses. This is followed by the blue lens and then the white lens comes last. This is a confirmation that the focal lengths associated with blue light is shorter than that associated with the red light. The data is a good confirmation of the phenomena. Further, the chromatic and spherical aberration were calculated with blue light giving a chromatic aberration of 1.71 and white lens giving a spherical aberration of 0.86.
The difference in focal length could be attributed to the fact that different colors have differing refraction index. In which case, blue light with short wavelengths has larger focal length while red light with long wavelengths has slightly shorter focal length. Further, the chromatic aberration amount is also dependent on the glass dispersion.
The major source of error could be attributed to the process of recording distance of images when the formed image was not in much clarity. By recording a poor image distance, this would interfere with calculation of the focal length. Consequently, this will lead to propagation of error thereby leading to a false focal length. Further, handling of the lenses might have also led to the experimental errors. Poor handling, especially when they are placed in wrong position might have also been source of errors.
Conclusion
As from the results, it comes out that there was success in meeting the major objectives of the lab. It comes out that the focal length can be easily figured out, through the help of jus finding the object and the image distance. From the data obtained, the thin lens equation is used together with the object and image distance to get the focal length. The thin lens equation was much successful since it we were able to compare the focal length between the blue, white and red lens. Through the help of the ray diagram, we were able to come up with the characteristics of the formed image. As from the results, red light have the longest focal average as compared to blue and white lenses. This is followed by the blue lens and then the white lens comes last. This is a confirmation that the focal lengths associated with blue light is shorter than that associated with the red light. The results also indicate that with the reduction in distance of the object, the image distance also decreased. These type of experiment proves to be of much use in study areas involving how to develop a telescope with the most effective lens. This is because it gives a background on the formation of images with lens. Apart from the formation of images, the experiment also gave a chance for directly determining spherical and chromatic aberration. , the chromatic and spherical aberration were calculated with blue light giving a chromatic aberration of 1.71 and white lens giving a spherical aberration of 0.86.
The major source of error could be attributed to the process of recording distance of images when the formed image was not in much clarity. By recording a poor image distance, this would interfere with calculation of the focal length. Consequently, this will lead to propagation of error thereby leading to a false focal length.
