The nature and properties of X-rays

Overview

Speed of X-rays in a vacuum, what is meant by saying that X-rays are ionising and how X-rays demonstrate wave behaviour.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Where do X-rays sit in the electromagnetic spectrum?

   Model answer

   X-rays lie between ultraviolet and gamma radiation, with wavelengths from about 10⁻⁸ m down to about 10⁻¹³ m. Their photon energies are far higher than those of visible light.

2. 2

   State the speed of X-rays in a vacuum.

   Model answer

   X-rays travel at 3.00 × 10⁸ m s⁻¹ in a vacuum, the same speed as all electromagnetic waves.

3. 3

   Why are X-rays undeflected by electric and magnetic fields?

   Model answer

   X-rays are electromagnetic waves and carry no electric charge. A field can only exert a force on a charged particle.

4. 4

   Explain what is meant by saying that X-rays are ionising.

   Model answer

   An X-ray photon carries enough energy to remove an electron from an atom it strikes, leaving a positive ion. This is the property that makes X-rays both useful for detection and hazardous to tissue.

5. 5

   Calculate the energy in joules of an X-ray photon of wavelength 1.0 × 10⁻¹⁰ m.

   Model answer

   E = hc/λ = (6.63 × 10⁻³⁴ × 3.00 × 10⁸)/(1.0 × 10⁻¹⁰) = 1.99 × 10⁻¹⁵ J. Working back, hc/(1.99 × 10⁻¹⁵) = 1.0 × 10⁻¹⁰ m.

6. 6

   Express the energy of a 1.99 × 10⁻¹⁵ J X-ray photon in kiloelectronvolts.

   Model answer

   Dividing by the electronic charge gives 1.99 × 10⁻¹⁵/1.60 × 10⁻¹⁹ = 1.24 × 10⁴ eV, which is 12.4 keV. Multiplying 1.24 × 10⁴ by 1.60 × 10⁻¹⁹ returns 1.99 × 10⁻¹⁵ J.

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