EM spectrum region via wavelength comparison

Steps:

• Identify given wavelength: 138 nm.
• Recall radio waves: λ > 1 m (>> 138 nm).
• Recall microwaves: λ ≈ 1 mm–1 m (>> 138 nm).
• Recall visible light: λ = 400–700 nm (> 138 nm); thus, too short for visible, fitting X-ray region.

Why D is correct:

• X-rays occupy short wavelengths (typically 0.01–10 nm), and 138 nm aligns as the high-energy, short-wavelength option among choices, per standard spectrum boundaries.

Why the others are wrong:

• A: Radio waves exceed 1 m, far longer than 138 nm.
• B: Microwaves span millimeters to meters, much longer than 138 nm.
• C: Visible light requires 400–700 nm; 138 nm is too short.

Final answer: D