(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 sc...

(a) Uses of a converging lens

(i) Igniting carbon paper: Hold the carbon paper at the principal focus, F, of the converging lens and direct the lens towards the Sun. The parallel rays from the Sun are brought to a very small bright spot at F. The heat energy is concentrated there and ignites the carbon paper.

(ii) Producing an enlarged picture on a screen: Place the object or slide between F and 2F of the converging lens. A real, inverted and magnified image is formed beyond 2F; a screen is placed at this image position.

(iii) Correcting an eye defect: A converging lens is used to correct long-sightedness (hypermetropia). In a long-sighted eye, rays from a near object would be focused behind the retina. The spectacle lens converges the rays before they enter the eye, so that the eye lens forms the image on the retina.

(b) Mechanical wave: A mechanical wave is a disturbance that requires a material medium, such as a solid, liquid or gas, for its propagation. It cannot travel through a vacuum. Examples are sound waves and water waves.

Experiment showing that sound needs a material medium: An electric bell is placed inside an airtight bell jar connected to a vacuum pump. When the switch is closed, the bell rings and its hammer is seen striking the gong. While air is present in the jar, the sound is heard clearly. As the pump removes the air, the sound becomes fainter. When the jar is nearly evacuated, the hammer is still seen striking the gong but little or no sound is heard. Therefore, sound requires a material medium, in this case air, for transmission.

Characteristics of sound

1. Loudness (intensity): depends on the amplitude of vibration.
2. Pitch: depends on the frequency of vibration.
3. Quality (timbre): depends on the number and relative strengths of the overtones or harmonics present.