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When light propagates in vaccum there is an electric field and a magnetic field. Which of the following is not true about these field?

1. They are constant in time
2. They have zero average value
3. They are perpendicular to the direction of propagation of light
4. They are mutually perpendicular

Correct answer: They are constant in time

Solution

In vacuum, light is an electromagnetic wave with electric and magnetic fields that vary sinusoidally in time and space. So they are not constant in time, while they do have zero average value and are perpendicular to each other and to the direction of propagation.

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