Optics 3 Question 22
22. A real image of a distant object is formed by a plano-convex lens on its principal axis. Spherical aberration
$(1998,2 M)$
(a) is absent
(b) is smaller if the curved surface of the lens faces the object
(c) is smaller if the plane surface of the lens faces the object
(d) is the same whichever side of the lens faces the object
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Answer:
Correct Answer: 22. (b)
Solution:
- In general spherical aberration is minimum when the total deviation produced by the system is equally divided on all refracting surfaces. A plano-convex lens is used for this purpose. In order that the total deviation be equally divided on two surfaces, it is essential that more parallel beam (or the incident and refracted) be incident on the convex side. Thus, when the object is far away from the lens, incident rays will be more parallel than the refracted rays, therefore, the object should face the convex side,but if the object is near the lens, the object should face the plane side. This has been shown in figure.
