Page 312 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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Electrical systems 295

Figure 3.81 Headlight bulbs

Figure 3.82 Concave refl ector Figure 3.83 Light source behind
(light source at the focal point) the focal point

halogen bulbs is mostly iodine and the glass envelope is made from fused silicon
or quartz. This allows higher temperatures and the shape of the envelope causes
the metal to condense back on the fi lament.
The object of the headlight refl ector is to direct the random light rays produced
Key fact
by the bulb into a beam of concentrated light, by applying the laws of refl ection.
Bulb fi lament position relative to the refl ector is important if the desired beam A headlight refl ector directs the
direction and shape are to be obtained. A refl ector is a layer of silver, chrome random light rays produced by the
bulb into a beam.
or aluminium deposited on a smooth and polished surface such as brass or
glass. Consider a mirror refl ector that ‘caves in’: this is called a concave refl ector
( Fig. 3.82 ). The centre point on the refl ector is called the pole, and a line drawn
perpendicular to the surface from the pole is known as the principal axis.
If a light source is moved along the principal axis, a point will be found where the
radiating light produces a refl ected beam parallel to the axis. This point is known
as the focal point, and its distance from the pole is known as the focal length. If
the fi lament is between the focal point and the refl ector, the refl ected beam will
diverge; that is, spread outwards along the principal axis ( Fig. 3.83 ).