In an experiment to measure the focal length of a concave mirror, a student placed an object in front of the mirror so that a real image was formed - Leaving Cert Physics - Question 3 - 2015

The position of the image was located by adjusting the screen until a sharp image of the object was formed. This was typically done by moving the screen back and forth until the image was at its clearest, indicating the exact distance from the mirror to the image. The sharpness of the image provides an indication of its correct position.

On the diagram:

• Label the distance 'u' as the distance from the object to the mirror.
• Label the distance 'v' as the distance from the image to the mirror. Both distances should be clearly marked to illustrate their respective measurements in centimeters as recorded in the table.

The focal length 'f' of the mirror can be calculated using the lens formula: $\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$ Substituting the values for one of the experiments (e.g., for u = 20 cm and v = 60 cm): $\frac{1}{f} = \frac{1}{20} + \frac{1}{60}$ Calculating the right side: $\frac{1}{f} = \frac{3 + 1}{60} = \frac{4}{60} = \frac{1}{15}$ Thus, the focal length is: $f = 15 \text{ cm}$

The student repeated the experiment to ensure accuracy and reliability in the results. By taking multiple measurements, the student could minimize errors and obtain an average value for the focal length, enhancing the validity of the findings.