The atmosphere of earth is full of
particles or molecules like N2, O2, O3, H2, H2O, dust particles etc. These are
so small that we cannot see them with naked eyes. Even with powerful microscope
of 100X or higher available in school labs, air particles are not visible (except
dust particles). When sunlight encounters these particles there is a change in
the direction of sun rays which lead to a phenomenon known as scattering of
light.The sunlight consists of
different frequencies from 430 – 770 THz (or wavelengths 390 -700 nanomoetres).
The colour of the scattered light depends upon the size of the encountered particles.
The process of selective scattering is known as Rayleigh scattering.
The size of the air particles has
size comparable to the wavelength of visible light at the blue end. These
particles are more suitable in scattering light of blue and violet colours as
compared to red colour. This is the reason why sky appears blue in colour. Due to pollution, large size
particles are introduced in the atmosphere. These particles are efficient in
scattering light of longer wavelengths also. This causes the pale blue or grey
colour of the sky in a polluted atmosphere. Watch the video shown below on youtube. This video shows how a laser light is scattered by water and talcom powder. The scattering of laser by water or talcom powder makes its path visible (known as Tyndall's effect) and this property can be used to study relection or refraction.
1. A concave mirror is used as a reflector of light in headlights of automobiles to obtain a parallel beam of light. In a similar way concave mirrors are also used in torch lights and search lights.
In such a case, the source of light such as bulb is placed at the focus of the concave mirror. The light rays which fall on the concave mirror are reflected parallel to the principal axis and thus a parallel beam is obtained.
Practically, there are two bulbs one above the focus to obtain low beam (to light the ground nearby) and the other bulb slightly below the focus to obtain a high beam (to illuminate a larger distance but this will blind the driver approaching from opposite side).
In some reflectors the position of the bulb can be shifted slightly to obtain a low beam/high beam.
2. A concave mirror is used as a dentist’s mirror. Dentists use a concave mirror to obtain the enlarged image of a tooth. The focal length of concave mirror used is large enough so that object (tooth) is placed between Focus and Pole. Thus a virtual, erect and magnified image of the tooth is obtained.
The position of object (tooth) is very important. Imagine what would happen if dentist view an inverted image using a concave mirror if it is placed between C and F. the image obtained will be magnified but inverted. In confusion, dentist may remove a healthy tooth.
3. Concave mirrors are used as concentrator of heat and light in solar furnace. A solar furnace can be constructed by using a huge concave mirror or an array of plane mirrors mounted on a curved surface giving rise to a concave shape. The huge concave mirror is directed towards the sun. The sun’s rays get focused at F. At focus F, the temperature will be very high as all heat rays (infra red) get converged there. The temperature can reach up to 3500°C which can be used to melt metals.
One such solar furnace is installed in Mount Louis in France.
4. A concave mirror can be used as a shaving mirror. Such a mirror will have a large focal length say 1m or 1.5m so that the person standing nearby would be placed between F and P. A virtual, erect and magnified image of the face is obtained which helps the person to see an enlarged image of his face while shaving.
Uses of Convex mirrors
1. It is used as a rear view mirror in automobiles.
Since a convex mirror provides a wider field of view and erect image of the object, it is a perfect choice for the rear view mirror.
The driver can view the wide view of traffic behind the vehicle.
The only disadvantage with the convex mirror is that the driver may be confused about the actual distance of the traffic behind his own vehicle. The image formed by the convex mirror is diminished and it gives an illusion that the object is very far. Even a closer object may appear very far in a convex mirror.
You can find this caution in every rear view mirror of automobile “Objects in the mirror are closer than they appear”.
2. Convex mirror are also used at the intersection of a busy traffic and at sharp curve.
