Know that many important systems of communications rely on electromagnetic radiation including: (a) mobile phones (cell phones) and wireless internet use microwaves because microwaves can penetrate some walls and only require a short aerial for trans

3.3 Electromagnetic Spectrum

Learning Objective

Understand that many important communication systems rely on electromagnetic radiation and recognise which part of the spectrum each system uses:

• Mobile phones (cell phones) and wireless internet – microwaves
• Bluetooth – radio waves
• Optical fibres (cable television and high‑speed broadband) – visible light or infrared

Why Different Parts of the Spectrum are Used

1. Microwaves for Mobile Phones and Wireless Internet

Microwaves have wavelengths of a few centimetres to a few millimetres, corresponding to frequencies roughly between \$300\ \text{MHz}\$ and \$30\ \text{GHz}\$. Their key properties for mobile communications are:

• They can penetrate non‑metallic walls (brick, concrete, wood) with moderate attenuation.
• Because the wavelength is short, a compact aerial (antenna) can be used for both transmission and reception.
• Higher frequencies allow larger data‑rate capacity, which is essential for modern mobile broadband.

2. Radio Waves for Bluetooth

Bluetooth operates in the \$2.4\ \text{GHz}\$ ISM band, which is part of the radio‑wave region (frequencies from about \$30\ \text{kHz}\$ to \$300\ \text{GHz}\$). Important characteristics include:

• Radio waves readily pass through walls and other obstacles, though the signal strength is reduced.
• The relatively low power (typically \$< 1\ \text{mW}\$) limits range to a few metres, which is suitable for personal‑area networks.
• Using a standard frequency band simplifies device design and reduces cost.

3. Visible Light / Infrared for Optical Fibre

Optical fibres transmit light in the visible (\$400\text{–}700\ \text{nm}\$) and near‑infrared (\$800\text{–}1600\ \text{nm}\$) regions. Glass is highly transparent to these wavelengths, giving the following advantages:

• Very low attenuation – signals can travel many kilometres without amplification.
• Immune to electromagnetic interference, providing high‑quality data transmission.
• Extremely high bandwidth, supporting cable television and high‑speed broadband services.

Comparison of Frequency Ranges and Typical Uses

Key Points to Remember

1. Microwaves can pass through many building materials and need only short antennas, making them ideal for mobile and Wi‑Fi communications.
2. Radio waves are excellent for short‑range, low‑power links such as Bluetooth because they easily penetrate walls, though the signal weakens.
3. Glass is transparent to visible and infrared light, allowing optical fibres to carry huge amounts of data over long distances with minimal loss.