Presentation on theme: "The nature of light The light of nature. The Sun is our star The Sun is our star Sunrise Sky and sea Sky and sea Colours of a rainbow Colours of a rainbow."— Presentation transcript:
The nature of light The light of nature
The Sun is our star The Sun is our star Sunrise Sky and sea Sky and sea Colours of a rainbow Colours of a rainbow Halos Lightning Sunset Aurora – northern and southern light Aurora – northern and southern light The Moon The Moon Stars Human light Human light
The Sun is our star The Sun is our star – it is classified as a yellow dwarf, because it is slightly smaller than the average star. The Sun provides the light for most natural phenomena of light, and it is of crucial importance for our life on Earth. The Sun is a giant sphere of gas – especially hydrogen and helium – where atoms of hydrogen fuse together to form helium. The Sun transform tons of hydrogen to helium every second. The nuclear process of the Sun result in both visible and invisible radiation – the infrared light and ultraviolet light. The light of our Sun is almost pure white, but it is in fact composed of many colours – the spectrum of the Sun.
The spectrum of colours becomes visible when the light is transmitted through a prism. The colours represent different wavelengths – therefore they are refracted at various angles. The spectrum of the Sun displays a number of black lines. These lines are called Fraunhofer lines or absorption lines, because they are formed by absorption of light by the matters of the Sun.
Sunrise s The sunrise often displays a reddish tint, but the colour is usually less distinct than the beautiful red sunset. During sunrise the Sun's light must pass through a much greater thickness of the atmosphere to reach an observer on the ground. This extra distance causes multiple scatterings of blue light, but relatively little scattering of red light; this is seen as a pronounced red-hued sky in the direction towards the sun.
Sky and sea The beautiful blue colour of a clear sky is actually an illusion – the atmosphere is transparent (if not polluted). The blue colour originate from the white light of the Sun – although the white light is composed of all colours. The blue colour is caused by scattering – Rayleigh scattering – where the various molecules of air and tiny dust particles scatter the blue and violet light more than any other colour of light. The blue and violet colours have the smallest wavelengths of visible light, which result in major scattering.
The Earth is called the blue planet – mainly because of the oceans, which show a distinctive blue colour, when viewed from space. It is the scattering of light by water molecules, that gives the blue colour of the sea. Even relatively shallow waters show the blue colouration, because water is more dense than air. The Danish seas are generally greyish green in colour, which is caused by a high concentration plankton and the abundance of clouds near the coastline. The sea may also reflect the colour of the sky.
Colours of a rainbow A rainbow is one of natures most loved phenomena of light. The are many tales of the rainbow – we all know, that there is a pot full of gold at the end of a rainbow. The colours arise when the white light of the Sun is refracted and reflected by water drops. The white light is a mixture of all colours, each with a characteristic wavelength. The water drops acts as small prisms, which split the white light into its coloured components. This picture shows a distinctive primary rainbow and a faint secondary rainbow.
In some cases You may see a double rainbow – one distinct and one with faint colours. This is caused by single and double reflection within the water drops.
Halos Halos, also known as icebows, are also optical phenomena that appear near or around the Sun or Moon. There are many types of optical halos, but they are mostly caused by ice crystals in cold cirrus clouds located high (5-10 km, or 3-6 miles) in the upper troposphere. The particular shape and orientation of the crystals is responsible for the type of halo observed. Light is reflected and refracted by the ice crystals and may split up into faint colours because of dispersion, similarly to the rainbow.
Lightning Lightning is a violent light phenomena accompanied by thunder. It has often been attributed to the anger of gods. Lightning are strong electrical currents, which originate from turbulent flows of air – especially during humid periods. The strong turbulence in a cloud result in a positive upper part and a negative lower part. The principle is the same as a balloon rubbed against woolly clothing. The large potential between cloud and ground – several million volts – may ionise the air and result in lightning. The lightning heats the air, which explodes in thunder.
