The trap here is geometric, not numerical. Asked to describe an electromagnetic wave, aspirants correctly say E and B are perpendicular to each other — and stop. Perpendicular to each other is only half the statement. Both must also be perpendicular to the direction of propagation. A candidate who holds only the first half will accept a described wave in which E oscillates along the travel direction, and will lose the mark.
NCERT Class 12 Physics Chapter 8 states it on page 204: in an electromagnetic wave the electric and magnetic fields are perpendicular to each other and to the direction of wave propagation. That double perpendicularity is what the word transverse means. A longitudinal wave — sound in air — oscillates its medium parallel to travel; an EM wave never does.
Three consequences follow, and NEET tests the consequences more often than the definition.
First, the triad is ordered, not merely mutually perpendicular. The direction of propagation is along E × B — that cross product, in that order. Swap the operands and you get the reverse direction, which is why "E and B are perpendicular" alone cannot fix where the wave goes.
Second, E, B and the propagation direction form a right-handed set. Given any two of the three directions, the third is determined.
Third, transverse waves can be polarised; longitudinal ones cannot. Polarisation of light is direct evidence of transverse character, and is the standard reason quoted for why sound cannot be polarised.
Note what transversality does not fix: the relative magnitudes of E and B, their units, or the wave's speed. Those belong to the characteristics topic. Here, work only in directions.
Watch-out: E and B oscillate in phase — they peak together and vanish together. Transverse geometry says nothing about a phase lag, and there is none.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
In a plane electromagnetic wave, the electric field vector is perpendicular to:
Answer: C. C is correct. NCERT Class 12 Physics Chapter 8, page 204, states that E and B are perpendicular to each other and to the direction of propagation — this double perpendicularity is the definition of transverse character.
Why A is wrong: A is wrong because it states only half the condition; stopping at mutual perpendicularity permits a wave whose E oscillates along the travel direction, which is not electromagnetic.
Why B is wrong: B is wrong because it omits the E ⊥ B requirement, which is equally part of the transverse statement.
Why D is wrong: D is wrong because it denies transversality altogether, which would make the wave longitudinal — a description NCERT explicitly rules out for electromagnetic waves.
Electromagnetic waves are classified as transverse waves. The property that establishes this classification is that:
Answer: B. B is correct. Transverse means the oscillating quantity is at right angles to the direction of travel; NCERT Class 12 Physics Chapter 8, page 204, defines the geometry this way.
Why A is wrong: A is wrong because speed is a magnitude property and says nothing about the direction of oscillation; a fast longitudinal wave is still longitudinal.
Why C is wrong: C is wrong because needing no medium distinguishes electromagnetic from mechanical waves, not transverse from longitudinal — sound needs a medium and is longitudinal, but so is a longitudinal wave in a solid.
Why D is wrong: D is wrong because energy and momentum transport is common to all travelling waves, including longitudinal ones.
Sound waves in air cannot be polarised, whereas light can. The correct reason is that:
Answer: B. B is correct. Polarisation requires a choice of oscillation direction within the plane perpendicular to propagation — available only for transverse waves. Light's transverse nature is described in NCERT Class 12 Physics Chapter 8, page 204.
Why A is wrong: A is wrong because speed is unrelated to polarisability; a slow transverse wave on a string can be polarised.
Why C is wrong: C is wrong because it names the mechanical-vs-electromagnetic distinction. A transverse wave on a stretched string needs a medium and can still be polarised, so medium-dependence is not the deciding factor.
Why D is wrong: D is wrong because wavelength does not determine polarisability; radio waves have far longer wavelengths than sound and are polarisable.
A plane electromagnetic wave propagates along the +z direction. Its electric field oscillates along the +x direction at a given instant. At that instant the magnetic field oscillates along:
Answer: A. A is correct. The propagation direction is along E × B, so with E along +x and propagation along +z, B must lie along +y, since x̂ × ŷ = ẑ. The mutual perpendicularity of E, B and propagation is given in NCERT Class 12 Physics Chapter 8, page 204.
Why B is wrong: B is wrong because it places B along the propagation direction, contradicting the transverse requirement that both fields be perpendicular to travel.
Why C is wrong: C is wrong because it makes B parallel to E, violating the E ⊥ B condition.
Why D is wrong: D is wrong because it also places B along the propagation axis (anti-parallel), which is still a longitudinal field component and is forbidden.
In a plane electromagnetic wave the electric field points along the +y axis and the magnetic field points along the +z axis at a certain instant. The wave travels along:
Answer: C. C is correct. Propagation is along E × B; with E along +y and B along +z, ŷ × ẑ = x̂, giving the +x direction. The ordered triad follows from the transverse geometry in NCERT Class 12 Physics Chapter 8, page 204.
Why A is wrong: A is wrong because it reverses the cross product, computing B × E instead of E × B; the order of the operands fixes the sign.
Why B is wrong: B is wrong because it places propagation along B, whereas B must be perpendicular to the direction of travel.
Why D is wrong: D is wrong because it places propagation along E, whereas E must be perpendicular to the direction of travel.
