Perbedaan Antara Hacker dan Cracker

Halo sobat blogger semua!!!!! Saya bisa menulis postingan ini setelah lama tidak diijinkan bermain internet oleh ibu saya. Selain itu, banyak sekali tugas sekolah yang harus dikerjakan. Maklum, saya adalah pelajar kelas 7 SMP.
Sekarang, saya akan menulis artikel tentang Perbedaan Antara Hacker dan Cracker. Ide ini muncul saat saya sedang membaca buku pelajaran, yaitu buku TIK. Saat saya membaca pada subbab “Dampak Negatif atau Kerugian Penggunaan TIK”, saya menemukan sebuah kotak yang mempunyai judul “Siapakah Hacker dan Cracker Itu?”. Saat saya membacanya, saya tertarik dan saya berniat untuk membuat artikel ini. Koq, Turfa banyak bacot yach? Tenang, sekarang saya akan menjelaskannya.
Peretas, yang biasa kita sebut hacker, adalah orang yang mempelajari, menganalisa, dan selanjutnya bila diinginkan, bisa membuat, memodifikasi, atau bahkan mengeksploitasi sistem yang terdapat di sebuah perangkat seperti perangkat lunak(software) dan perangkat keras(hardware) komputer seperti program komputer, admininstrasi dan hal-hal lainnya, terutama keamanan.
Selain itu, hacker juga merupakan sebutan untuk orang atau sekelompok orang yang memberikan sumbangan bermanfaat untuk dunia jaringan dan sistem operasi, membuat program bantuan untuk dunia jaringan dan komputer. Hacker juga bisa dikategorikan pekerjaan yang dilakukan untuk mencari kelemahan suatu sistem dan memberikan ide atau pendapat yang bisa memperbaiki kelemahan sistem yang ditemukannya.
Sedangkan cracker adalah sebuah sebutan untuk orang yang mencari kelemahan sistem dan memasukinya untuk kepentingan pribadi dan mencari keuntungan dari sistem yang dimasuki seperti pencurian data, penghapusan, dan banyak yang lainnya.
Terminologi hacker muncul pada awal tahun 1960-an di antara para anggota organisasi mahasiswa Tech Model Railroad Club di Laboratorium Kecerdasan Artifisial Massachusetts Institute of Teknology (MIT). Kelompok mahasiswa tersebut merupakan salah satu perintis perkembangan teknologi komputer dan mereka berkutat dengan sejumlah komputer mainframe. Kata hacker pertama kalinya muncul dengan arti positif untuk menyebut seorang anggota yang memiliki keahlian dalam bidang komputer dan mampu membuat program komputer yang lebih baik ketimbang yang telah dirancang bersama.
Kemudian pada tahun 1983, istilah hacker berubah menjadi negatif. Pasalnya, pada tahun tersebut untuk pertama kalinya FBI menangkap kelompok kriminal komputer The 414s yang berbasis di Milwaukee AS. 414 merupakan kode area lokal mereka. Kelompok yang kemudian disebut hacker tersebut dinyatakan bersalah atas pembobolan 60 buah komputer, dari komputer milik Pusat Kanker Memorial Sloan-Kettering hingga kompute milik Laboratorium Nasional Los Almos. Satu dari pelaku tersebut mendapatkan kekebalan atas testimonialnya, sedangkan 5 pelaku lainnya mendapat hukuman masa percobaan.
Hacker memiliki konotasi negatif karena kesalahpahaman masyarakat akan perbedaan istilah tetang hacker dan cracker. Banyak orang memahami bahwa hackerlah yang mengakibatkan kerugian pihak tertentu seperti mengubah tampilan suatu situs web(web defacing), menyisipkan kode-kode virus, dan sebagainya. Padahal, mereka adalah cracker. Crackerlah yang menggunakan celah-celah keamanan yang belum diperbaiki pleh pembuat perangkat lunak(bug) untuk menyusup dan merusak suatu sistem. Atas alasan ini biasanya para hacker dibagi menjadi 2 golongan, yaitu White Hat Hacker, yakni hacker yang sebenarnya dan Black Hat Hackers, yaitu para cracker ‘sang perusak sistem’.

