Question

Distance between object and screen is fixed and equal 120 cm. The converging lens of focal lengths 35 cm is positioned between object and screen in attempts to get clear image on the screen. Based on the data given we may conclude that:

There are two positions of lens which will provide sharp image on the screen.

There is one position of the lens which will provide sharp image on the screen.

No position of the lens will provide sharp image on the screen.

There are some positions of the lens providing the sharp image on the screen but there is not enough information to give more exact answer.Distance between object and screen is fixed and equal 120 cm. The converging lens of focal lengths 35 cm is positioned between object and screen in attempts to get clear image on the screen. Based on the data given we may conclude that:

a. There are two positions of lens which will provide sharp image on the screen.

b. There is one position of the lens which will provide sharp image on the screen.

c. No position of the lens will provide sharp image on the screen.

d. There are some positions of the lens providing the sharp image on the screen but there is not enough information to give more exact answer.

why is answer c

Answer 1

Referring to the figure,

x cannot be less than f=35 cm since with those distances, the image wouldn't be real.

Now consider f<x< 70(Radius of curvature).

where v is the distance between the lens and the image.

which is very much beyond the screen(check it by imaging on the figure)

(say)

(again beyond the screen)

(say)

(again beyond the screen)

Similarly, as we increase x the image is never formed on the screen no matter what x we choose. So "No positions of the lens will provide the sharp image on the screen".