What experiment suggested that atoms are not solid, but that they may have a hard nucleus encircled by negative electrons?

A 5 mw laserpointer beam has a diameter of 3 mm. The wavelength of laser light is 680 nanometers. Estimate the number of photons per second that hit the laser spot on the wall.

Niels Bohr suggested that an atom emits light as an electron, orbiting the atom, moves closer to the nucleus. How did Bohr explain the fact that spectra of chemical elements (such as Hydrogen) consists of discrete colors rather than the whole continuous spectrum?

The human retina can respond to the light energy of a single photon! The wavelength of visible light is of the order of 500 nm. Compare the energy of a photon of visible light to that of a 100 gm rifle bullet travelling at the speed of sound (340 m/s.)