Required practical activity 8: make observations to identify the…

The idea

Required practical 8 measures waves in two places. In a ripple tank, a motorised dipper makes straight ripples whose shadows fall on paper below; freeze them with a stroboscope or a photograph, measure across ten wavelengths with a rule and divide by ten, read the frequency from the motor's signal generator or count ripples passing a mark over 10 s, then use wave speed = frequency × wavelength. For waves in a solid, a vibration generator drives a stretched string until a clear standing pattern forms; the distance between adjacent nodes is half a wavelength, so λ = 2 × that spacing, and the frequency is read from the signal generator. Repeat each measurement and take a mean, and quote the resolution of the rule you used when you report the wavelength.

What earns the marks

• Measuring across ten wavelengths and dividing by ten.
• Using a stroboscope or a photograph to freeze the ripple pattern before measuring.
• Reading the frequency from the signal generator, or counting waves passing a point in a measured time.
• Stating that the wavelength on the string is twice the distance between adjacent nodes.
• Calculating the speed using wave speed = frequency × wavelength.

Common mistakes and how to put them right

• Measuring a single ripple, so the percentage uncertainty in the wavelength is large.

• Taking the node-to-node spacing on a vibrating string as a whole wavelength.

• Counting waves for one second by eye instead of counting over ten seconds and dividing.

• Forgetting to convert the measured wavelength from centimetres to metres before finding the speed.

Exam tips

• Name the apparatus precisely: ripple tank, dipper, signal generator, stroboscope, metre rule.
• Say how you reduce uncertainty, by measuring many waves or many oscillations, not by 'being careful'.
• Quote the equation you will use before describing the measurements, so the plan reads purposefully.