Question

the two thin plano-convex lenses, each of focal length 40cm are combined to form a bioconvex lens. how far from thislens should an object be placed such that its inverted image of unit magnification is formed on the other side of the lens?

Answer 1

Answer:

f=40cm

feq=

80

40×40

=20cm

the image formed when object is placed at the center of curvature is real, inverted and unitly magnified.

c=2f=2×20=40cm

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Answer 2

Given:

Focal length, f = + 40 cm

Magnification, m = - 1 (as the image will be real and inverted due to convex lens)

To Find:

Object distance, i.e., u

Calculation:

- Applying the relation between magnification, focal length and object distance, we get:

m = f / (f + u)

⇒ u = f × (1 - m) / m

⇒ u = 40 × {1 - (-1)} / (-1)

⇒ u = - 80 cm

- So the object should be placed at a distance of 80 cm from the lens to get the required image.