Class 12th

For BCC unit cell

√3a = 4r

a=4r√3

Numerous scientists contributed to the discovery of this concept, but the key was introduced by Albert Einstein in 1905. This concept was used to support the experiment of the photoelectric effect to prove that light could be as a wave too. Then this name photon was given 21 years later, in 1926 by Gilbert N. Lewis.

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.