Procedure
Type A is a convex lens with shorter diameter
B is a convex lens with longer diameter
METHOD I,
We put the lens in front of the window and then try to obtain a sharp image on the screen.
When we get a sharp image, then measure the distance between the screen and the lens and name it as F.
The distance, F, is the focal length of the lens.
Method II
First, put a light source with a transparent object F in front of a convex lens.
Then adjust the distance between the lens and the light source and mark down the distance as D when you can form a sharp image on the screen allocated right beside the light source. The length D is the focal length
METHOD III
By putting the light source in front of the lens and then adjust the distance between the lens and light source and the lens and the screen. Obtain a sharp image and mark down the distance of U and V.
Then plot a graph with U and V to find out the focal length of the lens by applying the equation.
Concave lens
Place the light source in front of the convex lens which is placed between the concave lens and the light source.
Then adjust the distance between the convex lens and the light source and the concave lens and the convex lens to obtain a sharp image on the screen.
Mark the distance between the two lens when and sharp image is formed on the screen as S.
Next minus F with S to get the focal length of the concave lens.
Data analysis
After the data analysis we obtain several stat. of the focal length.
Convex lens A: 22.8cm was obtain from method I
22.3cm from method II
19-23.4cm from method III
Convex lens B: 5.1 cm was obtain from method I
5.15cm from method II
5.45-5.68cm from method III
Concave lens: 9.9cm-12.8cm
Conclusion
In this experiment, I think that the most accurate method of finding the focal length of convex lens is method I. It is because for I the image can be easily obtain. So, we can get the right focal length accurately.
In the experiment of finding the focal length of concave lens the result is quite difficult to obtain. Since the light rays need to travel a long distance and we cant measure the size of the image and make comparison with the object. It is because we do not whether there are any other factors are affecting the result.
Discussion
Since the meter rules have their scale 1mm. So the error of our result should have about +- 0.05cm error.
Secondly the result will be more accurate if we can minimize the distance when we obtain the focal length of the convex lens by method III and the focal length of concave lens. It is because the image that we obtained is slightly tolerated.
Actually, whenever we test the focal length of the lenses we need to consider the degree of the curve of the lens. Since some light rays may refracted near the edge of the lenses and the refracted ray may thus get another focal length and thus resulting a unclear image.
On the other hand the nodal inside the lenses is another interesting topic that students should know. Although we cannot apply the knowledge of it in this experiment, I think most of us should know more about it. It is because this is another factor that affecting the degree of refraction of light rays in lenses.
Lastly, since light ray consist of 7different colours,and what we need to know that the distortion of the rays may happen. So we need to make the total distance of the experiment much smaller.
