Question

5. Reflected waves for detecting objects: Airplane radar, bat echolocation. (Chapter 4, Exercise 15) Airplanes can be located by radar, which detects radio waves reflected back toward a transmitter. In analogous fashion, bats can locate insects by making high-pitched squeaks and listening for echoes. a) Using concepts from this chapter, can you explain why radar works much better with microwaves (typical wavelength 10 cm) than it would in the AM radio band (wavelengths more than 100 m)? b) Can you explain similarly why a bat can succeed [in finding food] using sound at ultrahigh frequencies (80 kHz, for example), but would fail miserably with 800 Hz signals?

Answer 1

a) It works on principle of wave reflection from a obstacle, if the size of obstacle is less than half of wave length wave can skip the object and does not reflect back.

If we use a wave of wave length 10cm, we can get the information of object with a resolution of 5cm width, hence any object whose thickness is more than 5cm can be detected.

If we use a wave of length 100m, we will get the information with resolvable width of 50m, there is a possibility of missing so many terristirial objects.

Hence for high resolution micriwaves are used in RADAR

b) Same is the case with bats, the insect size is in milimeters and less, hence they need a high frequency waves who's wavelength is very less.

For 80 kHz frequency wave length of Ultrasonic wave is nearly 4mm whereas for 800Hz it is 40 cm. Hence 800 Hz is not a useful wavelength for bats to detect insects.