The strong turbulence in a thunder- cloud creates friction between ice crystals, water droplets and dust resulting in static electricity. If the potential between cloud and ground is sufficiently high – a lightning evolves. The lightning seeks the easiest way to ground – i.e. a tree. Lightning facts Length ~0,3 – 3 km. But lightning between clouds may be over 100 km. Voltage ~ V max V Current ~ A max A Effect ~ kW Energy ~250 kWh Temperature ~ °C max °C
A typical cloud-ground lightning flash initiates inside the storm. The electric field cause a negatively charged channel to emerge from the storm base toward the ground. It takes the stepped leader 50 milliseconds to reach its full length. As the stepped leader approaches the ground, its strong, negative charge repels all negative charge within the immediate strike zone, while attracting vast amounts of positive charge. The stepped leader actually induces electric channels up from the ground known as "streamers. The electric potential of the stepped leader is connected to the ground and the negative charge starts flowing DOWN the established channel. An electric current wave, called a "return stroke", shoots UP the channel as a brilliant pulse. It takes the current about 1 microsecond to reach 30,000 amperes. The stroke actually travels FROM the ground INTO the cloud! It takes the current about 1 microsecond to reach 30,000 amperes. The stroke actually travels FROM the ground INTO the cloud!
A thundercloud may produce lightning with intervals of a few seconds – and the lightning often strike the same spot! Lightning is very dangerous and kill many people each year. If you hear thunder or see lightning, you should go into a house or a car. If you are unable to do so, you should avoid trees and to minimize your risk of being struck, crouch down on the balls of your feet – The lightning crouch. When inside a house or a car, you should avoid all apparatus with electric wiring – including the phone.
Even a plane might be struck by lightning.
Sunset The sunset may be a beautiful display of red colours – and the colouration is usually more distinct than a sunrise, due to a higher humidity. During sunset the Sun's light must pass through a much greater thickness of the atmosphere to reach an observer on the ground. This extra distance causes multiple scatterings of blue light, but relatively little scattering of red light; this is seen as a pronounced red-hued sky in the direction towards the sun. The scattering is mainly caused by air and water molecules.
Sunset at Mars. Mars has almost no atmosphere which result in a faint bluish sunset, as the blue colours of light are scattered the most. In the daytime, the sky on Mars is completely transparent. Dust and volcanic gases also scatters the light. Big volcanic eruptions like Mt. St. Helens give rise to especially strong colouration of the sunset. The effect last more than one year.
Aurora – northern and southern lights Northern lights are one of the most beautiful sights in the night. Aurora is typical of the circumpolar regions of both the north and south hemisphere. Aurora is caused by an interplay of the solar wind, the upper atmosphere and the magnetic field of Earth. The Aurora is usually seen as greenish bands waving over the sky. More rarely the colours are red or violet. The colour depends on the composition of the atmosphere – red in km, green in km and violet in km.
The solar wind is composed of hydrogen plasma – protons and electrons – flowing from the Sun. The activity of the Sun varies and may result in a solar storm with a high number of particles. The magnetic field transports the charged particles towards the polar regions of Earth. Within the ionosphere the particles interact with air molecules – especially oxygen and nitrogen. Interaction with oxygen produces red and green colours, whereas nitrogen produce green colours.
The Moon Moonlight is very romantic, but it is also give rise to tales of superstition and magic phenomena. The Moon is the lightest object in the sky at night. Half the Moon is always lit by the Sun, but the trajectory of the Moon results in different views from the Earth. Actually the Moon is darker than Earth, but it reflect part of the Sun light, which make it appear light in the dark night. Mercury, Venus, Mars, Jupiter and Saturn may also be visible at night due to reflected light from the Sun.
Stars The night sky is constantly changing, but the apparent movement of the stars is due to the rotation of Earth. Star signs has been known for thousand of years – today the brightest stars are placed into 88 star signs. The stars display a great variety – the distance to Earth varies, they have different ages, size and composition. Blue stars have a surface temperature up to °C, while our Sun only have a temperature of 5.500°C at the surface. The Magellan Cloud viewed from the Hubble telescope.
Human light Once upon a time the night belonged to the Moon and stars…… Today electric lights of roads and cities become more and more dominant. Sometimes we think it is beautiful and sometimes we call it light pollution. Electric light is great, when used with a purpose. It is very important that we use the light where it is needed. To many lights are poorly designed, resulting in a waste of energy and light pollution. In the cities it may be difficult to view even the brightest star, because our electric light blind us.
Even from space we can see the human lights. This picture clearly shows the position of the big cities of Earth.