Ray Optics and Optical Instruments - Result Question 45

47. If fV and fR are the focal lengths of a convex lens for violet and red light respectively and FV and FR are the focal lengths of concave lens for violet and red light respectively, then we have [1996]

(a) fV<fR and FV>FR

(b) fV<fR and FV<FR

(c) fV>fR and FV>FR

(d) fV>fR and FV<FR

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Answer:

Correct Answer: 47. (a)

Solution:

  1. (a) 1f=(μ1)(1R11R2)

According to Cauchy relation

μ=A+Bλ2+Cλ4 Hence fλ.

Hence, red light having maximum wavelength has maximum focal length.

fv<fr and also Fv>Fr as focal length is negative for a concave lens.



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