181. Dioptre is the unit of power of
   (a) convex mirrors
   (b) concave mirrors
   (c) concave lenses
   (d) lenses irrespective of types
Ans: (d)
182. The power of a lens is
   (a) equal to its focal length
   (b) equal to its radius of curvature
   (c) equal to the reciprocal of its focal length (in metres)
   (d) equal to twice its focal length
Ans: (c)
183. The wavelengths of the visible spectrum extend from
   (a) 1000 Å to 6000 Å approximately
   (b) 4000 Å to 7500 Å
   (c) 16000 Å to 25000 Å
   (d) 8000 Å to 16000 Å
   Note: 1 Å (Angstrom) = 10-10m
Ans: (b)
184. Arrange the following regions of the spectrum of electromagnetic
   radiations in the descending order of their frequencies.
   I.      Infrared waves
   II.     X-rays
   III. Microwaves
   IV. Visible spectrum
   (a) I, II, Ill, IV
   (b) IV, II, I, III
   (c) II, IV, I, III
   (d) II, Ill, IV, I
Ans: (c)
185. The colour of the light which travels through glass with minimum speed