Basics of LIGHT

1. What is dispersion? How is it caused?

The phenomenon of the separation or splitting up of composite light into its constituent colours is called dispersion.
Dispersion is caused on account of different refractive indices of the medium for different colours.

2. What is spectrum?
The band of colours (wavelengths), obtained by dispersion of a narrow beam of composite light is called spectrum.

3. Draw a diagram showing the dispersion of light.

4. Give an experiment to show the recombination of colours.
Newton tested his supposition by placing two identical prisms P1 and P2. P2 was kept inverted with respect to P1. He found that colors of the spectrum produced by the prism P1 recombined in P2. White light emerges from P2.

5. How do you say that the colours in white light are not manufactured by a prism?

Newton obtained a spectrum of sunlight on a card with a hole at one point. He placed another prism behind the hole to find whether there was further splitting of colour. But the transmitted light had the same colour as before. The above experiments established that sunlight consists of different colours. They are separated but not manufactured by a prism.

6. Mention the range of wavelengths of visible spectrum.
Its range is about 400 nm to 750 nm.

7. What is meant by a pure spectrum? Explain.
A pure spectrum is a spectrum in which the constituent colours occupy their respective positions and are distinct.

8. What is a spectroscope?
Spectroscope is an instrument to obtain pure spectrum of a polychromatic light using a prism.

9. Explain the working of a spectroscope.

The light source, whose spectrum has to be investigated, is placed in front of the slit S. Light from the slit is rendered parallel by the lens CL. These rays pass through the prism, get dispersed and enter the telescope B. The telescope helps to observe a magnified image of them spectrum.

10. Mention the parts of a spectroscope.
Collimator and Telescope

11. List three uses of a spectroscope.
1. It is used for spectrographic analysis of spectra.
2. It is used for identifying the elements present in the solar atmosphere.
3. Specto chemical analysis is used in crime detection.

12. What is a continuous emission spectrum?
Sunlight, light from a filament lamp, molten iron or a candle flame all gives rainbow like spectrum. This is called continuous emission spectrum.

13. What is a line emission spectrum?
When gases or vapours are made to emit light. In this case, the spectrum consists of a number of sharp, bright coloured lines against a dark background. The lines are the slit images for different colours. This is called line emission spectrum

14. What is absorption spectrum?
The corresponding spectrum has a number of dark lines or bands, against the background of continuous spectrum. Such a spectrum is called absorption spectrum

15. What are Fraunhofer lines?
The solar spectrum has several dark lines in its spectrum. These are called Fraunhofer lines.

16. Explain Raman Effect.
Raman found that when a beam of monochromatic light was passed through organic liquids such as benzene, toluene etc, and the scattered light was no longer monochromatic. The scattered light contained higher and lower frequencies in addition to that of incident light. This phenomenon is called Raman Effect.

17. What is the important difference between Rayleigh scattering and Raman scattering?
In 1871, Lord Rayleigh had given the correct explanation of the blue colour of the sky on the basis of scattering of light by molecules in the atmosphere. He showed that the intensity of scattered light in any a medium is inversely proportional to the fourth power of its wavelength. This type of scattering, known as Rayleigh scattering, is also called coherent scattering because there is no change in wavelength of light.
Raman found that when a beam of monochromatic light was passed through organic liquids such as benzene, toluene etc, and the scattered light was no longer monochromatic. The scattered light contained higher and lower frequencies in addition to that of incident light. This phenomenon is called Raman Effect.

18. Mention one application of Raman Effect.
1. Raman Effect has been observed and studied in a great number of liquids, vapours, gases and some solids.
2. Raman Effect is very useful to understand the structure of molecules that constitute matter.