This chapter is treated as a tool for understanding the chapters ahead. In 19th century, the Munich optician Joseph von Fraunhofer found as many as 600 dark lines that exist in the spectrum of sunlight. These dark lines were later explained as absorbed light by atoms in the surface of the star. It provided information about the constituent elements of the sun's surface. Spectral measurements can be analyzed to find out the temperature of the star.
Spectral wavelength is measured to find the speed of the star relative to earth.
The planetary model of the atom consists of a tiny massive nucleus at the center and orbiting electrons surrounding the nucleus. Atom as a whole is electrically neutral. The nucleus contains protons that are positively charged, the electrons that are negatively charged and the neutrons that are electrically neutral particles. Protons and neutrons are the particles that make up most of the mass of an atom. So, we may say that a typical atom is 1x10 -10 m.