A lamp fimament is made of tungsten. Why do we get a continuous spectrum rather than a tungsten line spectrum when light from an incandescent lamp is viewed with a spectroscope?
Very few, if any, tungsten atoms are excited; therefore, there is no line spectrum.
When we observe the light emitted from an incandescent lamp through a spectroscope, we would expect to see a line spectrum consisting of specific wavelengths corresponding to the emission lines of tungsten. However, we actually observe a continuous spectrum instead. This is because of the unique properties of incandescent light and the thermal nature of the lamp filament.
To understand why we see a continuous spectrum, we need to consider how incandescent lamps work. These lamps have a tungsten filament that is heated to a high temperature by passing an electric current through it. As the filament gets hotter, it emits light due to thermal radiation.
Thermal radiation from a hot object, like the tungsten filament, follows Planck's law of blackbody radiation. According to this law, a hot object emits radiation over a wide range of wavelengths, rather than just a few specific ones. The intensity of radiation at each wavelength depends on the temperature of the object.
Therefore, when the tungsten filament is heated to a high temperature, it emits electromagnetic radiation across a broad range of wavelengths, resulting in a continuous spectrum. This spectrum contains a continuum of colors that blend seamlessly together, without any distinct lines or gaps.
In contrast, line spectra occur when atoms or molecules emit light at specific wavelengths due to electronic transitions between energy levels. For example, when an element is excited or heated, its electrons move to higher energy levels and then emit photons of specific energies as they return to lower energy levels. This results in a line spectrum where we observe discrete lines at specific wavelengths.
However, in an incandescent lamp, the tungsten atoms in the filament are in a highly excited and disordered state due to the high temperature. This disorder prevents the atoms from transitioning between energy levels in a way that produces distinct lines. Instead, the random thermal motion of the atoms in the filament causes a continuous range of energy transitions, and thus a continuous spectrum of light is emitted.
In summary, we observe a continuous spectrum rather than a tungsten line spectrum when light from an incandescent lamp is viewed with a spectroscope due to the thermal nature of the lamp filament. The high temperature of the tungsten filament causes it to emit light according to Planck's law of blackbody radiation, resulting in a broad range of wavelengths being emitted rather than specific lines.