Question

The charge on a piece of metal can be "watched" fairly easily by connecting it to an electroscope, a device with thin leaves that repel when a net charge is present You place a large excess negative charge on a piece of metal, then separately shine light source of two pure but different colors at it. The first source is extremely bright, but the electroscope shows no change in the net change. The second source is feeble, but the charge disappears. Appealing to as few fundamental claims as possible, explain to your friend what evidence this provides for the particle nature of light.

Short answer

The disappearance of charge when the second light source of low intensity falls on the metal surface and no change in net charge when the first light source of higher intensity falls on the metal suggest that the light has a particle nature.

Step-by-step solution

Given data

An electroscope is connected to a metal piece having a large negative charge.

Concept used

Thermal equilibrium: It is a state of equilibrium in which the temperature of both the objects becomes the same and there is no further transfer of heat.

Electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field propagating through space, carrying electromagnetic radiant energy.

Experiment

An electroscope is a device to detect the net charge on a given metal by showing deflection. The metal shines by two light sources of different colors and different intensities. The first light source is extremely bright while the second one is feeble. There is no deflection in the electroscope for the first light source but the charge disappears for the second light source.

Different colors correspond to different wavelengths. The first light source is bright signifying higher intensity or higher amplitude of the wave. The second light source is feeble signifying lower intensity or lower amplitude of the wave.

The result of the experiment  is purely a wave

If the light is considered a pure a wave, then the higher amplitude wave would have higher energy and would have caused the electrons to leave the surface of the metal thus causing a deflection in the electroscope. Also, the feeble light source having relatively very less energy wouldn't be able to excite the electrons and cause its emission from the metal surface.

Result of experiment

The experiment shows exactly the opposite case. The feeble light source has caused the electrons to completely leave the surface of the metal and thus, the charge disappears. This clearly means that light is not purely a wave. The light behaves like a particle having enough energy to excite the electrons forcing them to leave the metal surface. The packets of energy don't depend on the intensity but on the wavelength of the light source. Therefore, a low-intensity wave has more energy than that of extremely bright light that causes the emission of electrons from the metal surface.