Alexa Rank Naik Tajam

Hai sobat blogger semua..... Apa kabar? Rasanya jari-jari saya sudah tidak kuat untuk membuat postingan baru dengan Bahasa Indonesia karena selama ini saya membuat postingan dalam Bahasa Inggris. Saya kira, dengan hanya banyak postingan dalam Bahasa Inggris, Paman Google akan dengan baik hati meng-approve **lamaran kerja** untuk menjadi publisher. Tapi, ternyata lamaran saya di tolak juga,,,

Setelah di sibukkan dengan kegiatan men-copas artikel Bahasa Inggris, yang ternyata gagal, iseng-iseng saya cek Alexa Rank blog ini. Alangkah kagetnya ketika saya lihat bahwa Alexa Rank blog ini sudah mencapai 6 juta-an dan sudah masuk nominasi *alah, bahasanya* blog Indonesia sekitar 70ribu-an. Padahal, beberapa hari yang lalu AR blog ini masih sekita 19ribu-an. Dengan penasaran, saya cek AR blog saya yang lain, yaitu "Blog Punya Turfa". Untuk blog saya ini, AR nya sudah menembus 5 juta-an. Tapi anehnya, dalam kategori negara Indonesia blog ini masih sekita 90 ribu-an. Coba bandingkan dengan blog saya yang http://anak-tanjungsari.blogspot.com, AR nya hanya sekitar 6 juta-an, tapi dalam kategori negara Indonesia sudah mencapai 70ribu-an. Sedangkan di blog http://turfa-a.blogspot.com, AR nya 5 juta-an, tapi dalam kategori negara Indonesia hanya menempati tempat 90ribu-an. Untuk yang punya jawaban, silahkan tinggalkan di komentar.
Terima Kasih

JADIKAN SABAR DAN SHOLAT SEBAGAI PENOLONGMU

Didalam QS Al Baqarah ayat 153 Allah SWT berfirman,
“Wahai orang-orang yang beriman jadikanlah sabar dan sholat sebagai penolongmu. Sesungguhnya Allah beserta orang-orang yang sabar. MENGAPA SABAR DAN SHOLAT SEBAGAI PENOLONG? Kata sabar lebih dari seratus kali disebut didalam Al Qur’an. Hal ini menunjukkan betapa pentingnya makna sabar. Karena sabar merupakan poros, sekaligus inti dan asas segala macam kemuliaan akhlak. Jika kita telusuri lebih lanjut ternyata hakekat seluruh akhlak mulia, sabar selalu menjadi asas atau landasaannya. Misalnya :

- ‘Iffah (menjaga kesucian diri) adalah merupakan bentuk kesabaran dalam
menahan diri dari memperturutkan syahwat.

- Syukur adalah bentuk kesabaran untuk tidak mengingkari nikmat dari Allah SWT.

- Qana’ah (merasa cukup dengan apa yang ada) adalah sabar dengan menahan diri
dari angan-angan dan keserakahan.

- Hilm (lemah lembut) adalah kesabaran dalam mengendalikan amarah.

- Pemaaf adalah sabar untuk tidak membalas dendam.

- Demikian pula keutamaan akhlak lainnya, semuanya bersumbu pada kesabaran

Dengan kata lain secara psikologis kita bisa memaknai kesabaran sebagai suatu kemampuan untuk menerima, mengolah, dan menyikapi kenyataan. Jadi sabar adalah upaya menahan diri dalam melakukan sesuatu atau tidak melakukan sesuatu untuk mencapai ridho Allah SWT.Maka orang yang sabar adalah orang yang mampu menempatkan diri dan bersikap optimal dalam setiap keadaan. Sabar bukanlah sebuah bentuk keputus asaan tapi merupakan optimisme yang terukur. Ketika menghadapi situasi dimana kita harus marah misalnya maka marahlah secara bijak dan diniatkan untuk kebaikan bersama.

