EM Waves & Matter

Interaction of EM Waves with Matter

Electromagnetic (EM) waves interact with matter in three main ways: reflection, absorption, and transmission.

• Reflection: EM waves bounce off the surface of a material. For example, visible light reflects off a mirror, allowing us to see our reflection.
• Absorption: EM waves transfer their energy to the material, often causing heating. A black surface absorbs most EM radiation, while a white surface reflects most.
• Transmission: EM waves pass through a material without being absorbed. For example, visible light passes through glass, making it transparent.

The effect of EM waves depends on the type of material:

• Opaque materials absorb or reflect EM waves, so little or no transmission occurs.
• Transparent materials allow transmission of certain EM waves (like visible light through glass).
• Translucent materials scatter EM waves, allowing some transmission but not clear images.

Different substances interact differently with EM waves depending on their atomic and molecular structure. For example, ultraviolet (UV) light is mostly absorbed by the ozone layer in the atmosphere, protecting life on Earth.

For instance, when sunlight hits a leaf:

• Some light is reflected, giving the leaf its green colour.
• Some light is absorbed, powering photosynthesis.
• Some light is transmitted through the leaf.

EM Waves and Atomic Structure

EM waves can cause electrons in atoms to move between energy levels. This happens through absorption or emission of radiation:

• Electron transitions: Electrons exist in specific energy levels (or shells) around the nucleus. When an electron absorbs EM radiation with the right energy, it jumps to a higher energy level (excited state).
• Absorption of radiation: Occurs when an electron gains energy from an EM wave and moves to a higher energy level.
• Emission of radiation: When an electron falls back to a lower energy level, it emits EM radiation with energy equal to the difference between the two levels.

This process explains phenomena like the colours seen in neon lights or the absorption lines in the Sun’s spectrum.

The energy of the EM wave absorbed or emitted corresponds to the difference between energy levels:

$$E = hf$$

where $E$ is the energy difference, $h$ is Planck’s constant, and $f$ is the frequency of the EM wave.

For example, if an electron absorbs a photon of frequency $5 \times 10^{14}$ Hz, the energy absorbed is:

$$E = 6.63 \times 10^{-34} \times 5 \times 10^{14} = 3.315 \times 10^{-19} \text{ J}$$

Effects of EM Waves on Matter

Different parts of the EM spectrum affect matter in different ways:

• Infrared radiation: Mainly causes heating. When IR waves are absorbed by a material, their energy increases the vibration of atoms and molecules, raising the temperature.
• Ultraviolet (UV), X-rays, and gamma rays: Have enough energy to ionise atoms and molecules. Ionisation means removing electrons, which can damage or change cells.

Biological impacts include:

• Infrared radiation can cause burns if intense enough.
• UV radiation can cause skin damage and increase the risk of skin cancer by damaging DNA.
• X-rays and gamma rays are used in medicine but can also cause cell damage and mutations if exposure is too high.

For example, sunlight contains UV radiation, which can cause sunburn if skin is exposed for too long without protection.

Applications of EM Waves

EM waves have many practical uses in medicine, communication, and remote sensing.

• Medical imaging and treatments:
  • X-rays are used to create images of bones inside the body.
  • Infrared cameras detect heat patterns for diagnosis.
  • Ultraviolet light is used to sterilise equipment.
  • Laser treatments use focused light to remove or repair tissue.
• Communication technologies:
  • Radio waves carry radio and TV signals.
  • Microwaves are used for mobile phone signals and satellite communication.
  • Infrared is used in remote controls and short-range communication.
• Remote sensing:
  • Satellites use microwaves and infrared to monitor weather and the environment.
  • Radar uses radio waves to detect objects and measure speed.