A standing wave pattern is formed when a plane electromagnetic wave from a transmitter falls on a perfect reflector at normal incidence. A receiver kept at a distance d from the reflector and along the line of normal incidence detects a minimum for a signal of frequency, =100 MHz . When the frequency is decreased to =99.9 MHz , the receiver records a maximum. Assuming c=3 10^8 ~ms ⁻¹ , the distance d is