Animation 1 (upper left)

Light can be imagine as a wave with a frequency, amplitude and a velocity (in the case of light it is the speed of light). The periodic time is the inverse of the frequency and the wavelength is the length traveled during the periodic time. The frequency gives the color (from red to violet) and the amplitude gives the brightness.

Animation 2 (lower left)

In a laser light is oscillating between two mirrors. One mirror is high reflection the other transmit a small part of the light. In the laser only certain types of frequencies can oscillate. If several colors are combined in the right way, small pulses are produced as output. (The square of the electric field is plotted.)

Animation 3 (upper right)

More and more colors produces smaller and smaller pulses. In the inset you see the frequency spectrum. It looks like a comb. If all frequencies have a stable phase relation to each other the laser is mode locked. (The square of the electric field is plotted.)

Animation 4 (lower left)

In the two photon spectroscopy two counter-propagating beams interact with a particle. If the particle interact at the same time with two beams, a two photon transition is possible, if the sum of the frequencies in the beams is equal to the transition frequency of the particle. Two photons are absorbed and the excited particle can decay via an third intermediate level emitting red and blue light.