An rectangular strip of length $2 f$ (where $f$ is focal length of the mirror) is placed on the principal axis of a concave mirror as shown in the figure. One end of the strip coincides with the centre of curvature. The height of the rectangular strip is very small in comparison to the focal length of the mirror. The shape of image of strip formed by concave mirror will be a

Rectangle

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Trapezium

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Triangle

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Square

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Solution

The correct option is C Triangle
An incident ray from the top of the rectangular strip will pass through the focus.
Hence, the images of all points for example $(O_{1}, O_{2}, O_{3}, . . .)$ on the top side of the strip will lie on this reflected ray at $(I_{1}, I_{2}, I_{3}, . . .)$ in between focus and centre of curvature. Thus, the image of this strip is a triangle as shown in the figure.

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