# WAEC Standard for Physics Alternative to Practical II

A concave mirror is placed on the base of a retort stand and a pin, **P**, is held horizontally above the mirror as shown in the diagram above. The position of **P **is varied until its image in the mirror when viewed vertically downwards, coincides with it. The distance, **d**, between the image and the base of the retort stand is measured and recorded.

The position of **P **is adjusted so that it is at a height, **h**, (> 15 cm) from the base of the retort stand. A second pin, **Q**, is used to locate the position of the image of **P**, using the method of n parallax. The distance, **y**, between the position of **Q **and the base of the retort stand is measured and recorded.

This procedure is repeated for **four **other positions of **P.**

In **figure 2**(a), shows the position **P **when its image formed by the concave mirror coincides with it.** **represents the distance **mi **between **P **and the positions of the illuminated object **Oi**.

In **figure 2**(b), shows the position of **P **relative to the base of the retort stand, while **figure 2**(c) shows the corresponding position **Q **above the base of the retort stand.

- State
**two**precautions that are necessary to ensure accurate results when performing this experiment.

(b) (i) Explain with the aid of a labelled diagram, how a concave mirror can be used to

produce an enlarged, upright image of an object.

(ii) A concave mirror of focal length 10 cm forms an image of an object placed 20 cm from its pole. Calculate the magnification produced.

**Observation**

Part (a) Performance was below average. Moss candidates evaluated **h-1 **and **y-1 **of the raw values instead of the converted values. Also, many candidates plotted the reciprocals of the raw values of **h **an **y **instead of converting them before finding the reciprocals.

Part (b) Candidates’ responded poorly in this part as many candidates could not differentiate a mirror from a lens and a convex mirror from a concave mirror.

The expected answers are that candidates should:

- correctly measure and record to at least 1 d.p and within tolerance of ± 0.1 cm

raw value of **d**

- correctly convert value of
**d** - correctly measure and record to at least 1 d.p and within tolerance of ± 0.1 cm

five values of **hraw**

- correctly convert five values of
**h** - correctly measure and record to at least 1 d.p and within tolerance of ± 0.1 cm

five values of **yraw**

- correctly convert five values of
**y** - correctly evaluate to at least 3 d.p five values of
**h-1** - correctly evaluate to at least 3 d.p five values of
**y-1**

show composite table with **h, y, h-1 **and **y-1**