Science (086) — AI-generated practice question

The student's claim is correct, provided both lenses are thin and placed in perfect contact.

Using the concept of power of a lens:

The net power of lenses placed in contact is the algebraic sum of individual powers:

$$P = P_1 + P_2 = (-4\text{ D}) + (+4\text{ D}) = 0\text{ D}$$

A power of 0 D means the focal length is infinite, so the combination neither converges nor diverges light. Light passes through without any bending, exactly like a flat glass plate.

Assumption that must hold: Both lenses must be thin lenses placed in perfect contact, so that the formula $P = P_1 + P_2$ applies.

Source: Light – Reflection and Refraction, Section 9.3.8 (Power of a Lens)

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• Examiners want: (1) correct use of $P = P_1 + P_2$, (2) the conclusion that P = 0 D → no bending, and (3) the thin-lens-in-contact assumption stated explicitly.
• Do not just say "they cancel out" — show the calculation.
• The word "flat glass plate" in the question is a conceptual description of infinite focal length; link it clearly to P = 0.