(a) (i) The power of a lens 'X' is – 2.5 D. Name the lens and determine its focal length in cm. For which…
(a) (i) The power of a lens 'X' is – 2.5 D. Name the lens and determine its focal length in cm. For which eye defect of vision will an optician prescribe this type of lens as a corrective lens ? (ii) "The value of magnification 'm' for a lens is – 2." Using new Cartesian Sign Convention and considering that an object is placed at a distance of 20 cm from the optical centre of this lens, state : (I) the nature of the image formed; (II) size of the image compared to the size of the object; (III) position of the image, and (IV) sign of the height of the image. (iii) The numerical values of the focal lengths of two lenses A and B are 10 cm and 20 cm respectively. Which one of the two will show higher degree of convergence/divergence ? Give reason to justify your answer.
Solution
(a) (i)
• Concave lens
• P=
𝑓(𝑚)
– 25 =
𝑓
f= = – 0·4 m = – 40 cm
– 2⋅5
• Myopia
(ii) (I) Real and inverted
(II) magnified image /size of image is
double the size of object
(III) beyond 2F/ on the other side as that of object ½×4
(IV) Negative
(iii)
• The lens with focal length 10 cm
• less focal length, more converging/diverging power
Page 9 of 12 X_086_ 31/1/1
OR
(b) (i)
( if arrows not marked , deduct half mark)
(ii) The ratio of sine of angle of incidence to the sine of angle of
refraction is a constant, for the light of a given colour and for a
given pair of media. /
(iii)
Convex Lens Concave Lens
(I) Object to be placed between Object can be placed
O and F anywhere in front of the lens
(II) Magnified image Diminished image