Sedangkan sholat adalah
ibadah yang diawali dengan takbir dan diakhiri dengan salam dengan gerakan dan bacaan tertentu seperti yang telah dicontohkan Rasulullah SAW. Sholat adalah ibadah paripurna yang memadukan olah pikir, gerak, dan rasa. Ketiganya terpadu secara serasi dan selaras dan saling melengkapi. Dalam sholat terintegrasi proses latihan meletakkan kendali diri secara proporsional, mulai dari gerakan, inderawi, aql, dan pengelolaan nafsu yang pada akhirnya akan menghasilkan jiwa yang bersifat muthma’innah. Orang yang memiliki jiwa muthma’innah inilah yang pada akhirnya akan mampu mengaplikasikan nilai-nilai sholat dalam keseharian yaitu nilai-nilai yang didominasi kesabaran paripurna. Prakteknya tercermin dalam sikap penuh syukur, pemaaf, lemah lembut, penyayang, tawakal, qana’ah, menjaga kesucian diri, istiqomah dsb. Dengan kata lain, orang yang sholatnya baik dalam hidupnya akan dipenuhi sifat sabar yang tercermin dalam tingginya akhlak dalam kehidupan sehari-hari.

Karena itulah maka Rasulullah, para sahabat, dan orang-orang shaleh menjadikan sholat sebagai istirahat, sarana pembelajaran, media pembangkit energi, sumber kekuatan dan pemandu untuk meraih kemenangan. Ketika mendapat rezeki melimpah, sholatlah ungkapan kesyukurannya. Ketika beban hidup makin berat, maka sholatlah yang meringankannya. Ketika rasa cemas membelenggu, sholatlah yang membebaskannya.

Maka tak heran bila khubaib bin Adi ketika akan menjalani eksekusi mati, dedengkot kafir quraisy memberi kesempatan untuk mengajukan permintaan terakhirnya. Apa yang dia minta? Ternyata yang diminta adalah kesempatan untuk sholat. Dengan kusyuk sholat dua rakaat ditunaikan. Selepas itu beliau berkata,”Andai saja aku tidak ingin dianggap takut dan mengulur-ulur waktu niscaya akan kuperpanjang lagi sholatku ini”.

Memang sholat yang baik akan menghasilkan kemampuan bersabar. Sebaliknya kesabaran yang baik akan menghasilkan sholat yang berkualitas yaitu terjadinya dialog dengan Allah SWT sehingga melahirkan kenikmatan, ketenangan yang tak terhingga di hati. Barangsiapa yang mampu merasakan nikmatnya berdialog dengan Allah SWT didalam Sholat maka niscaya Allah SWT akan membuka lebar-lebar pintu pertolonganNya. Sudahkan sholat kita demikian?

Oleh karena itu marilah kita berusaha menegakkan sholat dan mewarnai kehidupan kita penuh kesabaran agar pintu pertolongan senatiasa terbuka lebar untuk kita. Amin.

...Semoga bermanfaat dan dapat diamalkan ...

Meninggalkan Shalat

Tanya : Tiga tahun silam, sama sekali aku tidak shalat. Setelah itu perilakunya berubah dan penuh harapan semoga Allah menerima taubatku. Agar taubatku menjadi taubatan nasuha, maka aku kembali rajin shalat termasuk berjama’ah di masjid serta berusaha meninggalkan segala hal yang menipu agamaku atau mencoleng ahlak. Bolehkah aku mengqodho shalat yang tertinggal selama tiga tahun itu dan bagaimana caranya? (R. Sy. Karyawan Koperasi Perancis)

Jawab :
Saudara tidak wajib qodlo atas apa yang telah lewat mengingat dua hal :
1. Meninggalkan shalat adalah murtad dan membuat pelakunya kafir menurut pendapat terkuat yang memegang nash al-Qur’an dan Hadits. Dengan demikian, kembalinya saudara kepada Islam, berarti dosa-dosa yang lalu terhapus sebagaimana firman Allah :

“Katakanlah kepada orang-orang yang kafir itu:”Jika mereka berhenti (dari kekafirannya) niscaya Allah akan mengampuni mereka tentang do-sa-dosa mereka yang telah lalu”.(Al-Anfaal: 38)

2. Orang yang meninggalkan ibadah (shalat) yang sudah tentu waktunya lalu bertaubat, maka ia tidak wajib mengqadlanya, karena ibadah yang telah tentu waktunya telah ditetapkan oleh pembuat syari’at (Allah) baik awal atau akhirnya. Nabi Shallallahu ‘Alaihi Wasallam bersabda :

Barangsiapa yang melakukan suatu amal yang tidak ada dasarnya dari kami, maka ia ditolak”.

