OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Wave behaviour: apply formulae relating velocity, frequency and wavelength
Applying the wave formulae means selecting v = f × λ and rearranging it to suit the quantity you need.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 5 of 12
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Applying the wave formulae means selecting v = f × λ and rearranging it to suit the quantity you need. Velocity v is in metres per second (m/s), frequency f in hertz (Hz, equivalent to s⁻¹) and wavelength λ in metres (m). To find velocity, multiply frequency by wavelength: 200 Hz × 1.5 m = 300 m/s. To find frequency, divide velocity by wavelength: 340 m/s ÷ 0.5 m = 680 Hz. To find wavelength, divide velocity by frequency: 3.0 × 10⁸ m/s ÷ 6.0 × 10¹⁴ Hz = 5.0 × 10⁻⁷ m. Convert prefixes first, so 2 kHz becomes 2000 Hz and 50 cm becomes 0.5 m, then substitute. Keep powers of ten in standard form and check that the answer's unit matches the quantity requested.
What earns the marks
- Selects the formula v = f × λ and rearranges it correctly for the quantity required.
- Converts values into SI units before substituting, for example kilohertz to hertz and centimetres to metres.
- Calculates velocity by multiplying frequency by wavelength and gives the result in metres per second.
- Calculates frequency by dividing velocity by wavelength and gives the result in hertz.
- Calculates wavelength by dividing velocity by frequency and gives the result in metres.
- Handles powers of ten correctly, for example 3.0 × 10⁸ m/s ÷ 6.0 × 10¹⁴ Hz = 5.0 × 10⁻⁷ m.
Common mistakes and how to put them right
Multiplies velocity by wavelength to find frequency.
Put it right: Divide velocity by wavelength instead.
Leaves a value in kilohertz or centimetres when substituting.
Put it right: Convert to hertz and metres first.
Inverts the powers of ten when dividing, giving a positive index where a negative one belongs.
Put it right: Subtract the indices so 10⁸ ÷ 10¹⁴ = 10⁻⁶.
Omits the unit on the final answer.
Put it right: State hertz, metres or metres per second as appropriate.
Exam tips
- Write the rearranged formula with the subject isolated before inserting numbers.
- Convert all prefixes to SI units at the start to avoid unit slips.
- Estimate the answer first so a misplaced power of ten is obvious.
Try the lesson
The start of the Wave behaviour: apply formulae relating velocity, frequency and wavelength lesson, made from checked teaching notes for this specification point.
Have a go first
A wave has a frequency of 10 Hz and a wavelength of 4 m. Using v = f × λ, what is its velocity?
Listen
Applying the wave formulae means selecting v = f × λ and rearranging it to suit the quantity you need. Velocity v is in metres per second (m/s), frequency f in hertz (Hz, equivalent to s⁻¹) and wavelength λ in metres (m). To find velocity, multiply frequency by wavelength: 200 Hz × 1.5 m = 300 m/s. To find frequency, divide velocity by wavelength: 340 m/s ÷ 0.5 m = 680 Hz. To find wavelength, divide velocity by frequency: 3.0 × 10⁸ m/s ÷ 6.0 × 10¹⁴ Hz = 5.0 × 10⁻⁷ m. Convert prefixes first, so 2 kHz becomes 2000 Hz and 50 cm becomes 0.5 m, then substitute. Keep powers of ten in standard form and check that the answer's unit matches the quantity requested. A strong answer earns marks for these points. Selects the formula v = f × λ and rearranges it correctly for the quantity required. Converts values into SI units before substituting, for example kilohertz to hertz and centimetres to metres. Calculates velocity by multiplying frequency by wavelength and gives the result in metres per second. Calculates frequency by dividing velocity by wavelength and gives the result in hertz. Exam tip: Write the rearranged formula with the subject isolated before inserting numbers.
See it
Key words
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Rearranging v = f × λ
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SI units and prefix conversion
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Calculations with powers of ten
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Selecting the correct formula for the required quantity
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See every definition
- Rearranging v = f × λ
- Selecting the formula and rearranging it to suit the quantity you need, whether velocity, frequency or wavelength.
- SI units and prefix conversion
- Converting prefixes first, so kilohertz becomes hertz and centimetres become metres, before substituting values.
- Calculations with powers of ten
- Keeping powers of ten in standard form and handling them correctly when dividing, so the answer's unit matches the quantity requested.
- Selecting the correct formula for the required quantity
- Choosing the form of the wave formula that gives the quantity needed, then substituting values converted into SI units.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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