Question

a teacher uses a convex lens of focal length 20cm in a projector to project the image of a slide on the laboratory wall she kept the projector at a distance of 70 cm from the wall find the distance between the lens and the slide

Answer 1

given,
image distance, v = 70 cm
focal length, f=20cm

we know that,
1/v-1/u=1/f
therefore,
1/70-1/u=1/20

1/70-1/2o=1/u

20-70/140=1/u

-50/140=1/u

-5/14=1/u

-14/5=u

-2.8cm=u



hope it helps you