Simulation · Physics · Class 12
An em wave entering a medium
From the lesson Speed in vacuum and in media in Electromagnetic Waves. Change the values and watch what happens.
An em wave entering a mediumPhysics · Class 12
The idea behind it
NCERT §8.3.2
- Electromagnetic waves are self-sustaining oscillations of E and B in free space. Unlike sound or waves on a string, they need no material medium: nothing material vibrates.
- In vacuum the speed is c = 1/√(μ₀ε₀). With μ₀ = 4π × 10⁻⁷ T m A⁻¹ and ε₀ = 8.854 × 10⁻¹² C² N⁻¹ m⁻², this is about 3 × 10⁸ m/s.
- In a material medium of permittivity ε and permeability μ these replace ε₀ and μ₀, and the speed becomes v = 1/√(με). The speed of light depends on the electric and magnetic properties of the medium.
- The refractive index of one medium with respect to another equals the ratio of the speeds of light in them (taken up in the next chapter).
- Measured with waves of many different wavelengths, the vacuum speed does not depend on wavelength; the results agree to a few m/s out of 3 × 10⁸ m/s. The value is known so precisely that it is used to define the standard of length.
- All electromagnetic waves, from radio waves to gamma rays, travel at the same speed c in vacuum.
- Electromagnetic waves carry energy from one place to another. Radio and TV signals carry energy, and light carries the sun's energy to the earth, which makes life on earth possible.