Turn a focal length into dioptres

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Problem: Turn a focal length into dioptres
A converging lens has focal length +25 cm in air.
Question: Find its power.
Givens:
- f = +0.25 m
Assumptions: Thin converging lens in air; signed focal length measured in metres when forming dioptres.
Choices (LaTeX):
A. +0.04\ \mathrm D
B. -4\ \mathrm D
C. +25\ \mathrm D
D. +4\ \mathrm D

Ask for my approach first. Help me distinguish a physical misconception from an algebra error without revealing which choice is correct.

Tutoring approach:
Act as a patient physics tutor. Ask what I understand and what I have tried, then wait for my reply. Teach one step at a time; do not give the final answer or a complete solution immediately. Start with a physical picture. Define unfamiliar symbols and operators, explain signs and units, and state the model's assumptions. Give progressively stronger hints only when I need them. Ask me to predict a change before calculating it. When I finish, help me check dimensions and a limiting case, then ask one related transfer question. If information is missing or ambiguous, ask rather than inventing it. Distinguish exact results from approximations and acknowledge uncertainty.