A plano-convex lens of refractive index 1.5 and radius of curvature 30 cm is silvered at the curved surface. Now, this lens has been used to from the image of an object. At what distance from this lens, an object be placed in order to have a real image of the size of the object? |
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a) |
20 cm |
b) |
30 cm |
c) |
60 cm |
d) |
80 cm |

A plano-convex lens of refractive index 1.5 and radius of curvature 30 cm is silvered at the curved surface. Now, this lens has been used to from the image of an object. At what distance from this lens, an object be placed in order to have a real image of the size of the object? |
|||||||

a) |
20 cm |
b) |
30 cm |
c) |
60 cm |
d) |
80 cm |

1 Answer

127 votes

A plano-convex lens behaves as a concave mirror if it’s one surface (curved) is silvered. The rays refracted from plane surface are reflected from curved surface and again refract from plane surface. Therefore, in this lens two refractions and one reflection occur. Let the focal length of silvered lens is Where focal length of lens before silvering focal length of spherical mirror. Hence from eq (i) Again given that, Size of object = size of image Thus, from lens formula Hence, to get a real image, object must be placed at a distance 20 cm on the left side of lens. |

127 votes

127