JEE MAIN - Physics (2017 - 9th April Morning Slot - No. 1)
In an experiment a convex lens of focal length 15 cm is placed coaxially on an optical bench in front of a convex mirror at a distance of 5 cm from it. It is found that an object and its image coincide, if the object is placed at a distance of 20 cm from the lens. The focal length of the convex mirror is :
27.5 cm
20.0 cm
25.0 cm
30.5 cm
Explanation
The given optical situation is depicted in the following ray diagram:
_9th_April_Morning_Slot_en_1_1.png)
In this case, the image forms 55 cm behind the convex mirror and then the reflection takes place due to the mirror image that forms at a distance (v) behind the mirror. Now, this image acts as an object for lens and the final image forms at a distance of 20 m from the lens if the focal length of convex mirror is as follows:
$${{55} \over 2} = 27.5$$ cm
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