Experiment 2 Locating the center of curvature using no-parallax method
Aim:
Find out the focal length of the mirror through focusing the image of objects on the center of curvature of the mirror.
Diagram of setup:
Procedure:
- The optical pin is placed close to the mirror. The vertical position of the pin is adjusted of the pin so that its tip is at the same level as the center of the mirror.
- The pin is moved away from the mirror along the principal axis until their separation is about twice the focal length. The position of the pin is adjusted until it coincides exactly with its image by no-parallax method until there is no relative motion between the image and the object pins i.e. they do not separate when the head is moved sideways.
- The distance between the mirror and the pin is measured as the pin is now at the center of curvature of the mirror. Hence, the focal length of the mirror is determined.
Precaution:
- Make sure that the mirror is perfectly vertical.
- If you cannot see the image of the pin, the probable reasons are
- You are too close to the pin, less than the least distance of distinct vision,
- The image occurs behind you
- The pin is not along the principal axis of the mirror.
Results:
The distance between the mirror and the pin: 30.8cm
Calculation/ Interpretation:
The center of curvature of the mirror: 30.8cm
The focal length of the mirror: 15.4cm
Experiment 3 Locating the center of curvature using an illuminated object
Aim:
Find out the center of curvature of the mirror through focusing the image of center of curvature on the screen alongside the object.
Diagram of setup:
Procedure:
- The lamp house is placed with an illuminated letter “F” in front of the mirror. The vertical position of the letter so that it is at the same level as the mirror.
- The screen is placed next to the lamp house. The position of the mirror is adjusted so that a sharp is formed on the screen.
- The distance between the mirror and the screen (i.e. the radius of curvature of the mirror) is measured. Hence, the focal length of the mirror is determined.
Precaution:
- Make sure that the mirror is perfectly vertical.
Results:
The distance between the mirror and the screen: 30.8cm
Calculation/ Interpretation:
The center of curvature of the mirror: 30.8cm
The focal length of the mirror: 15.4cm
Sources of error:
- It is difficult to determine the sharpest image on the screen. We could often find a range of sharp images.
Experiment 4 Graphical method using an illuminated object
Aim:
Determine the focal length of a concave mirror by graphical method.
Diagram of setup:
Procedure:
- The lamp house is placed at a distance greater than the radius of curvature of the concave mirror. The position of the screen is adjusted so that a sharp diminished image is formed. The object distance u and the image distance v is measured
- Step 1 is repeated to capture 3 other diminished images and 4 other magnified images on the screen. In each case the object distance u and the image distance v is recorded.
- A graph of 1/v against 1/u is plotted. The best-fit straight line is drawn and it is produced to intersect both axes.
- The intercepts on both axes are found. The mean intercept is calculated. Hence, the focal length of the concave mirror is determined.
Precaution:
- Make sure that the mirror is perfectly vertical.
Results:
Calculation/ Interpretation:
Sources of error:
- It is difficult to determine the sharpest image on the screen. We could often find a range of sharp images.
Conclusion:
There are a number of methods in finding the focal length of the concave mirror.
