A microwave oven operates at 2.45 GHz. (a) Find the wavelength. (b) Explain why food heats but the glass turntable stays cool, and why metal must not be put inside. (c) A police radar gun uses 10.5 GHz microwaves; explain in one line how it measures speed. Step 1: λ = c/f = 3.00×10⁸/2.45×10⁹ ≈ 0.122 m ≈ 12 cm. Standing waves of this size fit in the oven cavity — the turntable rotates food through hot spots. Step 2: Microwaves heat by rotating polar molecules (mainly water); glass and dry ceramic have few mobile polar molecules, so they absorb little. Metal reflects microwaves and sharp edges concentrate the field enough to spark (arcing) — hence no metal inside. Step 3: The radar measures the Doppler shift of the reflected microwaves: the frequency shift is proportional to the car's speed toward/away from the gun.