Light

1. Light 10th Grade - Physics

2. Module Objectives • Refractionthrough a prism • Dispersion • Recombination • Uses of spectroscope • Raman effect

3. Light - Recap • Light is a form of energy • Travels in straight lines • Refractionis due to change in velocity • Refractive index of a medium = velocity of light in vacuum : velocity of light in medium • Composite light splitsintoits constituent colors, whenpassedthrough a prism • Rainbowis a spectrumformed in nature

4. Difference in refractive indexCauses illusionalbend in straight objectDifferent medium has differentrefractive index and hence change in angle of bend • .

5. Dispersion 1. Seperationor splitting up of composite light into its constituent colours is called dispersion. 2. Medium that seperates light to its constituent colours is called dispersive medium. 3. Sunlight is made up of various colours. Can be splitted to the same sequence of colours as that of rainbow when passed through prism 4. Dispersion is found by Newton 5. Vacuum/Air is not a dispersive medium • .

6. Spectrum • Band of colours/wavelength, obtained by dispersion of a narrowbeam of composite light iscalled Spectrum • VIBGYOR – Violet, Indigo, Blue, Green, Yellow, Orange, Red • .

7. Spectrum experiments by Newton Colouris not manufactured by prism. Sunlight consistsdifferentcolours Newton test confirmedthat by placingtwoprisms in inverted positions, foundthatcolours of spectrumproduced by Prism 1 recombined in Prism 2 thusemerging as White light. • . http://www.youtube.com/watch?v=6_HroTxaZe0 Verydifficult to seperatecolours as colourcoming out of smallholeisdiverging

8. Visible spectrum • 400 nm (violet) to 750 nm (Red) • Wavelengthbelow the wavelength of violet iscalled ultraviolet • Wavelengthabove the wavelength of rediscalledinfrared

9. Pure/Impure Spectrum • Can youfind out distinctive yellowcolour in the belowspectrum? • Can youfind out distinctive bluecolour in the belowspectrum? • If weprojectanyspectrumproduced by a prism on a screen, colours are alwaysoverlapping • Spectrums are impure – colours are not distinct • Pure Spectrum – Spectrum in which the constituent coloursoccupythier respective positions and are distinct • How to obtain a pure spectrum ???

10. Spectroscope • Instrument to obtain pure spectrum of a polychromatic light using a prism. • Lenses are placedbefore and after the prism to minimise overlapping of colours. • Light from the slitisrenderedparallel by the collimeter. • These rays passthroughthe prism, getdispersed and enter the telescope. • Telescopehelps to observe a magnified image of a spectrum.

11. Spectroscope - Terminologies • Spectrometer – a circularscaleisprovided to find position of the telescope. Spectrometerisused to measure angle of prism, refractive index and dispersion. • Spectrograph – a camera used in place of the eye-piece to have a permanent record. • Direct vision spectroscope – the spectrum of light from a source isseen in the direction of the source. It is a pocketsized instrument meant for quick observation of spectra.

12. Differenttypes of Spectra • Continuousemissionspectrum • Sunlight, light from a filament lamp, molteniron or a candleflame all giverainbowlikespectrum • Line emissionspectrum • Occurswhendifferentgases or vapours are made to emit light. The spectrumconsists of a number of sharp, bright coloured linesagainst a dark background. Lines are slit images of differentcolours. • Patterns of linesisdifferent for differentelements. Once patterns are known, the elementspresent in any source canbeidentified. • This processhelps to find the elementspresent in the sun and other stars. Also, itis possible to findapprox. amount of eachsubstance present, by measuringintensities of the lines. • This technique iscalledspectrochemicalanalysisand isbeingused in industries, medical crime detectionetc • Absorption spectrum • When composite light passes through a semi-transparent substance, someparticularcolours of the incident radiation are absorbed. Hence the transmitted light lacksparticularcolours. The correspondingspectrum has a number of darklines or bands, against the background of continuousspectrum.

13. Different types of SpectraContd… • Fraunhofer lines • When white light from a carbon arc lampispassedthrough sodium vapour, then the continuousspectrumwill have twodarklines in the yellowregion. The solarspectrum has severaldarklines in itsspectrum. These are due to the absorption of certain colours by the elements in the solaratmosphere. • The study of these Fraunhofer lines has enabled us to identify the elementspresent in the solaratmosphere • Fraunhofer lines

14. Tyndall and Rayleigh • When a beam of light wasallowed to passthrougha homogeneous and transparent medium, a portion of the incident light gotdeflectedsideways or scattered. • Wavelength of the scatteredbeamisfound to be the same as that of the incident light. • Lord Rayleigh – 1871 – Showedthat the intensity of scattered light in any medium isinverselyproportional to the fourth power of itswavelength. This type of scatteringisknownas Rayleigh/coherentscattering– no change in wavelength of light

15. Based on Tyndall and Rayleigh • Sunset and sunrise are red and the skyisblue. • Phenomena due to scattering of light by the molecules of the atmosphere. • Intensity of scattered light increasesrapidly as the wavelengthdecreases. • When sunlight passes throughatmosphere, bluecolourisscatteredmost. So skyappearsblue. • Atsunset and sunrise, sunlight passes through maximum length of atmosphere. Much of the blueistakenaway by scattering. The light thatreachesearth’s surface lacks in bluecolour. Hencesunset and sunriseappearsreddish.

16. Raman Effect • C.V. Raman – 1928 – Studied the scattering of light by liquidswith the intention of reproducingbluecolour of the sea and fo the sky. • A beam of monochromatic light waspassedthroughorganicliquidssuch as benzene, tolueneetc, the scattered light was no longer monochromatic. Scattered light containedhigher and lighterfrequencies in addition to that of incident light. This phenomenoniscalled Raman effect. • Raman wasawarded Nobel prize in 1930. • February28thisobserved as National Science Day, everyyear in our country to commemorateRaman’sdiscovery. • Raman effectispredictedtheoritically by Smekal as early as 1923. Raman is first to observe itexperimentally. So sometime, Raman effectisalsocalled as Smekal – Raman effect.

17. Raman Effect • Special spectroscopes are required to obseve Raman effect. • In Raman effect, there are additionalfrequenciesapartfromthat of incident light. For thisreason, Raman scatteringiscalledincoherentscattering. • Raman gave satisfactoryexplanation of incoherentscattering on the basis of quantum theory of radiation. • Raman effect has been observed and studied in greatnumber of liquids, vapours, gases and somesolids. • Raman effectisveryuseful to understand the structure of moleculesthatconstitutematter.

18. Exercises • Explain Light Refractionbased on the belowexamples.

19. Exercises • Whatis dispersion? How isitcaused? • Mention the range of wavelengths of visible spectrum. • Whatismeant by a pure spectrum? Explain. • Whatis a spectroscope? • Mention the parts of a spectroscope. • List three uses of spectroscope. • Whatis a continuousspectrum? • Whatis a line emissionspectrum? • What are Fraunhofer lines? • Explain Raman effect. • Whatis the important differencebetween Rayleigh scattering and Raman scattering? • Mention one application of Raman effect.