Experiment 10 : Lenses

Introduction:

In this experiment, the image appeared on the white board would be analyzed based on the distance, as well as the length of the distance taken from different numbers of the focal point.

Before the experiment was started, the focal point of the lens had to be obtained from an infinite source, which the chosen source was the sun. In order to find the focal point, one point would be found very intense where the light from the sun converged. After the focal point being obtained, the object distance would be varied so that the image appeared on the white board could be observed and analyzed. The object distance would be changed into the distance of one, two, three, four, and five times of the focal point. Since the object height would always be constant, which was also the height of the Socket lamp with V-shaped filament. Therefore, the magnification, M, would depend on the height of the image since its equation is the image height divided by the object height. With the different object distance and with the focal point, the image distance and the image height should be only varied based on the object distance. Therefore, with the experimental observations, the theory of lens should be agreed experimentally.

Materials:

Socket lamp with V-shaped filament

Large converging lens

Large split lens or masking tape

Lens holder for large lens

cardboard

Meter stick

the image would appear on the white board, and the clearest image had to be obtained 

the light source would go through the lens

The object (also the light source)

The object distance started from the light source

The image appeared on the white paper, the image height could be obtained

The thick of the lens produced reflections in itself

Clearly the size and the height of the image was different

Data:

Converging lens		Focal Point:     9.22(cm)
# of focal point distance	Object distance do, cm	Image distance di, cm	Object height ho, cm	Image height hi, cm	M	Type of image
5f	46.1± 0.5	8.5± 0.5	9.0± 0.5	2.2± 0.5	~7/30	Real /Inverted
4f	36.8± 0.5	9.7± 0.5	9.0± 0.5	3.1± 0.5	~1/3	Real /Inverted
Reverse the lens	36.8± 0.5	9.7± 0.5	9.0± 0.5	3.1± 0.5	~1/3	Same as above
3f	27.7± 0.5	10.0± 0.5	9.0± 0.5	3.4± 0.5	~1.1/3	Real /Inverted
2f	18.4± 0.5	11.8± 0.5	9.0± 0.5	6.0± 0.5	~2/3	Real /Inverted
1.5f	13.8± 0.5	17.2± 0.5	9.0± 0.5	12.0±0.5	~4/3	Real /Inverted

Equation for Magnification = h'/ h or -s'/s

                        in this case, the magnification would be obtained by calculating h_i/h_o

---The image was observed to be the same when the object distance was kept constant, and the lens was reversed.

---When the top of the lens was masked with the tape without moving the lens, the image would still from on the white board, however, the image would become dimmer compared to the original appeared image.

---When the object was put a distance to 0.5 f, there was no image appeared on the white board. Then when the image was observed through the lens, the image turned to be erect. Therefore, in this case, the image was virtual since it could not appear on the white board and this virtual image should appear behind the object since the object was inside the focal point which could not form a real image the other side of the lens.

The picture below shows how the object distance is less than the focal point length:

 

Conclusion:

From the observations based on the experiment, it showed us that when the object distance was further than the focal point, the image formed on the white board, and from the magnifications calculated above, the sequence of the magnification was 4/3, 2/3, 1.1/3, 1/3, 7/30 with the increase of the object distance. Hence, we notice that the image size decreased as the object distance increased. However, even if we did not test the distance the same as the focal point, the image should be real and erect but might probably be too small to see. Based on the experiment, when the object image decreased and became smaller than the focal point, the image would be erect but not form on the whiteboard. The image would then be virtual and on the same side as the object.
In addition, after the experiment, when parts of the lens was masked, the image could still be formed on the whiteboard, however, the image was dimmer. That's because the light was still be able to go through the lens, however, the amount of the light went through the lens was less, so that the image appeared was dimmer.

From all the tests in this experiment, it showed us that the light was converging light with the application of convex lens. If the light could not converge because of the object distance, then the virtual image would appear, in other word, the light would virtually converged the opposite direction of the incident light, since it was not technically converging, the real image could not be formed.