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Electromagnetic Waves is the smallest chapter in JEE Main physics and it almost always carries one question. Nearly all of them are recall: the order of the spectrum, which band is used for what, the direction of E and B, and the idea of displacement current. One short sitting is enough to lock this chapter down for good. Ten questions below. Give them ten honest minutes.
What this chapter carries in JEE Main
- Usually 1 question every year, which is 4 marks.
- The spectrum and the uses of each band are asked most often.
- Wave properties, radiation pressure and displacement current fill the rest.
What the syllabus says
In the syllabus (Unit 15, Electromagnetic Waves): Displacement current, electromagnetic waves and their characteristics, the transverse nature of electromagnetic waves, the electromagnetic spectrum from radio waves to gamma rays, and applications of electromagnetic waves.
Taken out in 2024: the whole Communication Systems chapter, which used to follow this one.
Taken from the NTA syllabus for JEE Main 2027. See the full syllabus, unit by unit.
Formulas you must know by heart
| Speed in vacuum | c = 1/√(μ0ε0) = 3 × 108 m/s |
| Speed in a medium | v = c/n |
| Field ratio | E0 = cB0 |
| Direction | E, B and the direction of travel are mutually perpendicular, in that order |
| Energy sharing | the electric and magnetic fields carry equal shares |
| Intensity | I = ½ε0E02c |
| Radiation pressure | I/c on an absorber, 2I/c on a reflector |
| Displacement current | Id = ε0 dΦE/dt |
| Spectrum, shortest first | gamma, X-ray, ultraviolet, visible, infrared, microwave, radio |
| Visible range | about 400 nm to 700 nm |
Radiation pressure is twice as large on a mirror as on a black surface, because a reflected wave reverses its momentum instead of merely giving it up. That doubling is the question JEE Main likes to build on.
Ten questions. Give yourself ten minutes.
Write your answers down before you scroll. A question you looked up is a question you have not answered.
Q1. An electromagnetic wave travels along the +x direction with its electric field along +y. Its magnetic field points along
- (a) +x
- (b) +y
- (c) +z
- (d) −z
Q2. In an electromagnetic wave, the energy carried by the electric field and by the magnetic field are in the ratio
- (a) 1 : 1
- (b) 2 : 1
- (c) 1 : 2
- (d) c : 1
Q3. The displacement current between the plates of a charging capacitor is
- (a) zero
- (b) equal to the conduction current in the wires
- (c) half the conduction current
- (d) twice the conduction current
Q4. An electromagnetic wave of intensity I falls on a perfectly absorbing surface. The radiation pressure on it is
- (a) I/c
- (b) 2I/c
- (c) Ic
- (d) I/c2
Q5. The wavelengths of radio waves used for broadcasting are of the order of
- (a) nanometres
- (b) micrometres
- (c) metres
- (d) picometres
Q6. Infrared radiation is best detected by a
- (a) photographic plate
- (b) thermopile
- (c) Geiger counter
- (d) human eye
Q7. The frequency of visible light of wavelength 600 nm is about
- (a) 5 × 1014 Hz
- (b) 5 × 1012 Hz
- (c) 5 × 1016 Hz
- (d) 2 × 1014 Hz
Q8. A television remote control works using
- (a) ultraviolet
- (b) infrared
- (c) X-rays
- (d) microwaves
Q9. The speed of an electromagnetic wave in a medium of refractive index n is
- (a) c
- (b) c/n
- (c) cn
- (d) c/n2
Q10. Maxwell’s equations predict that electromagnetic waves travel in vacuum at a speed
- (a) μ0ε0
- (b) 1/√(μ0ε0)
- (c) √(μ0ε0)
- (d) 1/μ0ε0
Answer key, with a hint for each
| Q1 | (c) | E × B must point the way the wave travels, and ŷ × ẑ = x̂. |
| Q2 | (a) | The two carry exactly equal shares of the energy, at every instant and in every such wave. |
| Q3 | (b) | That is precisely why Maxwell introduced it — so that the current would not simply stop at the gap. |
| Q4 | (a) | The wave hands over its momentum. On a mirror the momentum is reversed, so the pressure doubles to 2I/c. |
| Q5 | (c) | From a few metres up to hundreds of metres. That is why broadcast aerials are so large. |
| Q6 | (b) | A thermopile responds to the heating effect, which is how infrared was discovered in the first place. |
| Q7 | (a) | f = c/λ = 3 × 108 / 6 × 10−7. |
| Q8 | (b) | A short range infrared beam, which is also why it will not work through a wall. |
| Q9 | (b) | n = c/v by definition, so the wave slows down in any medium. |
| Q10 | (b) | Putting in the measured values gives 3 × 108 m/s, which is how light was identified as an electromagnetic wave. |
Now mark yourself honestly
JEE Main marks the way it marks. Use the same rule here:
- +4 for a right answer
- −1 for a wrong one
- 0 for a blank
So ten questions here are out of 40. In the real paper, 25 Physics questions are out of 100.
If you scored 32 or more, the basics of this chapter are secure. Move to harder questions, not more of these.
If you scored below 24, do not feel bad about it. You have just found out something useful before the exam did. Go back to the formula table above, learn it properly, and sit these ten again tomorrow.
And if you guessed a few and they landed, count those as wrong. A guess that works once will not work in January.
The full chapter, on a clock
Ten questions tell you a little. Twenty-five tell you the truth.
The Foundation paper for Electromagnetic Waves is waiting for you on the portal: 25 questions, 45 minutes, marked out of 100, with the answer key and a hint for every question. Sit it tonight and you will know exactly where this chapter stands.
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