Question

a) A spectrum is obtained by emitting light from sodium vapour. Name the type of the spectrum.
b) A monochromatic light is passed through benzene. What change can be observed in the wavelength of scattered light? Write one application of this phenomenon.

Answer 1

Hint: Line spectrum is an electromagnetic spectrum that is produced by excited atoms and molecules. This spectrum has discrete lines. Scattered light through benzene has wavelength less or more than the incident light.

Complete step by step answer:
a) The spectrum produced has discrete lines. So the spectrum produced by sodium vapor is line spectrum.
b) When a monochromatic light is passed through benzene it gets scattered and consists of light of smaller and larger wavelengths in addition to the wavelength of incident light. Thus, the scattered light contains different colours. This phenomenon is used in the study of molecular structure.

Additional information:
A sodium-vapor lamp is a gas-discharge lamp that uses sodium in an excited state to produce light at a characteristic wavelength near 589 nm. There are two types of such lamps; low pressure lamps and high pressure lamps. Low-pressure sodium lamps are a highly effective source of electrical light, but their yellow light can only be used for outdoor lighting, such as street lamps, where they are profoundly used. High-pressure sodium lamps emit a broader spectrum of light than the low-pressure lamps, but they still have poorer color intensity than other types of lamps. Low-pressure sodium lamps only give monochromatic yellow light and so inhibit color visibility at night.

Note: Students should keep in mind that the spectrum is produced by excited atoms and molecules. Therefore the spectrum produced would have discrete lines which are basically line spectrum. As the size of benzene is large so scattering of the monochromatic light takes place.