Question

plz answer fast
find foval length
and with using all the observation​

Answer 1

you can see, case (iii) ; here both the object and the image distances are 30cm .but we know this is possible only case when object kept at 2f ,so, 2f = 30cm => f = 15cm

(b) use lens formula for finding incorrect observations.

1/v - 1/u = 1/f

see last observation (v)

u = -15

f = +15

then, 1/v = 1/15 - 1/15 = 0 = 1/∞ => v = ∞

but v = 70cm is given hence, observation (v) is wrong.

(c) obviously case 3 , in this case size of image is equal to size of object due to image distance distance is equal to object distance from pole.

Answer 2

here is your answer

(a) u=30 cm, v = 30 cm

This is possible if the object is placed at 2f

So, 2f =30 cm, f=15 cm

(b) u=15 cm, v=70 cm is incorrect. This is because if the object is at focus then the image is formed at infinity.

(c)In (lii) case, because the object is at the center of curvature.