Dalam masalah seperti ini tidak berlaku hadits bahwa barangsiapa yang tidur atau lupa hingga tertinggal shalat, maka shalatlah ketika telah ingat. Dan tidak berlaku pula firman Allah yang menegaskan :

“Dan barangsiapa sakit atau dalam perjalanan (lalu ia berbuka), maka (wajiblah baginya berpuasa) sebanyak hari yang ditinggalkannya “. (Al-Baqoroh; 185)

Sebab menangguhkan kewajiban di sini dibenarkan alasannya. Mengqodlo sesuatu yang dibenarkan alasannya sama nilainya dengan menunaikannya. Dengan demikian, saudara tidak wajib qodlo atas apa yang telah lalu.

----------

Tanya : Seseorang telah meninggalkan shalat, namun tetap mengakui kewajibannya dan tetap memegang dua kalimat syahadat, wajibkah ia mandi dan mengucapkan kembali syahadat atau cukup dengan taubat dan meminta ampun saja ?

Jawab : Jika seseorang meninggalkan shalat dan berakibat kafir lalu ia bertaubat dan kembali shalat, amka hendaknya ia mandi dulu, sebab ia bertaubat dari kekafiran sebagai mana halnya orang masuk Islam, baik dianggap wajib atau dianjurkan menurut perbedaan pendapat yang ada. Sedangkan kalimat syahadatain tak perlu dituturkan sebab telah diakuinya.

Maraji':
257 Tanya Jawab Fatwa-fatwa Al-'Utsaimin, Muhammad Al-shaleh Al-'Utsaimin, Gema Risalah Press dengan alih bahasa Prof. Drs. KH. Masdar Helmy

HOW DO ANIMALS SPEND THE WINTER?

The weather gets colder, days get shorter and leaves turn color and fall off the trees. Soon, winter is here. Snow covers the ground. People live in warm houses and wear heavy coats outside. Our food comes from the grocery store. But what happens to the animals?

MIGRATE

Animals do many different, amazing things to get through the winter. Some of them "migrate." This means they travel to other places where the weather is warmer or they can find food.

Many birds migrate in the fall. Because the trip can be dangerous, some travel in large flocks. For example, geese fly in noisy, "V"-shaped groups. Other kinds of birds fly alone.

How do they know when it is time to leave for the winter? Scientists are still studying this. Many see migration as part of a yearly cycle of changes a bird goes through. The cycle is controlled by changes in the amount of daylight and the weather.

Birds can fly very long distances. For example, the Arctic tern nests close to the North Pole in the summer. In autumn, it flys south all the way to Antarctica. Each spring it returns north again.

Most birds migrate shorter distances. But how do they find their way to the same place each year? Birds seem to navigate like sailors once did, using the sun, moon and stars for direction. They also seem to have a compass in their brain for using the Earth's magnetic field.

Other animals migrate, too. There are a few mammals, like some bats, caribou and elk, and whales that travel in search of food each winter. Many fish migrate. They may swim south, or move into deeper, warmer water.

Insects also migrate. Some butterflies and moths fly very long distances. For example, Monarch butterflies spend the summer in Canada and the Northern U.S. They migrate as far south as Mexico for the winter. Most migrating insects go much shorter distances. Many, like termites and Japanese beetles, move downward into the soil. Earthworms also move down, some as far as six feet below the surface.

ADAPT

Some animals remain and stay active in the winter. They must adapt to the changing weather. Many make changes in their behavior or bodies. To keep warm, animals may grow new, thicker fur in the fall. On weasels and snowshoe rabbits, the new fur is white to help them hide in the snow.

Food is hard to find in the winter. Some animals, like squirrels, mice and beavers, gather extra food in the fall and store it to eat later. Some, like rabbits and deer, spend winter looking for moss, twigs, bark and leaves to eat. Other animals eat different kinds of food as the seasons change. The red fox eats fruit and insects in the spring, summer and fall. In the winter, it can not find these things, so instead it eats small rodents.

Animals may find winter shelter in holes in trees or logs, under rocks or leaves, or underground. Some mice even build tunnels through the snow. To try to stay warm, animals like squirrels and mice may huddle close together.

Certain spiders and insects may stay active if they live in frost-free areas and can find food to eat. There are a few insects, like the winter stone fly, crane fly, and snow fleas, that are normally active in winter. Also, some fish stay active in cold water during the winter.

