How Does Wi-Fi Pass Through Walls?

You can’t see through a wall, yet your phone can often connect to a Wi-Fi router in another room. How can a signal pass through something that blocks light?

Wi-Fi is radio

Wi-Fi sends information using radio waves, which are a kind of electromagnetic wave. Visible light is also an electromagnetic wave, but radio waves have a much longer wavelength, and that changes how they behave when they meet different materials. Wi-Fi mostly uses the 2.4 and 5 gigahertz bands. Whether a wave passes through a material depends on how that material responds to that particular wavelength, which is why a wall can block light and still let radio waves through.

What walls do to the signal

Materials such as wood, plasterboard and glass let radio waves through, although the signal gets weaker as it does. Brick and concrete weaken it more, especially thick walls. Metal is the biggest problem. It reflects radio waves, so metal cupboards, lift shafts and reinforced concrete can block a signal strongly. Water absorbs these waves too, so a large fish tank or a crowded room can also reduce the signal.

Distance and bouncing

The signal also weakens with distance, spreading out as it travels. Radio waves bounce off surfaces as well, so what reaches your phone is often a mixture of the direct signal and several reflections. The 5 gigahertz band can carry data faster, but walls generally absorb it more and it tends not to reach as far as 2.4 gigahertz.

Getting a better signal

• Put the router in a central, open position, rather than in a cupboard or behind the television.
• Keep it away from metal objects, thick walls and appliances such as microwave ovens, which use a similar frequency band.
• Raise it off the floor if you can.
• In a large home, consider adding a second access point or a mesh system.

Key points

• Wi-Fi uses radio waves, which pass through many materials that block visible light.
• Metal and thick concrete block the signal most, and wood and glass block it least.
• The signal weakens with distance and with every obstacle in the way.
• Where you place the router matters.