A thin lens of focal length f and its aperture has a diameter d. It forms an image of intensity 1. Now the central part of the aperture upto diameter (d//2) is blocked by an apaque paper. The focal length and image intensity would change to

A thin lens of focal length f and its aperture has a diameter d. It fo

1.	A thin lens of focal length f and its aperture has a diameter d. It forms an image of intensity 1. Now the central part of the aperture upto diameter (d//2) is blocked by an apaque paper. The focal length and image intensity would change to
Answer» Solution :Focal length of lens does not depends on the aperture of lens, It only depends on the (i) Refractive index of material lens and SURROUNDING medium and (ii) Radius of curvature For intensity `I prop (aperture DIAMETER)^2` `implies I_(BLOCKED) prop (d/2)^2 and I_2 prop d^2` Hence, `I_(blocked)=I_0/4` Final image intensity `=I_(origi NAL)-I_(blocked)=I_0-I_0/4=(3I_0)/4`