“Radio Men In Very Unusual X-ray Gowns”
Order of the spectrum
Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma, in order of decreasing wavelength and increasing frequency and energy.
Class 12 Physics · Gujarat Board, CBSE, GUJCET, JEE and NEET
Electromagnetic Waves formulas
A short chapter with few formulas and reliable marks. Most questions ask for the order of the spectrum, the uses of one band, or the relation between the electric and magnetic field amplitudes, so it rewards memory more than method.
14 formulas, 3 memory hooks, and the 5 mistakes that cost the most marks.
The wave
| What | Formula | Symbols and notes |
|---|---|---|
| Speed in vacuum | c = 1/√(μ₀ε₀) = 3 × 10⁸ m s−1 | the same for every wavelength |
| Speed in a medium | v = 1/√(με) = c/n | n is the refractive index |
| Field amplitudes | E₀ = cB₀ | the electric field is far larger in number, not in effect |
| Directions | E, B and the direction of travel are mutually perpendicular | E × B points the way the wave goes |
| Energy density | u = ½ε₀E² + B²/2μ₀ | shared equally between the two fields |
| Intensity | I = ½ε₀cE₀² | power per unit area |
| Displacement current | Id = ε₀ dΦE/dt | Maxwell's addition, which completes Ampere's law |
The spectrum, longest to shortest
| What | Formula | Symbols and notes |
|---|---|---|
| Radio waves | > 0.1 m | radio, television, mobile communication |
| Microwaves | 0.1 m to 1 mm | radar, microwave ovens |
| Infrared | 1 mm to 700 nm | heating, remote controls, night vision |
| Visible light | 700 nm to 400 nm | the only part the eye detects |
| Ultraviolet | 400 nm to 1 nm | absorbed by the ozone layer, causes sunburn |
| X-rays | 1 nm to 10−3 nm | medical imaging |
| Gamma rays | < 10−3 nm | from nuclei, used to treat cancer |
Taught by Darshit Bhatt, founder and lead faculty at Darshit Sir Institute. Instrumentation and Control engineer, with a master's degree and a chartered degree, teaching in Paldi, Ahmedabad since 2006.
Know the formula but not when to use it?
Darshit Sir has taught this in Paldi since 2006, in the classroom and live online.
Memory hooks
How to remember this chapter.
“E is c times B”
The two field amplitudes
E₀ = cB₀, so the numbers differ by a factor of 3 × 10⁸, even though the two fields carry equal energy.
“ROYGBIV shrinks”
Wavelengths inside visible light
Red is the longest visible wavelength and violet the shortest, so violet carries the most energy per photon.
Where marks go
The mistakes that cost the most.
- Thinking an electromagnetic wave needs a medium. It does not, which is how sunlight crosses space.
- Writing the spectrum in the wrong direction. Check against a known pair: radio is longest, gamma shortest.
- Assuming the electric field matters more because E₀ is a much bigger number. The energy is shared equally.
- Forgetting that the speed is the same for all electromagnetic waves in vacuum, whatever the frequency.
- Leaving out displacement current when asked why Ampere's law needed correcting.
Test yourself
Do you actually know these?
Ask DSI
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Tell us the chapter and your class. We will tell you honestly whether our batch helps.
Related
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Bring the student's last two report cards. You will get an honest read on where they stand and which batch actually helps, from the person who will do the teaching.