Physics

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.

X_C  = 1/ ωC

On increasing w; XC decreases so Z decreases, current in circuit increases.

Yes, it strongly depends on the wavelength of the light wave. It is directly proportional to the length i.e. longer the wavelength, more will be the amount of diffraction. Similarly, short wavelength of the light will mean less diffraction comparatively.

Reflection is the concept of bouncing back of light waves when passed through a surface. Whereas, diffraction is a term used when light waves spread in different directions on being passed through an obstacle. Reflection requires a smooth surface whereas diffraction requires the edge of an object.