If the linear magnification of the objective and eyepiece convex lenses of a compound microscope are 4 and 7…

Analyze the question

A compound microscope has two lenses: the objective and the eyepiece. We are given the magnification each produces and asked for the overall magnification of the instrument.

Concept

For a compound microscope, the total (overall) magnification is the product of the magnifications of the two lenses:

M = M_objective × M_eyepiece

This is the standard approach in JAMB, where "linear magnification" of each lens is given and the combined magnification is requested. The angular magnification of the eyepiece alone would be a separate idea, but the overall magnification of the microscope is the product of the two.

Solution

• Magnification of objective, M₀ = 4
• Magnification of eyepiece, Mₑ = 7

M = M₀ × Mₑ = 4 × 7 = 28

Answer: The overall magnification is 28, which is option D.

Why not the others?

• A (2) and B (3): not obtainable by any sensible combination of 4 and 7.
• C (11): this comes from adding 4 + 7, which is a common mistake. Magnifications of lenses in series multiply, they don't add.

Exam tip

When lenses are used in series (compound microscope, telescope with intermediate image), multiply the magnifications. Remember: "Microscope stages multiply." If you see an option that is the sum, it's usually the trap answer.