HIBERNATE

Some animals "hibernate" for part or all of the winter. This is a special, very deep sleep. The animal's body temperature drops, and its heartbeat and breathing slow down. It uses very little energy. In the fall, these animals get ready for winter by eating extra food and storing it as body fat. They use this fat for energy while hibernating. Some also store food like nuts or acorns to eat later in the winter. Bears, skunks, chipmunks, and some bats hibernate.

AND MORE

Cold-blooded animals like fish, frogs, snakes and turtles have no way to keep warm during the winter. Snakes and many other reptiles find shelter in holes or burrows, and spend the winter inactive, or dormant. This is similar to hibernation.

Water makes a good shelter for many animals. When the weather gets cold, they move to the bottom of lakes and ponds. There, frogs, turtles and many fish hide under rocks, logs or fallen leaves. They may even bury themselves in the mud. They become dormant. Cold water holds more oxygen than warm water, and the frogs and turtles can breath by absorbing it through their skin.

Insects look for winter shelter in holes in the ground, under the bark of trees, deep inside rotting logs or in any small crack they can find. One of the most interesting places is in a gall. A gall is a swelling on a plant. It is caused by certain insects, fungi or bacteria. They make a chemical that affects the plant's growth in a small area, forming a lump. The gall becomes its maker's home and food source.

Every type of insect has its own life cycle, which is the way it grows and changes. Different insects spend the winter in different stages of their lives. Many insects spend the winter dormant, or in "diapause." Diapause is like hibernation. It is a time when growth and development stop. The insect's heartbeat, breathing and temperature drop. Some insects spend the winter as worm-like larvae. Others spend the winter as pupae. (This is a time when insects change from one form to another.) Other insects die after laying eggs in the fall. The eggs hatch into new insects in the spring and everything begins all over again.

Source : Science Made Simple

THE ATMOSPHERE

WHAT IS THE ATMOSPHERE?

The atmosphere is the mixture of gases and other materials that surround the Earth in a thin, mostly transparent shell. It is held in place by the Earth's gravity. The main components are nitrogen (78.09%), oxygen (20.95%), argon (0.93%), and carbon dioxide (0.03%). The atmosphere also contains small amounts, or traces, of water (in local concentrations ranging from 0% to 4%), solid particles, neon, helium, methane, krypton, hydrogen, xenon and ozone. The study of the atmosphere is called meteorology.

Life on Earth would not be possible without the atmosphere. Obviously, it provides the oxygen we need to breath. But it also serves other important functions. It moderates the planet's temperature, reducing the extremes that occur on airless worlds. For example, temperatures on the moon range from 120 °C (about 250 °F) in the day to -170 °C (about -275 °F) at night. The atmosphere also protects us by absorbing and scattering harmful radiation from the sun and space.

Of the total amount of the sun's energy that reaches the Earth, 30% is reflected back into space by clouds and the Earth's surface. The atmosphere absorbs 19%. Only 51% is absorbed by the Earth's surface.

We are not normally aware of it but air does have weight. The column of air above us exerts pressure on us. This pressure at sea level is defined as one atmosphere. Other equivalent measurements you may hear used are 1,013 millibars, 760 mm Hg (mercury), 29.92 inches of Hg, or 14.7 pounds/square inch (psi). Atmospheric pressure decreases rapidly with height. Pressure drops by a factor of 10 for every 16 km (10 miles) increase in altitude. This means that the pressure is 1 atmosphere at sea level, but 0.1 atmosphere at 16 km and only 0.01 atmosphere at 32 km.

The density of the lower atmosphere is about 1 kg/cubic meter (1 oz./cubic foot). There are approximately 300 billion billion (3 x 10**20, or a 3 followed by 20 zeros) molecules per cubic inch (16.4 cubic centimeters). At ground level, each molecule is moving at about 1600 km/hr (1000 miles/hr), and collides with other molecules 5 billion times per second.

The density of air also decreases rapidly with altitude. At 3 km (2 miles) air density has decreased by 30%. People who normally live closer to sea level experience temporary breathing difficulties when traveling to these altitudes. The highest permanent human settlements are at about 4 km (3 miles).

LAYERS OF THE ATMOSPHERE

The atmosphere is divided into layers based on temperature, composition and electrical properties. These layers are approximate and the boundaries vary, depending on the seasons and latitude. (The boundaries also depend on which "authority" is defining them.)

