Carbon, silicon and germanium have four valence electrons each. At room temperature which one of the following statements is most appropriate ?

Photon of frequency $$v$$ has a momentum associated with it. If $$c$$ is the velocity of light, the momentum is

Two lenses of power $$-15$$ $$D$$ and $$+5$$ $$D$$ are in contact with each other. The focal length of the combination is

In a Young's double slit experiment the intensity at a point where the path difference is $${\lambda \over 6}$$ ( $$\lambda $$ being the wavelength of light used ) is $$I$$. If $${I_0}$$ denotes the maximum intensity, $${I \over {{I_0}}}$$ is equal to