Question 1

What is the phase relationship between the electric and magnetic fields in an EM wave?

Accepted Answer

They are in phase — both reach their maximum, zero, and minimum at the same instant and place. Their vectors are perpendicular in space, but there is zero time-phase difference between them. Students often confuse the 90° spatial angle with a 90° phase lag — they are different things.

Question 2

How do the electric and magnetic fields create each other in light?

Accepted Answer

A changing electric field produces a magnetic field (Ampère–Maxwell), and a changing magnetic field produces an electric field (Faraday). In a wave each one's change regenerates the other, so the disturbance sustains itself and travels forward without needing a medium.

Question 3

Why does an electromagnetic wave not need a medium to travel?

Accepted Answer

Because the fields are each other's source. The oscillating E feeds B and the oscillating B feeds E, so the wave carries its own restoring mechanism through empty space — unlike sound, which needs particles to push.

Question 4

What is the relationship between E₀ and B₀ in an electromagnetic wave?

Accepted Answer

Their ratio equals the speed of light: E₀/B₀ = c ≈ 3×10⁸ m/s. That's why the electric amplitude is numerically far larger than the magnetic one, even though both carry the wave equally.

Question 5

Are electromagnetic waves transverse or longitudinal?

Accepted Answer

Transverse. Both E and B oscillate perpendicular to the direction the wave travels (and perpendicular to each other). This is why EM waves can be polarized, unlike longitudinal sound waves.

Question 6

In which direction does an EM wave travel relative to E and B?

Accepted Answer

Along the direction of E×B (the cross product). If E points up and B points sideways, the wave moves along the third mutually perpendicular axis — given by the right-hand rule.

Electromagnetic Wave — How E and B Create Each Other

Practice

Learned it? Now prove it under the clock.