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For a radiation of 6 GHz passing through air, the wave number (number of waves) per 1 m length is _______ (1 GHz=10⁹ Hz).

Asked in GSEB Board March 2019 · Wavelength, frequency and photon energy

• (1) 5
• (2) 3
• (3) 20
• (4) 30

Step-by-step solution

Why the other options are wrong

• (1) 5 is the wavelength in centimetres: λ=c/u=0.05 m=5 cm. The question asks how many such waves fit in a metre, not how long one is.
• (2) 3 is only the mantissa of c; no step here produces it. Sanity check: 1/λ=u/c=(6×10⁹)/(3×10⁸)=20.
• (4) 30 is the count for a 9 GHz radiation. At 6 GHz each wave is longer, so fewer of them fit into a metre.

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