Physics · Atoms · NEET
The Sun's hot dense surface (photosphere) gives a continuous spectrum with all colours. But cooler gases in the Sun's outer atmosphere absorb certain wavelengths, so the spectrum we actually see also has thin dark lines (Fraunhofer lines). So sunlight is basically a continuous spectrum with an absorption line spectrum on top of it.
A bulb filament is a hot solid. In a solid the atoms are packed close and interact strongly, so their energy levels merge into wide bands, allowing all wavelengths to be emitted, giving a continuous spectrum. In a low-pressure gas the atoms are far apart and act independently. Each atom has sharp, separate energy levels, so it emits only a few fixed wavelengths, giving a line spectrum.
An electron in an atom can only sit in fixed energy levels. When it jumps from a higher level to a lower one, it emits a photon whose energy exactly equals the gap between the two levels (E = E_high minus E_low). Since only certain jumps are allowed, only certain photon energies, and therefore only certain wavelengths, appear. Each wavelength shows up as one bright line.
A line spectrum can be either. An emission line spectrum is bright lines on a dark background (gas emits light). An absorption line spectrum is dark lines on a bright continuous background (a cool gas absorbs certain wavelengths from white light passing through it). The dark lines fall at exactly the same wavelengths as the gas's bright emission lines.
No. A continuous spectrum comes from any hot dense source: hot solids, hot liquids, and hot dense (high-pressure) gases all give it. The key is that the atoms are close together and interact, so all wavelengths get emitted. Only low-pressure (rarefied) atomic gases give clean line spectra.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A continuous spectrum has all wavelengths with no gaps and comes from hot dense sources (solids, liquids, dense gases). A line spectrum has only a few discrete wavelengths, shown as separate lines, and comes from excited low-pressure atomic gases.
Any hot dense source: an incandescent bulb filament, molten metal, a hot solid, or a high-pressure hot gas. All of these emit every wavelength, producing a continuous band of colours.
Each element has its own unique set of energy levels, so it emits a unique set of wavelengths (line pattern). No two elements share the same pattern, so the line spectrum identifies the element, just like a fingerprint identifies a person.
Atomic hydrogen excited at low pressure gives a line spectrum, a set of discrete bright lines (such as the Balmer series in the visible region), not a continuous band.
When white light (continuous spectrum) passes through a cool gas, the gas absorbs its characteristic wavelengths, leaving dark lines in the continuous background. This pattern of dark lines is the absorption spectrum, and its lines match the gas's emission lines.