LAYERS BASED ON COMPOSITION

Homosphere

· The lowest 100 km (60 miles), including the Troposphere, Stratosphere and Mesosphere.

· Contains 99% of the atmosphere's mass.

· Molecules do not stratify by molecular weight.

· Although small local variations exist, it has a relatively uniform composition, due to continuous mixing, turbulence and eddy diffusion.

· Water is one of two components that is not equally distributed. As water vapor rises, it cools and condenses, returning to earth as rain and snow. The Stratosphere is extremely dry.

· Ozone is another molecule not equally distributed. (Read about the ozone layer in the Stratosphere section below.)

Heterosphere

· Extends above homosphere, including the Thermosphere and Exosphere.

·Stratified (components are separated in layers) based on molecular weight. The heavier molecules, like nitrogen and oxygen, are concentrated in the lowest levels. The lighter ones, helium and hydrogen, predominate higher up.

LAYERS BASED ON ELECTRICAL PROPERTIES

Neutral atmosphere

· Below about 100 km (60 miles)

Ionosphere

· Above about 100 km

· Contains electrically charged particles or ions, created by the absorption of UV (ultraviolet) light.

· The degree of ionization varies with altitude.

· Different layers reflect long and short radio waves. This allows radio signals to be sent around the curved surface of the earth.

· The Aurora Borealis and Aurora Australis (the Northern and Southern Lights) occur in this layer.

· The Magnetosphere is the upper part of the ionosphere, extending out to 64,000 km (40,000 miles.) It protects us from the high energy, electrically charged particles of the solar wind, which are trapped by the Earth's magnetic field.

LAYERS BASED ON TEMPERATURE

Troposphere - Height depends on the seasons and latitude. It extends from ground level up to about 16 km (10 miles) at the equator, and to 9 km (5 miles) at the North and South Poles.

· The prefix "tropo" means change. Changing conditions in the Troposphere result in our weather.

· Temperature decreases with increasing altitude. Warm air rises, then cools and falls back to Earth. This process is called convection, and results in huge movements of air. Winds in this layer are mostly vertical.

· Contains more air molecules than all the other layers combined.

Stratosphere - Extends out to about 50 km (30 miles)

· The air is very thin.

· The prefix "strato" is related to layers, or stratification.

· The bottom of this layer is calm. Jet planes often fly in the lower Stratosphere to avoid bad weather in the Troposphere.

· The upper part of the Stratosphere holds the high winds known as the jet streams. These blow horizontally at speeds up to 480 km/hour (300 miles/hour)

· Contains the "ozone layer" located between 15 - 40 km ( 10 - 25 miles) above the surface. Although the concentration of ozone is at most 12 parts per million (ppm), it is very effective at absorbing the harmful ultraviolet (UV) rays of the sun and protecting life on Earth. Ozone is a molecule made of three oxygen atoms. The oxygen molecule we need to breathe contains two oxygen atoms.

· The temperature is cold, about -55 °C (-67 °F) in the lower part, and increases with increasing altitude. The increase is caused by the absorption of UV radiation by the oxygen and ozone.

· The temperature increase with altitude results in a layering effect. It creates a global "inversion layer", and reduces vertical convection.

Mesosphere - Extends out to about 100 km (65 miles)

· Temperature decreases rapidly with increasing altitude.

Thermosphere - Extends out to about 400 km ( 250 miles)

· Temperature increases rapidly with increasing altitude, due to absorption of extremely short wavelength UV radiation.

· Meteors, or "shooting stars," start to burn up around 110-130 km (70-80 miles) above the earth.

Exosphere -Extends beyond the Thermosphere hundreds of kilometers, gradually fading into interstellar space.

· Density of the air is so low that the normal concept of temperature loses its meaning.

· Molecules often escape into space after colliding with one another.

WHY IS THE SKY BLUE?

On a clear sunny day, the sky above us looks bright blue. In the evening, the sunset puts on a brilliant show of reds, pinks and oranges. Why is the sky blue? What makes the sunset red?

To answer these questions, we must learn about light, and the Earth's atmosphere.

