Microwaves have longer wavelength than visible light, and a 20 cm slit is wider than what you would use to observe diffraction from visible light (where the slit would need a width similar to a hair). Microwaves will produce the same type of diffraction pattern as discussed in the background for this Investigation, where the equation

Along wire of diameter 1 mm is submerged in an oil bath of temperature ∞ 31°c. the wire has an electrical resistance per unit length of 0.01 ω/m. if a current of 103 a flows through the wire and the convection coefficient is 500 w/m2·k, what is the steady-state temperature of the wire? from the time the current is applied, how long does it take for the wire to reach a temperature that is within 1°c of the steady-state value? the properties of the wire are 8000 kg/m3, 500 j/kg·k, and 20 w/m·k.