Question
Two plane mirrors are set at right angles to each other. A coin is placed in-between these two plane mirrors. How many images of the coin will be seen?
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Answer
When 2 plane mirrors are set at right angles to each other and a coin is placed in-between these two plane mirrors, then three images will be formed, see the diagram below: 
The formula for calculating the number of images, when two are kept at an angle θ, is given as: $\Big(\frac{360}{\theta}\Big)-1$ Now, $\theta=90$ degree. Thus, the number of images formed will be$=\Big(\frac{360}{\theta}\Big)-1$
$=\Big(\frac{360}{90}\Big)-1$
$=4-1$
$=3$
The formula for calculating the number of images, when two are kept at an angle θ, is given as: $\Big(\frac{360}{\theta}\Big)-1$ Now, $\theta=90$ degree. Thus, the number of images formed will be$=\Big(\frac{360}{\theta}\Big)-1$$=\Big(\frac{360}{90}\Big)-1$
$=4-1$
$=3$
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