What is a Range Finder?

                    A range finder is an instrument used to measure long distances for a number of purposes, especially, by surveyors and the army. They are mainly of two types: optical range finder and laser range finder. Radar is also a form of non-optical range finder. It measures the time lapse of an electromagnetic echo and translates the time into distance. 

                    The military range finders are usually long tubes with eyepieces at the centre. The lenses and prisms are located at each end of the tube. The operator looks through the eyepiece and adjusts the prisms so that the target can be sighted through both the ends of the tube. The difference in the direction of the two lines of sight from the ends of the tube is called the parallactic angle. The angle depends upon the distance of the target. The angle is measured on a dial from which the distance of the target can be read directly.

                    There are two types of optical range finders, coincidence and stereoscopic. In the coincidence range finder, the operator looks through a single eyepiece and sees two images of the target. By turning a knob, these two images move together. When this happens, the distance to the target can be read on a dial. 

                    In case of a stereoscopic range finder the operator looks through a pair of eyepieces. One sees a single image of the target as well as a marker that seems to be floating in space near the target. The operator turns a knob until the marker seems to be at the same distance away as that of the target. Then the distance is read on a dial.

                    The laser range finder works on the principles of radar. In this a beam of laser is sent towards the target and the reflected beam is received by the receiver. The time taken by the laser beam from the range finder to the target and back is measured. This time when multiplied by the velocity of the laser beam gives a reading that is doubles that of the distance of the target. When this figure is divided by two, the required distance is found out. These range finders have automatic arrangements for showing the distance of the target. In modern warfare the laser range finder is absolutely necessary to locate the targets and the enemy positions. Spy satellites are used for the same purposes.