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The de Broglie wavelength of a ball of mass 120 g moving with a speed of 20 m s⁻¹ is ______.

Asked in GSEB Board August 2020 · de Broglie relation and its graphs

• (1) 2.76×10⁻³⁴ A
• (2) 2.76×10⁻³⁴ m
• (3) 2.76×10⁻²⁴ m
• (4) 2.76×10²⁴ A

Step-by-step solution

Why the other options are wrong

• (1) Right number, wrong unit: λ = h/(mv) in SI units comes out in metres. The same length in angstrom is 2.76×10⁻²⁴ A, since 1 m = 10¹⁰ A.
• (3) 2.76×10⁻²⁴ is the wavelength expressed in angstrom but labelled in metres -- the number was converted and the unit was not.
• (4) The power of ten has the wrong sign: 2.76×10⁻³⁴ m is 2.76×10⁻²⁴ A, not 10²⁴. A positive power would make a ball's wavelength astronomically large.

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