An 8 astronomical telescope has a 30 cm focallength objectiv

An 8 astronomical telescope has a 30 cm focal-length objective lens. After looking at stars, an astronomer moves the eyepiece 1.0 cm farther away from the objective to focus on nearer objects. What is the distance to the nearer objects?

Solution

The lens equation is : 1/v + 1/u = 1/f

For far away objects : u = infinity

So, 1/u = 0

So, f = v = 30 cm <----- image location is at focal plane

After moving the eyepiece 1 cm away, the image distance = v = 30 +1 = 31 cm

So, 1/u + 1/31 = 1/30

So, u = 930 m <-------answer