DSE Physics | Electromagnetic Waves and Communication: A Tutor Guide

DSE Physics | Electromagnetic Waves and Communication: A Tutor Guide | Three Ways to Use c equals f lambda

2026-11-03

Tutor lesson-prep guides · Secondary Physics

Students can recite c equals f lambda and still lose the decimal place the moment 2.4 GHz appears, because the prefix was never converted. This lesson does three things only: convert prefixes, use one equation in three directions, and judge energy and penetration qualitatively, with 10 questions, 25 marks and 40 minutes.

Objectives — Students can: (1) convert 100 MHz to 1 times 10 to the 8 Hz and 2.4 GHz to 2.4 times 10 to the 9 Hz; (2) use c equals f lambda to get an FM wavelength near 3.0 metres, a WiFi wavelength near 0.125 metres and visible light near 600 nanometres; (3) explain why higher frequency means more energy and weaker penetration; (4) work out 4 minutes and 2.5 marks per question from 10 questions, 25 marks and 40 minutes, and finish the exit check in five minutes.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Wave equation	Which equation links speed, frequency and wavelength?	c equals f times lambda
Prefixes	How many hertz is 1 GHz?	1 times 10 to the 9
Speed of light	How fast do electromagnetic waves travel in a vacuum?	About 3 times 10 to the 8 metres per second
Timing	With 25 marks in 40 minutes, how many marks per minute?	About 0.63

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Prefix warm-up	Write MHz, GHz and nm as powers of ten	All three correct
8-18 min	Three directions	Find wavelength, frequency and speed	Write the same equation three times
18-30 min	Spectrum order	Radio through to gamma rays	State the frequency and energy trend
30-42 min	Communication uses	FM, WiFi, optical fibre	Say why each band is chosen
42-52 min	Independent practice	Three questions in ten minutes	Mark answers and label the error type
52-60 min	Exit check	Four questions spoken, notes closed	Pass only when all four are right

03 Teaching script (read it out)

Scene 1: Convert the prefix before touching the equation

One hundred megahertz is 1 times 10 to the 8 Hz and 2.4 gigahertz is 2.4 times 10 to the 9 Hz. Substituting the raw number produces an answer three orders of magnitude out, and students then blame the equation. Require one line at the top of every answer that writes f in hertz, and that line is the error guard. Worked example: 3 times 10 to the 8 divided by 1 times 10 to the 8 gives 3.0 metres.

Ask for 2.4 GHz in hertz and what happens without the conversion.

Scene 2: One equation, three directions

c equals f lambda has only three quantities, so there are only three question shapes: frequency to wavelength, wavelength to frequency, and both to speed. All are rearrangements of one equation, so three memorised formulas are unnecessary. Have the student run all three directions on a single question, which cuts the time from two minutes to forty seconds. Keep the units honest: metres for lambda, hertz for f.

Ask the student to recover the frequency from a wavelength of 0.125 metres.

Scene 3: Higher frequency means more energy and weaker penetration

The spectrum runs radio, microwave, infrared, visible, ultraviolet, X-ray and gamma, with frequency and photon energy rising together, yet penetration is not monotonic. Radio diffracts well, which is why indoor reception works; 2.4 GHz WiFi penetrates walls better than 5 GHz but carries less data; visible light at 600 nanometres will not pass through paper. Use the pair of ideas, energy up and penetration down, to explain why X-rays image bones rather than carry calls.

Ask why 5 GHz is faster but worse through walls.

04 Question types and worked steps

Type	Example	Steps	Answer
Wavelength	Given the frequency	Convert the prefix, apply c equals f lambda, state units	Answer in metres or nanometres
Frequency	Given the wavelength	Convert to metres, divide, write hertz	Prefix conversion shown
Spectrum order	Low to high frequency	Recall seven bands, order them, state the trend	Energy rises with frequency
Application	Why a band suits a use	Name the frequency, the penetration or diffraction, the conclusion	One cause, one effect

Sanity check (teach this): Order check: 3 times 10 to the 8 divided by 2.4 times 10 to the 9 is about 0.125 metres, and 12.5 metres means an inverted division or a lost prefix. Forty divided by ten is four minutes per question; twenty-five divided by forty is about 0.63 marks per minute, and 6.3 means a misplaced decimal. Visible light must land between 400 and 700 nanometres, so 600 metres means the nanometre conversion was dropped.

05 Independent practice (3 questions)

Question	Answer	What to watch
Wavelength of a 2.4 GHz signal	About 0.125 metres	Prefix converted first
Marks per minute for 25 marks in 40 minutes	0.625 marks	Decimal point in place
How photon energy changes as frequency rises	It increases	Not mistaken for a decrease

06 Common mistakes and what to say

Mistake	Student writes	Say this
Substituting without converting	“2.4 divided by 3 times 10 to the 8 gives a very small number.”	“2.4 is gigahertz, so write 2.4 times 10 to the 9 hertz first. Without that the answer is nine orders out.”
Mixing units	“Centimetres for wavelength and megahertz for frequency is fine.”	“The equation only accepts metres and hertz. Convert both, or the answer misses by orders of magnitude.”
Treating penetration as monotonic	“Higher frequency penetrates further, so X-rays travel furthest.”	“X-rays penetrate flesh but are absorbed quickly. Radio carries signals because of diffraction and the atmospheric window, not because of high energy.”
Reversing the spectrum	“Infrared has a higher frequency than ultraviolet.”	“Ultraviolet sits above visible light and infrared below it. Once red to violet is fixed, both sides follow.”

07 Homework (paper, section and question numbers)

Physics chapter index — Revise waves, then do practice questions 1 and 2
Gravitation and circular motion — Question 3 also needs units converted first
Radioactivity and half-life — Questions 4 and 5 drill order-of-magnitude checks

08 Exit check (last 5 minutes)

Question	Answer	Decision
Wavelength of a 100 MHz signal	About 3.0 metres	Error: prefix not converted
Marks per minute for 25 marks in 40 minutes	0.625	Error: decimal point moved
How energy changes as frequency rises	It increases	Error: said it decreases
Visible light wavelength range	About 400 to 700 nanometres	Error: written in metres

09 Tutor FAQ

My student always skips the prefix conversion. How do I train it?

Require the first line of every answer to be the frequency in hertz and nothing else. Ten questions later the step becomes automatic.

Must the whole spectrum be memorised?

The order of the seven bands and their relative positions, yes. Only radio, microwave and visible light need approximate numerical ranges.

Why are communication questions mostly explanatory?

Because the syllabus tests band selection: diffraction, penetration and bandwidth. The calculation is a threshold and the explanation carries the marks.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.