電磁波--NASA

無所不在的電磁波

每天一些你看不到、摸不到、甚至感覺不到的東西圍繞著你，轟炸你。無論你走到哪裡。 它無臭無味。 然而你每天每時每刻都在使用它並依賴它。 沒有它，你所知道的世界就不可能存在。 它是什麼？ 電磁輻射。

電磁波

這些波的光譜從非常短的伽馬射線到X射線、紫外線、可見光波，甚至更長的紅外線、微波，再到可以測量到比山脈還要長的無線電波。 這個頻譜是資訊時代和現代世界的基礎。 您的收音機、遙控器、簡訊、電視、微波爐，甚至醫生的 X 光檢查，都依賴電磁波譜內的波。 電磁波（或 EM 波）與海浪相似，兩者都是能量波 - 它們傳輸能量。 電磁波是由帶電粒子振動產生的，具有電和磁特性。 但與需要水的海浪不同，電磁波以恆定的光速穿過太空真空。 電磁波像海浪一樣有波峰和波谷。 波峰之間的距離就是波長。

電磁波的波長

雖然有些電磁波波長非常長，以公尺為單位測量，但許多電磁波波長都很小，以十億分之一公尺(\(10^{-9}\) m)...奈米(nm, nano meter)為單位測量。 一秒鐘內通過給定點的波峰數量被描述為波的頻率。 每秒一個波或週期稱為赫茲。 長電磁波，例如無線電波，具有最低頻率並且攜帶較少能量。 增加能量會增加波的頻率並使波長更短。伽瑪射線是光譜中最短、能量最高的波。 所以，當你坐著看電視時，不僅有電視發出的可見光波射入你的眼睛，還有從附近電台發射的無線電波； 以及承載手機通話和簡訊的微波爐； 以及來自鄰居 WiFi 的電波； 以及行駛過的汽車中的 GPS 裝置。 現在有來自各個光譜的混亂波浪穿過你的房間！ 這麼多的波浪圍繞著你，你怎麼可能看你的電視節目？ 與將收音機調諧到特定廣播電台類似，我們的眼睛會調諧到電磁頻譜的特定區域，並且可以檢測波長為 400 至 700 奈米（光譜的可見光區域）的能量。 物體看起來有顏色是因為電磁波與其分子相互作用。 可見光譜中的某些波長被反射，其他波長被吸收。 這片葉子看起來是綠色的，因為電磁波與葉綠素分子相互作用。 長度在 492 到 577 奈米之間的波被反射，我們的眼睛將其解釋為葉子是綠色的。

電磁波在天文學和氣象科學上的應用

我們的眼睛看到葉子是綠色的，但無法告訴我們葉子如何反射紫外線、微波或紅外線。 為了更多地了解我們周圍的世界，科學家和工程師設計了一些方法，使我們能夠「看到」電磁光譜中稱為可見光的一小部分。 來自多個波長的數據幫助科學家研究地球上各種令人驚奇的現象，從季節變化到特定的棲息地。 我們周圍的一切都會根據其成分不同地發射、反射和吸收電磁輻射。 顯示電磁光譜某區域內這些交互作用的圖表稱為光譜特徵。 特徵模式（如光譜中的指紋）使天文學家能夠識別物體的化學成分並確定溫度和密度等物理特性。 NASA 的史匹哲太空望遠鏡觀測到 32 億光年外的星系中存在水和有機分子。 利用 SOHO 衛星以多個波長觀察太陽，科學家可以研究和了解與太陽耀斑和噴發相關的太陽黑子，這些太陽黑子對衛星、太空人和地球上的通訊有害。 透過利用電磁頻譜不同波中包含的獨特訊息，我們不斷了解更多關於我們的世界和宇宙的信息














Electromagnetic wave (NASA)

Something surrounds you. Bombards you some of which you can't see, touch, or even feel. Everyday. Everywhere you go. It is odorless and tasteless. Yet you use it and depend on it every hour of every day. Without it, the world you know could not exist. What is it? Electromagnetic radiation. These waves spread across a spectrum from very short gamma rays, to x-rays, ultraviolet rays, visible light waves, even longer infrared waves, microwaves, to radio waves which can measure longer than a mountain range. This spectrum is the foundation of the information age and of our modern world. Your radio, remote control, text message, television, microwave oven, even a doctor's x-ray, all depend on waves within the electromagnetic spectrum. Electromagnetic waves (or EM waves) are similar to ocean waves in that both are energy waves - they transmit energy. EM waves are produced by the vibration of charged particles and have electrical and magnetic properties. But unlike ocean waves that require water, EM waves travel through the vacuum of space at the constant speed of light. EM waves have crests and troughs like ocean waves. The distance between crests is the wavelength. While some EM wavelengths are very long and are measured in meters, many are tiny and are measured in billionths of a meter...nanometers. The number of these crests that pass a given point within one second is described as the frequency of the wave. One wave - or cycle - per second, is called a Hertz. Long EM waves, such as radio waves, have the lowest frequency and carry less energy. Adding energy increases the frequency of the wave and makes the wavelength shorter. Gamma rays are the shortest, highest energy waves in the spectrum. So, as you sit watching TV, not only are there visible light waves from the TV striking your eyes...But also radio waves transmitting from a nearby station; and microwaves carrying cell phone calls and text messages; and waves from your neighbor's WiFi; and GPS units in the cars driving by. There is a chaos of waves from all across the spectrum passing through your room right now! With all these waves around you, how can you possibly watch your TV show? Similar to tuning a radio to a specific radio station, our eyes are tuned to a specific region of the EM spectrum and can detect energy with wavelengths from 400 to 700 nanometers, the visible light region of the spectrum. Objects appear to have color because EM waves interact with their molecules. Some wavelengths in the visible spectrum are reflected and other wavelengths are absorbed. This leaf looks green because EM waves interact with the chlorophyll molecules. Waves between 492 and 577 nanometers in length are reflected and our eye interprets this as the leaf being green. Our eyes see the leaf as green, but cannot tell us anything about how the leaf reflects ultraviolet, microwave, or infrared waves. To learn more about the world around us, scientists and engineers have devised ways to enable us to 'see' beyond that sliver of the EM spectrum called visible light. Data from multiple wavelengths help scientists study all kinds of amazing phenomena on Earth, from seasonal change to specific habitats. Everything around us emits, reflects and absorbs EM radiation differently based on its composition. A graph showing these interactions across a region of the EM spectrum is called a spectral signature. Characteristic patterns, like fingerprints within the spectra allow astronomers to identify an object's chemical composition and to determine such physical properties as temperature and density. NASA's Spitzer space telescope observed the presence of water and organic molecules in a galaxy 3.2 billion light years away. Viewing our Sun in multiple wavelengths with the SOHO satellite allows scientists to study and understand sunspots that are associated with solar flares and eruptions harmful to satellites, astronauts and communications here on Earth. We are constantly learning more about our world and Universe by taking advantage of the unique information contained in the different waves across the EM spectrum



授課教師
陳永忠 ycchen@thu.edu.tw