This is why most metals reflect visible light and appear shiny. 'Warm' electrons When the effects of the electron thermal speed are taken into account, the electron pressure acts as a restoring force as well as the electric field and the oscillations propagate with frequency and wavenumber related by the longitudinal Langmuir wave: called the Bohm–Gross dispersion relation. If the spatial scale is large compared to the Debye length, the oscillations are only weakly modified by the pressure term, but at small scales the pressure term dominates and the waves become dispersionless with a speed of . For such waves, however, the electron thermal speed is comparable to the phase velocity, i.e., so the plasma waves can accelerate electrons that are moving with speed nearly equal to the phase velocity of the wave. This process often leads to a form of collisionless damping, called Landau damping. Consequently, the large-k portion in the dispersion relation is difficult to observe and seldom of consequence. In a bounded plasma, fringing electric fields can result in propagation of plasma oscillations, even when the electrons are cold. In a metal or semiconductor, the effect of the ions' periodic potential must be taken into account. This is usually done by using the electrons' effective mass in place of m. Plasma oscillations and the effect of the negative mass Plasma oscillations may give rise to the effect of the “negative mass”. The mechanical model giving rise to the negative effective mass effect is depicted in Figure 1. A core with mass is connected internally through the spring with constant to a shell with mass . The system is subjected to the external sinusoidal force . If we solve the equations of motion for the masses and and replace the entire system with a single effective mass we obtain: where . When the frequency approaches from above the effective mass will be negative. The negative effective mass (density) becomes also possible based on the electro-mechanical coupling exploiting plasma oscillations of a free electron gas (see Figure 2). The negative mass appears as a result of vibration of a metallic particle with a frequency of which is close the frequency of the plasma oscillations of the electron gas relatively to the ionic lattice . The plasma oscillations are represented with the elastic spring , where is the plasma frequency. Thus, the metallic particle vibrated with the external frequency ω