How does a burglar's alarm work?

The alarm consists of a photoelectric cell connected in a closed electrical circuit and an infrared source of light. The light from the infra red source is continuously incident on the photoelectric cell, maintaining a constant current in the circuit. The device is kept concealed near the place to be protected.

If a burglar intercepts the invisible infrared rays, the photoelectric current stops. This automatically operates a relay, which sends an electric current in another circuit connected to a bell. The bell starts ringing and alarm is raised.

What are exposure meters and lux meters?

Exposure meter is a device used along with a camera to determine the correct time of exposure for taking good photographs. It is actually a photocell based on photoelectric effect. The apparatus is enclosed in a case having two windows. The light is incident on the photocell through one window. Through the other window, the deflection in the milliammeter can be seen.

When light falls on the photocell, electron current is set up and is seen as a deflection on the milliammeter.

The milliammeter scale of the exposure meter is calibrated to give the value of required time of exposure directly. If the light is poor, the time of exposure is increased. If the light is bright, the time of exposure is decreased.

Intensity of light is measured in lux. A lux meter is an instrument used to measure the intensity of light. The construction and working of this instrument is the same as exposure meter. The only difference is that, the scale of the milliammeter is calibrated in terms of lux and candela and it can directly measure the intensity of light.

How does a remote control work?

Remote control works on the principle of photoelectric effect. The remote transmitter has an infrared light. The light signal has instructions according to what you press on the remote button. The receiver is connected to a photoelectric cell. The infrared light is used to generate electrons and hence electric current. The current signal follows the same information that has been received from the remote controller. The current signal is then passed on to electronic circuit which takes action on the command sent.

What are night vision goggles and how do they work?

All objects emit radiations in the infrared region. A photoelectric cell, sensitive to infrared radiation absorbs light and converts it into electric current.The images are electronically enhanced on a screen, which is the screen of the goggles.This is how night vision goggles work. Armies on duty use the night vision goggles at nights.The same principle is found in many other equipments such as thermal imaging cameras.