Q.What is the wavelength range of visible light in nanometers?

Visible light is the narrow band of the electromagnetic spectrum, roughly $4\times10^{-7}$ m to $8\times10^{-7}$ m in wavelength, that the human eye can detect directly -- which is exactly why this particular range is singled out and named 'visible light' in the first place. It is produced by the rearrangement of the outer orbital electrons of atoms and molecules as they move between energy levels (atomic excitation), for example inside a gas-discharge tube.

Different wavelengths within this band are perceived as different colours, running continuously from violet at the shortest wavelength (around 380-450 nm) through blue, green, yellow and orange to red at the longest wavelength (around 620-750 nm). The visible light emitted or reflected by objects around us is our primary everyday channel of information about those objects and about our surroundings generally. Visible light also has a special status in astronomy: together with radio waves, it is one of only two bands able to pass through the Earth's atmosphere and reach the ground without being significantly absorbed, which is why optical (and radio) telescopes can be built and operated from the ground, whereas telescopes for every other band of the spectrum must be carried aboard satellites; even so, optical …