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If the wavelength of light is 4000 A, then the number of waves in 1 mm length will be _____.

Asked in GUJCET 2015 · Wavelength, frequency and photon energy

• (1) 2500
• (2) 25
• (3) 250
• (4) 25000

Step-by-step solution

Why the other options are wrong

• (2) comes from taking 1 A=10⁻⁸ m, so λ=4×10⁻⁵ m and N=(10⁻³)/(4×10⁻⁵)=25. An angstrom is 10⁻¹⁰ m.
• (3) comes from taking 1 A=10⁻⁹ m, one power too large: λ=4×10⁻⁶ m gives N=250.
• (4) comes from measuring the length as 1 cm instead of 1 mm: (10⁻²)/(4×10⁻⁷)=25000.

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