THE ATMOSPHERE

The atmosphere is the mixture of gas molecules and other materials surrounding the earth. It is made mostly of the gases nitrogen (78%), and oxygen (21%). Argon gas and water (in the form of vapor, droplets and ice crystals) are the next most common things. There are also small amounts of other gases, plus many small solid particles, like dust, soot and ashes, pollen, and salt from the oceans.

The composition of the atmosphere varies, depending on your location, the weather, and many other things. There may be more water in the air after a rainstorm, or near the ocean. Volcanoes can put large amounts of dust particles high into the atmosphere. Pollution can add different gases or dust and soot.

The atmosphere is densest (thickest) at the bottom, near the Earth. It gradually thins out as you go higher and higher up. There is no sharp break between the atmosphere and space.

LIGHT WAVES

Light is a kind of energy that radiates, or travels, in waves. Many different kinds of energy travel in waves. For example, sound is a wave of vibrating air. Light is a wave of vibrating electric and magnetic fields. It is one small part of a larger range of vibrating electromagnetic fields. This range is called the electromagnetic spectrum.

Electromagnetic waves travel through space at 299,792 km/sec (186,282 miles/sec). This is called the speed of light.

The energy of the radiation depends on its wavelength and frequency. Wavelength is the distance between the tops (crests) of the waves. Frequency is the number of waves that pass by each second. The longer the wavelength of the light, the lower the frequency, and the less energy it contains.

COLORS OF LIGHT

Visible light is the part of the electromagnetic spectrum that our eyes can see. Light from the sun or a light bulb may look white, but it is actually a combination of many colors. We can see the different colors of the spectrum by splitting the light with a prism. The spectrum is also visible when you see a rainbow in the sky.

The colors blend continuously into one another. At one end of the spectrum are the reds and oranges. These gradually shade into yellow, green, blue, indigo and violet. The colors have different wavelengths, frequencies, and energies. Violet has the shortest wavelength in the visible spectrum. That means it has the highest frequency and energy. Red has the longest wavelength, and lowest frequency and energy.

LIGHT IN THE AIR

Light travels through space in a straight line as long as nothing disturbs it. As light moves through the atmosphere, it continues to go straight until it bumps into a bit of dust or a gas molecule. Then what happens to the light depends on its wave length and the size of the thing it hits.

Dust particles and water droplets are much larger than the wavelength of visible light. When light hits these large particles, it gets reflected, or bounced off, in different directions. The different colors of light are all reflected by the particle in the same way. The reflected light appears white because it still contains all of the same colors.

Gas molecules are smaller than the wavelength of visible light. If light bumps into them, it acts differently. When light hits a gas molecule, some of it may get absorbed. After awhile, the molecule radiates (releases, or gives off) the light in a different direction. The color that is radiated is the same color that was absorbed. The different colors of light are affected differently. All of the colors can be absorbed. But the higher frequencies (blues) are absorbed more often than the lower frequencies (reds). This process is called Rayleigh scattering. (It is named after Lord John Rayleigh, an English physicist, who first described it in the 1870's.)

WHY IS THE SKY BLUE?

The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air.

However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue.

As you look closer to the horizon, the sky appears much paler in color. 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 color of the sky near the horizon appears paler or white.

THE BLACK SKY AND WHITE SUN

On Earth, the sun appears yellow. If you were out in space, or on the moon, the sun would look white. In space, there is no atmosphere to scatter the sun's light. On Earth, some of the shorter wavelength light (the blues and violets) are removed from the direct rays of the sun by scattering. The remaining colors together appear yellow.

Also, out in space, the sky looks dark and black, instead of blue. This is because there is no atmosphere. There is no scattered light to reach your eyes.

WHY IS THE SUNSET RED?

As the sun begins to set, the light must travel farther through the atmosphere before it gets to you. More of the light is reflected and scattered. As less reaches you directly, the sun appears less bright. The color of the sun itself appears to change, first to orange and then to red. This is because even more of the short wavelength blues and greens are now scattered. Only the longer wavelengths are left in the direct beam that reaches your eyes.

The sky around the setting sun may take on many colors. The most spectacular shows occur when the air contains many small particles of dust or water. These particles reflect light in all directions. Then, as some of the light heads towards you, different amounts of the shorter wavelength colors are scattered out. You see the longer wavelengths, and the sky appears red, pink or orange.

Source : Scince Made Simple