InterviewSolution
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InterviewSolution
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A converging mirror forms a real image of height 4 cm, of an object of height 1 cm placed 20 cm away from the mirror. Calculate the image distance. What is the focal length of the mirror ? |
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Answer» Here, height of image, `h_(2) = -4 cm`, minus sign is for real image, which is inverted i.e. below the principal axis. Height of object, `h_(1) = + 1 cm`. Object distance, `u = -20 cm`, minus sign for the object distance is measured from pole in a direction opposite to the direction of incidence of light. We have to calculate the image distance, `v = ?` As `m = h_(2)/h_(1) = -v/u`, putting the values given above, we get `(-4)/1 = (-v)/(-20)` or `v = -80 cm` Negative sign of v indicates that the image is on the side of the object. It must be real and inverted. To calculate focal length of the mirror, we use mirror formula :`(1)/(f) = (1)/(u) + (1)/(v)` putting `u = -20 cm`, and `v = - 80 cm`, `(1)/(f)=(1)/(-20)+(1)/(-80) = (-4-1)/(80) = (-5)/(80)` `f = (-80)/5 = -16cm` Negative sign of f indicates that mirror is concave. |
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