Electromagnetic spectrum, imaging and applications Notes

The Electromagnetic Spectrum Organizes All Types Of "Light"

1. Light is only a small part of a much bigger family of waves.
2. The electromagnetic spectrum is the continuous range of all electromagnetic (EM) waves, from extremely long wavelengths (radio) to extremely short wavelengths (gamma rays).
3. Even when we cannot see a wave, it still exists and can be useful.
4. A key feature of EM waves is that, in a vacuum, they all travel at the same speed, the speed of light: $$c = 3.0 \times 10^8 \text{ m s}^{-1}$$
5. They are also all transverse waves, meaning the oscillations are perpendicular to the direction the wave travels.

Wavelength, Frequency, And Energy Are Linked

1. Electromagnetic waves are described using measurable wave quantities.
2. Wavelength is the distance between two corresponding points on a wave.
3. Frequency is the number of wave cycles passing a point each second.
4. For electromagnetic waves in a vacuum:
   1. Speed is constant
   2. Wavelength and frequency are inversely related
5. For any wave, $$c = f\lambda$$
6. So if $c$ is fixed (as it is for EM waves in a vacuum), then shorter $\lambda$ implies larger $f$, and longer $\lambda$ implies smaller $f$.

Order of the Electromagnetic Spectrum

1. The spectrum is ordered systematically.
2. From longest wavelength / lowest frequency to shortest wavelength / highest frequency:
   1. Radio waves
   2. Microwaves
   3. Infrared
   4. Visible light
   5. Ultraviolet
   6. X-rays
   7. Gamma rays
3. As you move along the spectrum:
   1. Wavelength decreases
   2. Frequency increases
   3. Energy carried by the wave increases

Visible Light Is A Small Window (400–700 nm)

1. Visible light is a small but important part of the spectrum.
2. Visible light is the only part of the electromagnetic spectrum that the human eye can detect.
3. It has wavelengths approximately between 400 nm (violet) and 700 nm (red).
4. Different colours correspond to different wavelengths.
5. Within this range:
   1. Violet is around 400 nm (shorter wavelength, higher frequency)
   2. Red is around 700 nm (longer wavelength, lower frequency)

Infrared Imaging Detects Thermal Radiation From Warm Objects

1. Just beyond red light (longer wavelength than 700 nm) is infrared (IR), meaning "below red".
2. IR is invisible to humans, but many cameras can detect near-infrared.
3. Infrared radiation is closely linked to temperature.
4. Infrared radiation has longer wavelengths than visible light.
5. All objects above absolute zero emit infrared radiation.
6. The hotter an object is, the more infrared radiation it emits.
7. Infrared radiation transfers thermal energy.

Ultraviolet Radiation has higher energy than visible light

1. Ultraviolet radiation has shorter wavelengths than visible light.
2. It carries more energy per wave.
3. Ultraviolet radiation is divided into:
   1. UVA
   2. UVB
4. Only some ultraviolet radiation reaches Earth’s surface.
5. UVB radiation can damage living tissue.
6. High exposure can cause:
   1. Skin cancer
   2. Eye damage
7. Human activities once reduced ozone levels.

Microwaves And Radio Waves Diffract, Enabling Communication

1. At longer wavelengths than infrared, we find microwaves and then radio waves.
   1. Microwaves: roughly from about 1 mm to 1 m wavelength.
   2. Radio waves: typically longer than 1 m (the boundary can overlap in different conventions).
2. One reason these long-wavelength waves are so useful is diffraction.
3. Waves with longer wavelengths diffract more, meaning they can bend and spread out around obstacles.
4. Because of diffraction, radio and microwaves can often be detected even when there is not a clear line of sight to the transmitter.
5. Microwaves are widely used for:
   1. mobile phone networks (for example common 4G/5G frequency bands)
   2. Wi-Fi and Bluetooth
   3. satellite communication (some microwave frequencies pass through the atmosphere well)
6. However, many microwave wavelengths are absorbed by water in the air, so some microwave signals are better for short-range links.
7. Other microwave frequencies are absorbed less and can travel further, supporting satellites or tall-mast communication.
8. Radio waves can also reflect from the ionosphere, increasing their range to thousands of kilometers.

X-Rays And Gamma Rays Have Higher Energy And Greater Risk

1. At even shorter wavelengths come X-rays and gamma rays.
2. X-rays are produced mainly by fast electrons (for example in an X-ray tube) and are used for medical imaging.
3. Gamma rays are produced in nuclear transitions (from the nucleus) and are useful in sterilization and cancer treatment, but require strong shielding.

Imaging Across The Spectrum Depends On Interaction With Matter

1. Visible imaging: depends on reflection and absorption of visible wavelengths by surfaces.
2. Infrared (thermal) imaging: depends on emission of IR due to temperature differences.
3. Microwave imaging and radar: depends on reflected microwaves, and can work in conditions where visible light is scattered (for example fog or cloud).
4. X-ray imaging: depends on differential absorption, dense materials (bone, metal) absorb more.

Ionizing Radiation and Safety

1. Some electromagnetic waves are ionizing.
2. X-rays and gamma rays are classified as ionizing radiation.
3. Ionizing radiation can remove electrons from atoms.
4. This can damage cells and DNA.
5. Exposure must be carefully controlled in medical and industrial uses.

Why Different Regions of the Spectrum Have Different Uses

1. Longer wavelengths interact less strongly with matter.
2. Shorter wavelengths penetrate materials more deeply.
3. Higher-energy waves carry greater risks but are useful for imaging.

Maxwell's Insight Linked Light To Electromagnetic Waves

1. Historically, people once thought light travelled instantaneously.
2. Later investigations showed it had a finite speed.
3. In the 19th century, James Clerk Maxwell showed that a wave made of oscillating electric and magnetic fields would travel at a speed very close to the measured speed of light.
4. He concluded that light is an electromagnetic wave, and that other EM waves should exist as well.

Why Ultrasound Is Suitable for Medical Imaging?

1. Ultrasound waves are non-ionizing, so they do not damage cells.
2. This makes ultrasound safer than X-rays or gamma rays for repeated use.
3. Ultrasound is especially useful for imaging soft tissues.
4. It is commonly used during pregnancy to monitor fetal development.