-
Calculate the focal length of the concave lens f by using the lens formula:
By using real-is-positive convention, u will be taken as negative.
Measurement of the focal length of a convex mirrorusing an auxiliary convex lens
- Position the screen to catch the real image formed by the auxiliary convex lens
-
Add the convex mirror between the screen and the convex lens and move it to and fro until an image I forms coincide with the object O.
The image I forms coincide with the object O due to the fact that the reflected beam reflects back along its original path when the convergent beam is incident normally on the mirror. The distance between C and the pole of the mirror P is then the radius of curvature r.
Procedure:
Estimating the focal length of the convex lens f :
-
The focal length of the convex lens f was estimated by using a distant object.
A. Measurement of the focal length of a concave lens using an auxiliary convex lens:
-
The convex lens was placed about 2f from the illuminated fine gauze grid and a sharp image was tried to be caught on the white screen.
-
The concave lens was then placed about half way between the convex lens and the screen, The distance between the concave lens and the screen u was measured. After that, The screen was moved away from the lens until a sharp image of the fine gauze grid was again obtained. The new distance between the concave lens and the screen v was measured.
-
Step 3 was repeated by moving the concave lens either farter away or towards the convex lens and the screen was repositioned so as to obtain a focused image again. Four more sets of values of u and v were obtained and the results were tabulated.
B. Measurement of the focal length of a convex mirrorusing an auxiliary convex lens
-
An image was formed by on the white screen by using a convex lens. The distance between the lens and the screen should be about 0.5m. And the distance d between the lens and the screen was measured.
- The convex mirror was placed between the lens and the screen. The position of the mirror was adjusted until a focused image of the source was obtained on the side of the light box and close to the grid. The distance x between the centres of the lens and the mirror was then measured.
- Step 1 and 2 were repeated by using a different initial position of the lens. 4 more sets of values of d and x were obtained and the results were tabulated.
Precaution:
- The battery should be set to its minimum value in the beginning because it could avoid overheating of the light bulb.
- The experiment should be set in a darkroom to make the image clearer.
Caution:
1. The lens and mirror should be handled carefully to prevent broken
Result:
-
The focal length of the concave lens f:
Average of f =
-
The focal length of the convex mirror f:
Radius of curvature of the convex mirror r = d – x
The focal length of the concave lens f = r/2
Average of f =
Disscusion:
- Sketching the image on the screen:
- Sources of error and improvement:
There were systematic errors in the experiment leading the results inaccurate.
- The sharpness of the images were observed by the human naked eyes
Since the sharpest image obtained was difficult to be observed by human naked eyes, there was a range of distance where could obtain relatively sharper images. As a result, the distances obtained were inaccurate and hence the results become inaccurate. To improve the experiment, the images should be observed by several observers instead of one.
- The wearing of the metre rule
The common zero error arises from using the metre rule from one end, which was worn. It is due to the deviation from “0” value at the “0” mark. So using the centre of the rule instead was needed.
-
Parallax error
A human error called parallax error arises when observers looked at the graduated scale like A and C. To reduce the error, Looking at the graduated scale right from above, like B to obtain the reading was necessary.
①
Conclusion:
Through the experiment, the focal length of the concave lens was found to be 17.25cm. The focal length of the convex mirror was found to be 11.04cm.
Reference:
① : Picture from http://www.jjjtrain.com/vms/Media/glossary_p/parallax.gif
S.6 Physics TAS Experiment 3 Chan Man Lok 6C (16) P.
