(a) Explain with the aid of a diagram how a converging lens could be used to : (i) ignite a piece of carbon paper ; (ii) produce an enlarged picture on a screen ; (iii) correct an eye defect.
(b) What is a mechanical wave? Describe with the aid of a diagram, an experiment to show that sound needs a material medium for transmission.
State 3 characteristics of sound and mention the factor on which each depends.
Explanation
(a)(i) 
When the parallel rays of light from the sun pass through the converging lens, they converge at the principal focus. This converging rays cause the paper to ignite after a while.
(ii) 
Production of an enlarged picture
as in the magnifying glass. I - enlarged image; O - object. Alternatively, as in the slide projector below
The lens causes the image to be magnified but inverted in this case.
(iii) Correction of eye defect:
Converging lens can be used for correcting long sightedness. Long sightedness occurs when the eyeball is too short. Rays from the normal near point (25cm from the eye) are brought to focus beyond the eyeball instead of on the retina. The correction is done as shown below.
(b) A mechanical wave is one that requires material medium for their propagation e.g. sound waves, water waves. Waves which do not require material medium for their propagation i.e. they can pass through vacuum are electromagnetic waves.
Experiment to show that sound requires material medium for transmission.
When the key is closed, current will flow through the electric bell and the hammer will be seen, striking the gong. Sound will thus be produced. However as the air inside the bell jar is gradually pumped out using a vacuum pump via the opening, the intensity of the sound produced by the electric bell gradually dies down even though the hammer is still seen striking the gong. When the air has been pumped out and the bell jar becomes empty (vacuum), the intensity of the sound dies almost completely. This shows that sound requires a material medium (air in this case) for transmission.
The three characteristics of sound are:
1. Intensity or Loudness: depends on amplitude.
2. Pitch : depends on frequency
3. Quality or Timbre : depends on overtones.