Question

the wavelength of ultrasonic waves in air is of the order of

Answer 1

The  frequency, wavelength, and speed of ultrasonic waves in air are related as

v = λ * f

So, λ = v / f

The speed of sound in air is 340 m/s and frequency of ultrasonic waves is 68000 Hz, so we get

λ = 340 / 68000

λ = 5 x 10⁻³ m

λ = 5 x 10⁻⁵ cm

Answer 2

Ultrasonic waves are of frequencies above the upper limit of audible frequency
I mean , frequencies of ultrasonic waves > 20,000 Hz
We can't accurate value of it , So, let's assume frequency of ultrasonic wave = 68,000 Hz [ this is because speed of sound is 340 m/s and 68 Is divisible by 34 ]
Now, we know, speed of sound = frequency × wavelength
As I mentioned , speed of sound = 340 m/s
frequency of ultrasonic wave = 68,000 Hz
Now, wavelength = 340/68,000 = 1/2 × 10⁻² m
= 0.5 × 10⁻² m
= 5 × 10⁻⁵ cm

Hence, answer should be 5 × 10⁻⁵ cm