1. Given data

• Convex lens focal length: $f_1 = +30\ \text{cm}$
• Concave lens focal length: $f_2 = -120\ \text{cm}$
• Object distance from convex lens: $u_1 = -60\ \text{cm}$

A plane mirror is placed to the right of the lenses as shown, so the light passes through the two lenses, reflects from the mirror, and then passes back through the same lenses.

2. Image formed by the convex lens in first pass

Using lens formula,

for the convex lens,

So, the convex lens alone would form an image $60$ cm to its right.

3. Effect of the concave lens and plane mirror

From the usual arrangement for this standard problem, the image formed by the convex lens lies at the plane mirror. Hence, rays incident on the plane mirror are converging to a point on the mirror.

A plane mirror forms the image of that point at the same distance behind the mirror. Thus after reflection, the reflected rays retrace symmetrically and again pass through the concave lens and convex lens.

Because the object point for the mirror is exactly on the mirror, the mirror sends the rays back so that the optical system retraces its path. Therefore the final image is formed back at the original object position.

So the final image is a real image at a distance

from the convex lens.

4. Check options

• A: $60$ cm from the convex lens ✅
• B: $60$ cm from the concave lens ❌
• C: $70$ cm from the convex lens ❌
• D: $70$ cm from the concave lens ❌

5. Final answer

The final image is formed at $60$ cm from the convex lens.