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प्रश्न
A convex lens of focal length 15 cm is placed on a horizontal plane mirror. A pin is placed above the lens such that no parallax is obtained between the pin and its image. Calculate the height of the pin from the lens. Explain with a neat ray diagram.
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उत्तर
Condition for No Parallax: When a pin is placed above the lens-mirror combination and its inverted image coincides with the pin itself without any parallax, it means the light rays are retracing their own paths.
Path of Light Rays: For light rays to reflect directly backward along the same path from a flat plane mirror, they must hit the mirror surface normally (perpendicularly at an angle of 90°).
Action of Convex Lens: This means the light rays emerging from the convex lens must be perfectly parallel to the principal axis.
Conclusion: A convex lens only produces parallel rays when the object is placed exactly at its focus (F). Therefore, the distance of the pin from the lens must be equal to its focal length.
Height of the pin = Focal length of the lens (f) = 15cm
