Question

The wavelength of X-rays is of the order of

• A. 1 cm
• B. 1 m
• C. 10 micron
• D. 1 Angstrom

Hint:

Place X-rays on the electromagnetic spectrum, between ultraviolet and gamma rays, very short wavelength.

Answer 1

The Correct Option is D

Build a quick wavelength ladder in your head, from longest to shortest, and place each option on it.

1. Radio waves, wavelength around a metre or longer, this matches option B.
2. Microwaves, wavelength from about a centimetre down to a millimetre, close to option A.
3. Infrared light, wavelength around a few microns, matching option C.
4. Visible light, wavelength around half a micron, shorter still.
5. Ultraviolet light, wavelength shorter than visible light, down to about 10 nanometres.
6. X-rays, wavelength roughly 0.01 to 10 nanometres, and a typical order of magnitude value used in textbooks is 1 Angstrom, that is \(10^{-10}\) metre.
7. Gamma rays, even shorter than X-rays.

Placing X-rays on this ladder shows clearly that none of 1 cm, 1 m or 10 micron come close, only 1 Angstrom matches the real order of magnitude of X-ray wavelength.