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Periscope
Periscopes are also used in armoured vehicles and whenever we want to see over an obstacle (e.g. over crowds of people). It is shown how the periscope works on the schematic diagrams here (the mirror-based periscope on the left and the prismatic one on the right).
The rays of light hit both mirrors at 45 degree angle. If we look insight we see a straight image of the real object we observe.
If we want to observe the whole horizon, we have to turn a periscope through 360 degrees. Why doesn't a simple device in which only the upper part would turn around exist? See on the diagram what will happen if we turn only the upper mirror/ prism through 180 degrees.
The image is reversed! Therefore, it is not very useful for observation purposes. The film shows you what happens in a "turn-around" periscope, like the blue one shown on the previous page.
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