Chapter 8: Microwaves — From Radar to Your Kitchen
8.1 RADAR: Seeing with Radio Waves
RADAR stands for RAdio Detection And Ranging. Developed during WWII, it remains one of the most important microwave applications.
A radar sends a short pulse of microwaves. When it hits an object, some energy bounces back. By measuring the time, direction, and frequency shift (Doppler effect), radar determines an object's distance, direction, and speed.
\[ d = \frac{c \times t}{2} \]
The factor of 2 is because the signal travels to the target and back.
| Radar Application | How It Uses Microwaves |
|---|---|
| Air Traffic Control | Tracks positions and altitudes of all aircraft |
| Weather Radar | Detects rain, snow, storms via water droplet reflections |
| Speed Cameras | Uses Doppler shift to measure vehicle speed |
| Ship Navigation | Helps ships avoid collisions in fog |
8.2 The Microwave Oven: An Accidental Discovery
In 1945, engineer Percy Spencer noticed a chocolate bar melting in his pocket near radar equipment. He tested popcorn — it popped! The microwave oven was born.
Microwave ovens work at 2.45 GHz. Water molecules are polar (like tiny magnets). The oscillating electric field makes them spin billions of times per second. This molecular friction generates heat.
📊 Diagram: A polar water molecule (H₂O) flipping back and forth in response to the alternating electric field of the microwave radiation, generating heat through molecular friction.
🧠 Think About It: Dry plates stay cool in the microwave but wet food gets hot — microwaves primarily heat water! Never run a microwave empty — the energy bounces around and can damage the magnetron.