Night vision glasses, used for seeing in the pitch dark night, use the little light available to form an image of sufficient brightness to be seen.
The image is first focussed, as in a camera, on to a window which is coated with special chemicals containing sodium, potassium, cadmium and oxygen compounds which emit electrons when illuminated.
The electrons so emitted are then accelerated by a series of powerful electric fields and made to fall onto another screen coated with a fluorescing chemical which glows recreating a much brighter image of the original scene.
Some night vision glasses make use of infrared radiation emitted by all objects even in the dark. These radiations are amplified in a similar way to make night vision possible.
Metals when present near a magnetic field create disturbance in the field. This is similar to the waves created by a stone thrown into a stream of water. Metal detectors are equipped with sensitive electronic circuitry to detect disturbances created by metal objects in the magnetic field. The hand held metal detectors have two coils which create magnetic fields of similar strengths. Whenever a metal object comes near any of the coils it disturbs the magnetic field there. This upsets the balance between the two fields and alerts an electronic circuitry. Some detectors do not have any coils, but instead sense the disturbances created by metal objects in the natural field present in the surroundings. The door-frame type of metal detectors works this way. They have a series of coils in the frame which continuously monitor the surrounding magnetic field. Any disturbance caused in the field is sensed and an alarm set off.
Locks are nothing but special latching devices in which the secured bolt is released only by a specially designed tool, the key. In an ordinary door latch, a bolt is pushed into a hole in the door-frame, and is then secured by turning the key. A similar principle works in a padlock or the portable lock. However, in a padlock, a projection called tumble-bolt is pushed into a notch in the shockle and made secure by a special arrangement of levers. The levers are placed at different heights and the tumble moves only when all the levers are moved at once. Notches of different heights are made on the key such that they are in alignment with the levers of different heights. Thus, when the key is turned, all the levers are operated together and the tumble-bolt is moved into the groove in the shockle or out of it. By different combinations of the levers of various heights, each lock is made to open with a specific key.
The operation of the parachute is based on the simple principle of balancing two opposing forces. There are two forces that act on any falling object —gravity and air resistance.
Unlike an aeroplane, helicopters have moving wings in the shape of spinning rotor blades. Air currents passing over the upper and lower surfaces of the spinning blades create low pressure above it and generate lift. So, a helicopter does not have to taxi on a runway to get the lift. The lift can be controlled by varying the pitch (angle) of the rotor blades. By increasing the pitch of the main rotor blades lift is increased and the craft climbs. Lowering the pitch of the blades reduces lift and gravity causes the helicopter to descend. Similarly, if the pilot holds the rotors pitch in such a way that the blades produce just enough lift to counteract gravity, the helicopter remains stationary in mid-air.