What is line spectrum?

We observe that when a ray of white light falls on a prism it experiences refraction twice. Once when it travels from the rarer medium (air) to a denser medium (glass) and again from the denser medium (glass) to a rarer medium (air). Finally, we observe a band of colors, called spectrum, formed out of a ray of white light. If we observe this spectrum more closely, the color having a smaller wavelength deviates the most and vice versa. Thus, a spectrum of colors ranging from red to violet is observed where red having the longest wavelength suffers the least deviation. This kind of spectrum is called a continuous spectrum as violet merges into blue, blue into green and so on.

However, the emission spectrum of atoms in the gas phase, do not exhibit a continuous spread of wavelength from one color to other. Rather, the emitted light consists of a specific wavelength having dark spaces existing between them. Such kind of spectra is known as atomic spectra or line spectra.