A student describes a wave in which E oscillates along +x, B oscillates along +y, and the disturbance advances along +x. The description is:
Answer: D. D is correct. Transversality requires both fields to be perpendicular to propagation; here E is parallel to it. NCERT Class 12 Physics Chapter 8, page 204, states both conditions.
Why A is wrong: A is wrong because mutual perpendicularity of E and B is only half the transverse condition — this is exactly the partial statement that lets a bad description through.
Why B is wrong: B is wrong because the presence of a medium does not relax the geometric requirement; transversality holds for electromagnetic waves in vacuum and in media alike.
Why C is wrong: C is wrong because the geometry is frequency-independent — it holds across the whole spectrum.
A plane electromagnetic wave travels along the +x direction. At one instant, the electric field at a point is directed along +y. At an instant half a period later, the directions of E and B at that same point are:
Answer: A. A is correct. Initially B is along +z, since ŷ × ẑ = x̂ gives the +x propagation. Half a period later both fields reverse together — they oscillate in phase — so E is along −y and B along −z, and (−ŷ) × (−ẑ) = +x̂ preserves the travel direction. The in-phase, mutually perpendicular geometry is in NCERT Class 12 Physics Chapter 8, page 204.
Why B is wrong: B is wrong because it reverses E while leaving B unchanged, implying a phase lag between the fields; E and B are in phase, and this combination would also reverse the propagation direction.
Why C is wrong: C is wrong because after half a period the field must have reversed, not returned to its original direction; it also introduces a phase difference between E and B.
Why D is wrong: D is wrong because it places E along the ±z axis, which would require E to have rotated out of its oscillation plane — the oscillation direction of E is fixed for a linearly polarised plane wave.
For a plane electromagnetic wave travelling along the +z direction, a student is told only that the magnetic field at some instant is along the −x direction. The student concludes that the electric field at that instant must be along the +y direction. This conclusion is:
Answer: C. C is correct. Propagation is along E × B. Testing E along +y: ŷ × (−x̂) = +ẑ, matching the stated travel direction. Given any two of the three directions, the right-handed set determines the third, as follows from the geometry in NCERT Class 12 Physics Chapter 8, page 204.
Why A is wrong: A is wrong because E along ±x would make E parallel or anti-parallel to B, violating E ⊥ B; the constraint on E is not absent.
Why B is wrong: B is wrong because (−ŷ) × (−x̂) = −ẑ, giving propagation along −z, which contradicts the stated +z direction. This reverses the cross-product order.
Why D is wrong: D is wrong because amplitude is a magnitude; the direction of E follows from geometry alone and is independent of how strong the fields are.
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Given.
A plane electromagnetic wave in vacuum travels along the −y direction. At a particular instant the magnetic field at a point P is directed along the +z direction.
Required.
The direction of the electric field at P at that same instant.
Concept.
In an electromagnetic wave, E, B and the direction of propagation are mutually perpendicular, and the propagation direction is along the ordered cross product E × B (NCERT Class 12 Physics Chapter 8, page 204). Given two of the three directions, the third is fixed.
Formula.
Direction of propagation ∝ E × B. (No algebraic formula from the dossier applies here — this topic is purely geometric. The three in-dossier formulas relating E, B, c and intensity belong to the characteristics topic and are excluded under Rule 7.)
Substitution.
Let Ê be the unknown unit vector along E. We require Ê × ẑ = −ŷ.
Calculation.
Test the candidates in turn. Using the standard right-handed relations x̂ × ŷ = ẑ, ŷ × ẑ = x̂, ẑ × x̂ = ŷ:
• Ê = +x̂: x̂ × ẑ = −(ẑ × x̂) = −ŷ ✔• Ê = −x̂: (−x̂) × ẑ = +ŷ ✘• Ê = ±ŷ: parallel or anti-parallel to the propagation axis — forbidden by transversality ✘• Ê = ±ẑ: parallel or anti-parallel to B — forbidden by E ⊥ B ✘
Only Ê = +x̂ survives. The unit vectors and the integer coefficients in the cross-product relations are exact by definition and do not enter any significant-figure count; this problem carries no measured quantity at all.
Final answer.
The electric field at P is directed along the +x direction.
Common trap.
The tempting move is to accept any direction perpendicular to B — both +x and −x satisfy E ⊥ B, and a student who checks only mutual perpendicularity will call the answer ambiguous or pick the wrong sign. The propagation direction is what breaks the tie, and it breaks it only if the cross product is taken in the order E × B. Reversing the operands returns the opposite direction and yields −x̂.
Similar NEET-style question.
A plane electromagnetic wave in vacuum has its electric field oscillating along the −z direction while the wave advances along the +x direction. At that instant the magnetic field is directed along: (a) +y (b) −y (c) +z (d) −x. [Answer: (a). Check: (−ẑ) × ŷ = +x̂.]
Self-sustaining oscillating E and B fields propagating through space. Speed in vacuum: c = 1/√(μ₀ε₀) ≈ 3 × 10⁸ m/s. E and B mutually perpendicular and both perpendicular to propagation direction (transverse).
-- NCERT Class 12 Physics, Ch. 8, p. 207More in Electromagnetic Waves: 2 exam traps and mistakes · 4 formulas · 3 question patterns from its other lessons.
No question in our NEET 2020–2025 set targets this topic directly.
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