NEET Physics Ray Optics and Optical Instruments Questions Solved


Two point light sources are 24 cm apart. Where should a convex lens of focal length 9 cm be put in between them from one source so that the images of both the sources are formed at the same place

(a)   6 cm                                   (b)   9 cm

(c)   12 cm                                (d)   15 cm

(a) 

The given condition will be satisfied only if one source (S1) placed on one side such that u < f  (i.e. it lies under the focus). The other source (S2) is placed on the other side of the lens such that u > f (i.e. it lies beyond the focus).

If  is the object for lens then   1f=1y-1x

1y=1x-1f                                                      ....(i)

If is the object for lens then

1f=1+y=1-24-x1y=1f-124-x       ....(ii)

From equation (i) and (ii)

1x-1f=1f-124-x1x+124-x=2f=29x2-24x+108=0. After solving the equationx=18cm, 6cm

 

 

 

 

 

 

 

 

 

 

Difficulty Level:

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