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

Which of the following transitions in hydrogen atoms emit photons of highest frequency ?

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