Waves and Electromagnetic Radiation
Sound waves
Say why sound needs a medium, use v = λν and ν = 1/T, work out an echo distance, and place a frequency in the infrasonic, audible or ultrasonic band.
Put a ringing electric bell inside a jar and pump the air out. The clapper can still be seen moving, but the sound fades to nothing; let the air back in and it returns. Sound needs something to travel through — a solid, a liquid or a gas — and what travels is energy passed from particle to particle, not the particles themselves.
Four quantities, one equation
Wavelength λ is the distance from one crest to the next, in meters. Frequency ν is how many density oscillations pass a fixed point each second, in hertz. Time period T is how long one oscillation takes, in seconds, and ν = 1 ÷ T. Multiply the first two and you get the speed: v = λ × ν.
Have a play
Two sound waves, drawn one above the other.
Tap any part of the picture.
Worked example
How far away does a wall have to be before you can hear its echo? Take the speed of sound as 340 m/s.
Start from the ear, not the wall: two sounds have to arrive at least 0.1 s apart to be heard as two.
Closer than that and the brain merges the reflection into the original. This is a fact about hearing, and it is what sets the distance.
Try it together
Now work between period, frequency, wavelength and speed.
A sound wave in air has a time period of 0.004 s. Answer with a bare number each time.
1.Use ν = 1 ÷ T. If the time period is 0.004 s, what is the frequency, in Hz?
Have a go
Have a go on your own. A wave has a wavelength of 1.5 m and a frequency of 220 Hz. What is its speed, in m/s? Answer with a number.
Ready to practice?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.