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| Spectra |
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| The two types of spectra are emission and absorption. |
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| When light emitted from a source is examined directly in a spectroscope, we observe the emission spectrum of the source. |
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| When continuous light from a source is made to pass through an absorbing substance and then examined in a spectroscope, we obtain the absorption spectrum of the substance. The dark lines are the missing wavelengths, which must have been absorbed by the substance. |
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| These are the dark lines observed in otherwise continuous spectrum of the sun. When continuous light of all wavelengths from the sun's atmosphere passes through its outer layers, certain wavelengths are absorbed by elements present in this layer and hence the dark lines are present in spectrum of the sun. |
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| Application of specter emission spectra of |
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| A (chromium) |
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| B (stain less steal) |
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| C (nickel). |
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| The lines in B show that stainless steel contains chromium and nickel in it. |
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| It is the natural spectrum of the sun's light when they fall on raindrops during or after a shower. |
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| 1) Observer must stand with his back towards the sun. The common centre of the concentric circular arc lies on the line joining the sun to the eye of the observer. The brightest bow seen is the primary rainbow, which is formed due to one total internal reflection and two refraction from the raindrops. |
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| When sunlight falls on small water droplets suspended in air during or after a rain it suffers total internal reflection and dispersion. |
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| The figure shows the path of a typical ray forming the rainbow. The ray suffers a refraction, a total internal reflection and then again a refraction. Dispersion takes place at both the refractions. |
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| The rays of a given color are strongly returned by the droplet in a direction that corresponds to a maximum deviation in its path. For light of red color this maximum deviation is 137.8o so that the angle is 180o-137.8o = 42.2o. For violet this angle is 40.6o and the other colors lie between the two. All the droplets on the surface of the core will return the light (red) to the eye subtending an angle of 42.2o and violet at an angle 40.6o. The other colors form their respective circle of intermediate radii. The rainbow is visible as an arc from the ground level but will appear as full circular one from an aeroplane. |
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