The distance between repeating parts of the pattern (the length of each single wave) is known as the wavelength (l). Wavelengths can be measured from anywhere to the next identical part of the wave.
Frequency is a measure of how often something happens in one second. The symbol of frequency is f and the units are hertz (Hz). The relationship between velocity, frequency and wavelength is:
Velocity (m/s) = frequency (Hz) x wavelength (m)
Light waves travel much faster than sound waves. The speed of a set of waves can be found by using information about the distance travelled and time taken or from information about the frequency and wavelength.
Distance, speed and time are connected by:
Speed (m/s) = Distance (m) / time (s)
Example An echo takes 1.5 s to reflect from the front wall of a school building 250m away. What is the speed of sound in air?
Distance for echo = 2 x 250 m = 500 m Time taken = 1.5 s
Speed = distance/time = 500/1.5 = 333m/s
This method is practical at school only for the measurement of the speed of sound.
Example Capital Radio broadcasts throughout the London area using radio waves of frequency 1548kHz, travelling at 300000km/s. To what wavelength would you tune a radio to receive Capital programmes?
Speed = 300000km/s = 300000000m/s Frequency = 1548 kHz (1 548 000 Hz)
Wavelength = speed/frequency = 300 000 000/1 548 000 = 194 m
Using your knowledge of the wave equation, and the relationship between frequency and time [f = 1/T], answer the following questions:-
- If the waves in the ocean are timed so that they come to shore every 1.74 seconds, what is the frequency of these waves? (Show your working and don’t forget units).
- A radio wave has a frequency of 2 MHz (2 MegaHertz = 2 000 000 Hz). Calculate the time period between successive waves.
- A radio station broadcasts with a wavelength of 160m. If the velocity of the radio signal is 3 x 108 m/s, calculate the frequency of the wave.
- If all the 1.2 billion people in China jump up and down at the same time, they will create a shock wave that will travel around the Earth in all directions. This wave will focus and peak on the other side of the Earth, exactly opposite China (around California). If the circumference of the Earth is 40 000km and the speed of sound on the Earth’s surface is 2850m/s, calculate how long it will take for California to crumble into the sea due to the shock wave.
Here are some more practice questions for you to try. We will then have an assessment on this next week. Wave equation practice questions
And here are the answers: Grade_9_Integrated_Science_1_docwaves_practice_questions_mark_scheme