Class 12th

Yes, they travel in a vacuum at the speed of light and can never be in still position. This is because they do not have any mass to do so and will not exist only if not in motion.

From Lenz law, current in the square loop will be in anticlockwise sense

The key difference lies in not their spelling but the properties they possess. Protons are positively charged particles which revolve around the nucleus of an atom and have a specific mass. Whereas, photons don't have any mass and are responsible for carrying out electromagnetic radiation. They travel free without any restriction.

Direction of E is perpendicular to equipotential surface.

Yes, both coherent and incoherent waves can be seen by the naked eye. However, we cannot detect whether the wave is coherent or incoherent. This is because the incoherent waves have different frequencies which cannot be distinguished by the human eye. We have can only see lights of specific wavelengths as per the human limit.

Density of nuclei is order of 10¹⁷ kg m^–3.

Yes, whether a wave is coherent or incoherent depends on its phase difference and frequency. For converting a coherent wave into an incoherent wave, you have to damage the stable phase of the wave. This can be done by making the medium tough or mixing them with incoherent waves.

Similarly, incoherent waves can be converted into coherent waves by making their frequencies same and having all of them into the same phase.

Photocell works on photoelectric effect.