X-rays and Ultrasound

OCR Applied Science • 3.1 Interactive Teaching Resource • Board Friendly Edition

1. The Atom and Electron Transitions

Key Ideas

  • Electrons exist in fixed energy levels.
  • Ground state: electron is in the lowest available energy level.
  • Excited state: electron has absorbed energy and moved up.
  • Ionised state: electron has gained enough energy to leave the atom.

Transition Rule

Electrons absorb energy to move up levels and emit photons when they fall down levels.

2. Electromagnetic Radiation and Photon Energy

EM Spectrum Explorer

X-rays

High frequency, short wavelength radiation used in medical imaging.

Pattern: moving from radio waves to gamma rays, wavelength decreases while frequency and photon energy increase.

Photon Energy

Electromagnetic radiation is transferred in packets of energy.

Quantum: a fixed packet of energy.

Photon: a quantum of electromagnetic radiation.

Exam link: higher frequency and shorter wavelength mean each photon has more energy.

3. X-ray Tube Simulator

What is happening?

Step 1: The heater warms the cathode. Electrons are released by thermionic emission.

Controls meaning

  • Higher current: more electrons hit the target each second, so intensity increases.
  • Higher voltage: electrons hit the target with more energy, so the X-rays are more penetrating.
Energy transfer: electrical energy → kinetic energy of electrons → X-ray radiation + heating of the target.

4. X-ray Attenuation

Attenuation Controls

Emergent intensity: 0.67 W m⁻²

5. Wave Motion

Wave Controls

Wave speed: 6 m s⁻¹

6. Ultrasound

Ultrasound Scan Story

Ready: The transducer sends a short pulse into the body, then listens for echoes.

  • Ultrasound is sound above 20 kHz.
  • The transducer both transmits and receives pulses.
  • Reflections occur at boundaries where acoustic impedance changes.
  • Large impedance change = stronger echo.

Why gel?

Air has a very different acoustic impedance from skin, so without gel most ultrasound reflects before entering the body.

7. Exam Practice Quiz

Question 1

What happens when an electron moves from an excited state to a lower energy level?

It absorbs a photon
It emits a photon
It becomes a neutron

Question 2

Increasing X-ray tube current mainly increases...

Beam intensity
Wavelength only
Acoustic impedance

Question 3

Why is gel used in ultrasound scans?

To cool the patient
To reduce reflection at the skin surface
To create X-rays