Assertion–Reason: Relationship between TE10 and TE01 in rectangular waveguides Assertion (A): In a rectangular waveguide, the TE01 mode is simply the TE10 mode rotated by 90°. Reason (R): Rotating a rectangular waveguide by 90° interchanges the broad (a) and narrow (b) dimensions.

Introduction / Context:
Mode indices (m, n) in a rectangular waveguide count field variations across the broad and narrow dimensions. TE10 and TE01 are closely related by symmetry and physical rotation of the guide, which swaps the roles of a and b.

Given Data / Assumptions:

• Dimensions: a (broad), b (narrow).
• Standard TE mode definitions: TE10 has one half-wave across a and none across b; TE01 has none across a and one across b.

Concept / Approach:
Physically rotating the guide by 90° interchanges a and b. The field pattern for TE10 in the rotated guide corresponds to TE01 in the original labeling. Hence the assertion is true, and the reason correctly explains it.

Step-by-Step Solution:

Verification / Alternative check:
Cutoff wavelengths confirm symmetry: λ_c(TE10) = 2a and λ_c(TE01) = 2b; rotation exchanges these values consistently.

Why Other Options Are Wrong:

• B: The reason directly explains the assertion; it is not incidental.
• C/D: Incorrectly negate a true relationship derived from symmetry.

Common Pitfalls:
Mixing TE and TM families; assuming rotation changes the mode family instead of relabeling indices.

Final Answer:
Both A and R are correct and R is correct explanation of A