Final answer:
The Doppler shift is a real effect, not a sensory illusion, observed when there is relative movement between a wave source and an observer. Aircraft should decrease speed when entering colder air to maintain a constant Mach number. Sonic booms occur when an object exceeds the speed of sound and might be heard without visual sighting of the plane. a) Real is correct option.
Step-by-step explanation:
The Doppler shift is a phenomenon that occurs when there is relative motion between a source of waves (like sound or light) and an observer. As the source approaches the observer, the observed frequency is higher than when it is moving away. This is true for any type of wave, including sound, light, and other electromagnetic waves. Therefore, the Doppler shift is real, not a sensory illusion.
When it comes to supersonic transport aircraft maintaining a constant Mach number, if the aircraft transitions from warm air to colder air where the speed of sound is lower, it should decrease its speed. This maintains the same ratio of the aircraft's speed to the new, lower speed of sound so the constant Mach number is preserved.
Occurrence of Sonic Boom
The occurrence of a sonic boom depends on the speed of the source. A sonic boom happens when an object moves through the air at a speed faster than sound, compressing air molecules and creating shock waves. These waves may reach an observer even when the plane itself is not visible, as it may have passed by before the boom is heard.