# Light reflected 90° out of phase, not 180°

• damosuz
In summary, the explanation for why light appears to move slower in glass is that the electrons in glass emit a light wave that is 90° out of phase with the light wave shaking them. This is often compared to a string attached to a rigid wall, but it is actually 90° out of phase rather than 180° as commonly mentioned. This information can be found in Feynman's lectures and on Wikipedia's page on refractive index.
damosuz
Classically, we explain why light appears to move slower in glass by saying that the electrons in glass emit a light wave which is 90° out of phase with the light wave shaking them. However, the electrons would have to emit this wave not only in the incident direction, but in the opposite direction as well, which would be the reflected wave. This means the reflected wave is 90° out of phase with the incident wave, and not 180° as it is mentioned everywhere (often by making an analogy with string attached to a rigid wall). What am I missing here?

damosuz said:
the electrons in glass emit a light wave which is 90° out of phase with the light wave shaking them
This is new to me. Can you mention the source?

## 1. What does it mean for light to be reflected 90° out of phase?

When light is reflected 90° out of phase, it means that the phase of the reflected light is shifted by 90 degrees compared to the incident light. This means that the peaks and troughs of the two waves are in opposite positions, resulting in destructive interference.

## 2. How is light reflected 90° out of phase different from being reflected 180°?

Light reflected 90° out of phase is different from being reflected 180° because in the former, the reflected light is shifted by half a wavelength compared to the incident light, resulting in destructive interference. In the latter, the reflected light is shifted by a full wavelength, resulting in constructive interference.

## 3. What causes light to be reflected 90° out of phase?

Light can be reflected 90° out of phase when it encounters a reflective surface with a thickness that is equal to half of the wavelength of the incident light. This causes the reflected light to be shifted by half a wavelength, resulting in destructive interference.

## 4. Can light be reflected 90° out of phase in all materials?

No, light can only be reflected 90° out of phase in certain materials that have specific optical properties. For example, this phenomenon can be observed in thin films or coatings, where the thickness of the material is crucial in determining the phase shift of the reflected light.

## 5. What is the significance of light being reflected 90° out of phase?

The significance of light being reflected 90° out of phase lies in its applications in various fields such as optics, electronics, and telecommunications. This phenomenon is used in the production of anti-reflective coatings, polarizing filters, and in controlling the phase of light in optical devices.

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