Colours In The Sky By Harsh Gupta 11

Colours In The Sky By Harsh Gupta 11 B 10/2/2020 1

Index l Key Knowledge – – – – Types of waves Wavelength Frequency Amplitude Refraction Colour Despersion Colour Components Rayleigh scattering coefficient 10/2/2020 l Why the sky is blue? l. Why isn’t the sky violet? l Why the sun is seen yellow? l Changes during sunset and sunrise? l. Why is the sky black at night? l. Why do we see stars as a small dots? 2

Types Of Waves There are two main types of waves: – Longitudinal wave: Wave where the particles move right and left along the wave. – Transverse Waves : Wave where the particles move up and down along the wave. 10/2/2020 3

What is Light? Light is a transverse wave which always has a trough and a crest. The distance between two adjacent troughs or crests is the wavelength of the wave. 10/2/2020 4

Wave Length The wavelength is the distance between repeating units of a wave pattern. It is represented by the greek letter lambda (λ). In a wave, the wavelength is the distance between peaks (Crests): 10/2/2020 5

Frequency is the number of waves made per second. This is usually measured in hertz (Hz). Here the frequency is 3 Hz: 10/2/2020 6

Amplitude The amplitude is the height of the wave. There is a positive amplitude and a negative amplitude. 10/2/2020 7

Refraction It is the bending of light when it passes from one medium to another medium. When the light enters a denser medium it slows down and when the light enters a less denser medium it speeds up. Incident ray - ray of light before it strikes the block. Refracted ray - ray of light after it has passed into the medium. Angle of incidence - angle between the normal and the incident ray. Angle of refraction - angle between the normal and the refracted ray. Emergent ray - ray that comes out on the opposite side of the block. 10/2/2020 8

Colour Dispersion is refraction of white light into its component colours when it passes through a prism because the different speed of the colours inside the prism. l Each colour is bent or refracted by a different amount. l The colour pattern that is seen is called the spectrum of white light. l 10/2/2020 9

Colour components A Prism can be used to refract white light into its component colours. This shows that white light is a mixture of the projected colours. VIOLET INDIGO Short Wave Length BLUE GREEN Long Wave Length YELLOW ORANGE RED 10/2/2020 10

Why is Sky Blue? l Optical Phenomenon. l It is an event that can be observed which results from the interaction of light and matter. l The blue in the sky we see is scattered blue light. 10/2/2020 11

Why is Sky Blue? l The blue colour of the sky is due to Rayleigh scattering. As light moves through the atmosphere – Most of the longer wavelengths pass straight through. – Shorter wavelength light is scattered by the gas molecules. l The scattered blue light is then radiated in different directions. It gets scattered all around the sky. 10/2/2020 12

Horizon To reach you, the scattered blue light must pass through more air. Some of it gets scattered away again in other directions. Less blue light reaches your eyes. The colour of the sky near the horizon appears paler or white. 10/2/2020 13

Rayleigh Scattering Rayleigh scattering (named after Lord Rayleigh or John William Strutt ) is the scattering of light by particles smaller than the wavelength of the light. It occurs when light travels in transparent solids and liquids, but is most prominently seen in gases. 10/2/2020 14

Rayleigh Law The amount of Rayleigh scattering that occurs to a beam of light, is dependent upon the size of the particles and the wavelength of the light. Considering this fact, he made the scattering coefficient. 10/2/2020 15

Rayleigh scattering coefficient The Rayleigh scattering coefficient ks is: where n is the number of scatters of diameter d; m is the index of refraction; and λ is the wavelength of the radiation. 10/2/2020 16

Summary So, in the atmosphere, blue photons get scattered across the sky to a greater extent than photons of a longer wavelength, and so one sees blue light coming from all regions of the sky. 10/2/2020 17

What about Violet? If shorter wavelengths are scattered most strongly, then why does not the sky appear violet, which has the shortest visible wavelength. – There is less violet in the light emitted from the sun as the light is not constant at all wavelengths, and additionally is absorbed by the high atmosphere. – Our eyes are also less sensitive to violet. Response curves for the three cones 10/2/2020 18

Sunset and Sunrise The bending of sunlight causes the sun to be seen after it has dropped below the horizon, both sunrise and sunset are daily optical illusions. 10/2/2020 19

Change in Colour As the sun begins to set, the light must travel further through the atmosphere before it reaches the surface. More of the light is reflected and scattered. As less reaches you directly, the sun appears less bright. 10/2/2020 20

Yellow Sun On Earth, the sun appears yellow. Some of the shorter wavelength light (the blues and violets) are removed from the direct rays of the sun by scattering. The remaining colours together appear yellow. 10/2/2020 21

White Sun The sun looks white in space because there is no atmosphere to scatter the sun's light. This is also partly why the space looks dark and black. 10/2/2020 22

Amazing Fact Why does the sky appear black at night as there are many stars all around Earth to give light to and make the night sky bright? 10/2/2020 23

Simple Answer The night sky is black because the stars are so far away that by the time it reaches us, the luminosity of the star is gone. It is supported by the Inverse Square Law. 10/2/2020 24

Inverse Square Law 10/2/2020 25

Complicated Answer The night sky is dark because of the Olbers’s Paradox. In simple language, the fact that the universe had a finite age and is expanding is the principle explanation of Olbers’s Paradox 10/2/2020 26

Olbers’s Paradox After a long calculations it says “The universe must be at least 6. 6 X 1015 years old in order to make the night sky as bright as the surface of the sun” According to studies, the universe is only about 8 – 12 billion years old. So we wont be holding our breath waiting for the night sky to be as bright as the daylight. 10/2/2020 27

Why Stars as dots? The smaller the θ , the smaller we see the object no matter what its size is. If you take the object of the same size further far, the object would seem smaller. 10/2/2020 28

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