Many compact cameras and some SLR cameras have built-in-flash. The flash cannot be detached from these cameras and may result in “red-eye” that is, the pupils of a subject’s eyes appear red-in colour photographs.

            Red-eye occurs when light from the flash is reflected off the back of the retina. (Subjects pupils are dilated and the On-camera flash actually illuminates the blood vessels in the eye). The only way to avoid this is to close down the pupils (contract the iris) by raising the room lights, or using “red-eye reduction flash mode”.

            The feature is available in sophisticated compacts. This mode fires a pre-flash or a bright lamp prior to exposure, which makes the iris contract and close down the pupils, thereby reducing the effect. In compacts where there is no red-eye reduction feature, the following technique may be employed to reduce the red-eye effect. The flash light may be bounced off the ceiling or wall, before it reaches the subject.

            Engineers have exploited the human eye’s ‘persistence of vision’ to display pictures on television screens.

            The sets flash every frame at a rate of 50 times a second. (This is the frame or refresher rate).That is, each frame appears on the screen for about 20 milliseconds making us feel that the picture is continuous. To see the picture being refreshed on the screen, just wink your eye rapidly and look at a TV screen. A black bar dividing the picture into two should be visible. (This is the simple stroboscopic effect.)

            Cameras have typically a shutter speed of 1/100th of a second, that is, the shutter stays open for only 10 milli seconds. As this time interval is much smaller than the TV frame rate, the shutter does not give enough time for the picture to be displayed next time.

            Hence, the camera freezes the TV picture including the dividing bar and captures it in the film. If you carefully observe the scenes in movies, you will notice that any TV set in them displays the picture with a bar black slowly moving across the picture. To photograph from a TV, use an SLR camera and set the shutter speed to 1/8th of a second (125 ms) for flicker free pictures. (Theoretically, the shutter speed should be just below the frame rate). Use a tripod to avoid hand shake. 

  Photocopier is an electromechanical device having a photoconductive cylindrical drum made of cadmium, selenium or an organic photoconductive material. (A photoconductive material changes its conductivity under light).

            Upon switching on the machine, the drum is positively charged and the heating section, at the exit of the copier, with a Teflon coated roller is heated to 230 to 320 degrees Celsius by a heating lamp. This temperature is maintained by means of a feedback control loop having a thermistor and an on-off control switch.

            The original document is placed on a glass plate and held flat. The paper fed through the feed section is pulled in by contact switches, rollers and a gear mechanism. As the paper touches a registration switch, an optical system equipped with a scanning lamp of 800-900 W is energized and driven by a motor, for scanning the document. An image of the document is reflected by various mirrors and lenses to the drum. Depending on the intensity of light received the photoconductive material loses its resistance at varying levels on its surface. That is, the positive charges on the drum are neutralized except in those areas representing the image). This results in a latent ‘charge image’ on the drum. A toner (negatively charged) is then pumped to the drum. Based on the ‘charge map’ the toner gets deposited and forms a ‘toner image’ on the drum. The drum then rotates and prints this image on the moving paper and ‘fixes’ it at a high temperature. Enlargement and reduction is achieved by adjusting the position of the lens (by varying the focal length) of the optical system.

  Chronometers are used to determine the speed of the vehicle with a hand held stopwatch. No radar, no loop just a plain and simple digital stop watch, and two points of a known distance apart. Start and stop switches are to be manually actuated when the vehicle crosses the points.

The speed will be displayed instantly. The second method is by using a Piezo Sensors, this method uses set of 3 rubber strips with a specific distance apart are build across the road or buried in the road. The time between compressions is measured to give the resulting speed of the vehicle. If the speed exceeds the camera associated with the system will photograph the vehicle.

In another method two beams of invisible infrared beams are emitted across the given traffic lane. The elector optic head directs the beams to two retro-reflectors mounted on the road surface.

            Every vehicle passing between the electro optic head and the reflectors breaks the beams and triggers the unit computer to measure the speed. If a vehicle exceeds speed limit, the high-resolution camera photographs the front or rear of the vehicle including its license plate.

            In another method, infra red diodes are pulsed at certain frequencies and to create radar beams. This beam is emitted via parabolic reflector at 15 degree spread and directed towards the traffic. The emitters are shielded to prevent external interference.

            They normally use K band in a frequency 24GHZ. The associated computer will measure speed and triggers camera to capture the image of erring vehicles.

            This works on the phenomenon of Doppler Effect. When a sound source and an observer are in relative motion with respect to the medium in which the waves propagate, the frequency of waves observed is different from the frequency of sound emitted by the source.

            This phenomenon is called Doppler Effect. This is due to the wave nature of sound propagation.    The principle of Doppler Effect is used to detect the speed of automobiles by traffic police. An electromagnetic wave of constant frequency is emitted by a source attached to the police van. The wave is reflected by the moving vehicle which in turn acts as a moving source.

            Stationary objects experience only the gravitational force. Such objects can be balanced only when the line of action (line connecting the centre of gravity of the object towards the earth) touches the ground at a point within the base of the object. If the base area is large, slight deviations due to external forces such as the wind will not push this point out of the base.

    So it is easy to balance such objects. In the case of a two wheeler, the base is very narrow, almost a line connecting the two points where the two wheels touch the ground. Hence any slight disturbance will push the line of action outside the base area leading to difficulty in balancing.

            During motion the situation is entirely different. In addition to gravity there is another force exerted by the engine. It acts in the forward direction.

            The resultant of these two forces is in the forward direction and so the vehicle moves forward independent of where the line of action cuts the ground. Hence it is easier to balance a moving vehicle.

New vessels are rubbed on the surface with an abrasive powder. This makes the surface almost uniformly leveled.

Almost all the light falling on the buffed surface gets reflected uniformly along certain directions depending on the shape of the vessel. This makes them shine.

  Upon continuous use, scratches are formed and so the surface becomes rough. Hence, the light falling on the surface is scattered randomly all around. Hence the reflectively of the surface is not as good as it was before. Dirt accumulating on the scratches, corrosion and oxidation of the metal surface dull the vessels further.

            The surface of any metal when scrubbed and polished would have a shinning appearance. This is a characteristic property of metal and is due to reflection of light.

            When exposed to air and moisture, most metals react with oxygen, carbon dioxide and moisture. Iron vessels rust and others such as copper and aluminum tarnish. When such vessels are used for cooking, this process gets quickened; aluminum gets coated with its oxide and becomes dull. Copper gets covered with its basic carbonate resulting in a green colour.

            We are familiar with an age old practice of cleaning such vessels with tamarind.

Mild organic acids present in such fruits and vegetables dissolve the coating and restore the sheen. Washing powders which are alkaline also aid in dissolving the dull coat.

Metallic tin is resistant to many of these reactions. That is it used to ‘tin’ copper vessels. Stainless steel does not get tarnished due to the presence of chromium in it.

Even silver vessels get tarnished due to the trace amounts of hydrogen sulphide present in the atmosphere. Such vessels also require polishing in this case with the more poisonous cyanides!!!