Focal Length Of A Concave Mirror

Aim: To observe the focal length of a concave mirror.

Materials:

· Concave Mirror

· Retort Stand

· 2 Pins in stands

· Candle

· Matches

· Ruler

· Screen

Procedure:

1. An approximate focal length was found using parallel light from a
distant object.

2. The mirror was placed in a stand. Pin (A) was moved in its stand,
in front of the mirror till there was no parallax between the pin and
its inverted image.

3. The distance from the object to the mirror was measured.

4. The object pin was placed between F and C. Then the second pin (B)
was moved about till there was no parallax between (B) and the image
(A). The distances of the image and objects were measured.

5. Step 4 was repeated for another position of (A).

6. The image of a light source was located using a screen. Then the
image and object distances were measured.

7. Step 6 was repeated twice.

8. The corresponding values of u and v were tabulated.

Results:

U

V

0.50 m

2.00 m

0.55 m

1.40 m

0.60 m

1.25 m

0.65 m

1.00 m

0.70 m

0.95 m

0.75 m

0.57 m

Data Analysis:

· The tabulated results of step 8 illustrated the image distance,
[IMAGE] and the object distance [IMAGE].

Graphical method

· The two variables, [IMAGE] and [IMAGE] were substituted into the
linear equation y = mx + c to form [IMAGE]= - [IMAGE] + [IMAGE].

· The data was plotted on the graph [IMAGE] versus [IMAGE], thus
proving the relationship between the variables and the linear
equation.

See Figure 1.1

Algebraic method

I. The mirror formula [IMAGE]=[IMAGE]+[IMAGE] was used to calculate f,
the focal length of the mirror for each of the results.

II. The average of the consistent results was then calculated.

See Figure 1.2

Conclusion:

The experiment enabled the physicist to locate the focal point of a
concave mirror. Through the use of obtained results, the graph [IMAGE]
versus [IMAGE] was formulated, proving that a linear relationship
exists between the two variables. However the concave mirrors used in
the experiment were too small in depth, which inhibited accurate
measurements due to human error. Consequently the experiment was an
overall success, the minor errors contained in the data did not affect
the outcome of the experiment, which succeeded to locate the focal
point of the mirror.