ELECTRO MAGNETIC SPECTRUM

Transcription

1 ELECTRO MAGNETIC SPECTRUM The Electro Magnetic Spectrum encompasses a continuous range of frequencies or wavelengths of electro magnetic radiation, ranging from long wave length, low energy radio waves to short wavelength, high frequency, high-energy gamma rays. Frequency is defined as the number of wave cycles that pass a particular point per unit time and is commonly measured in Hertz (cycles per second), wavelength defines the distance between adjacent points of the electro magnetic wave that are in equal phase (wave crests). German physicist Maxwell Planck proposed that atoms absorb or emit electromagnetic radiation only in certain bundles termed quanta. Albert Einstein used the term photon to describe these electromagnetic quanta. Planck determined that energy of light was proportional to its frequency ie as the frequency of light increases, so does the energy of light. Although electromagnetic radiation is now understood as having both photon (particle) and wave-like properties, descriptions of the electromagnetic spectrum generally utilizes wave related terminology ie frequency and wavelength. Electromagnetic fields and photons exert forces that can excite electrons. As electrons transition between allowed orbitals, energy must be conserved. This conservation is achieved by the emission of photons when an electron moves from a higher potential orbital energy to lower potential orbital energy. Accordingly, light is emitted only at certain frequencies characteristic of every atom and molecule. Correspondingly, atoms and molecules absorb only a limited range of frequencies and wavelengths of the electromagnetic spectrum and reflect all other frequencies and wavelengths of light. These reflected frequencies and wavelengths are often the actual observed light or colours associated with an object. Electromagnetic radiation differ significantly in their properties in their means of production and also in the way we observe then, they have certain other features in common. The following are common features. (i) They all can be described in terms of oscillating electric (E) and magnetic B fields perpendicular to each other and hence are called electromagnetic (E.M) radiation. (ii) These electric and magnetic fields are found to oscillate perpendicular to the direction of propagation of radiation. All electromagnetic waves are transverse in nature. (iii) All these radiations travel with the same speed, that is the speed of light (C) in vacuum. The velocity (C) in related to frequency (η) and wavelength (λ). C = ηλ = m/s The waves travelling with velocity of light and consisting of oscillating electric and magnetic fields perpendicular to each other and also perpendicular to the direction of their propagation are called the electromagnetic waves. Such waves with different ranges of frequency constitute an electromagnetic spectrum.

2 The difference in properties of different types of E.M radiations arise only from the difference in their wavelengths (λ) or frequency (η). Therefore, the name given to particular region of electromagnetic spectrum is based on the range of its wavelengths or frequencies. The variation in the wave lengths of electromagnetic radiation is a continuous one and the various regions overlap and hence are not sharply defined. The energy of electromagnetic radiation. (ie photon) is inversely proportional to wavelength.

3 Electromagnetic Spectrum Diagram

4 Here are the different types of radiation in the EM spectrum, in order from lowest energy to highest. Radio waves: Radio waves with wavelengths ranging from hundred meters to about one millimeter. They are used for transmission of data via modulation. Radio waves are used in television, mobile phones, wireless networking and amateur radio. They are produced by the electromagnetic oscillators of low frequency. These radiations are created when electrons are a accelerated in a suitable electronic circuit. They can travel long distances carrying messages. These are also received from extra terrestrial sources. Mapping of the radio emissions from extra terrestrial sources is known as "radio-astronomy". This has provided information about the universe which is not obtainable using an optical telescope. Still higher frequency radio waves with wavelengths of a few centimetres can be utilized for RADAR imaging. Microwaves: Microwaves with wavelengths in the range 10µm to 10m these waves are commonly produced by electromagnetic oscillators with high frequency in electric circuits. They are used in telemetry, microwave ovens and RADAR. These are also used to transmit telephone conversations. The satellite communications also utilises microwaves. Infrared Radiation (IR): Infrared radiation comprises the region of the electromagnetic spectrum where the wavelength of light is measured region from 0.7 µm to about 100µm. They change their states of rotational or vibrational motion. Heat from hot bodies travels in the form of IR radiations and therefore all hot bodies are sources of IR radiation. These radiations can be detected by devices like thermopiles, bolometers, thermometers etc. which are sensitive to heat. These radiation are used in physiotherapy and to take photographs of objects in darkness. Visible radiation: Above infrared in frequency comes visible light. This is the range in which the sun and stars similar to emit most of their radiations, these radiation stimulate the rod and comes in the human with the eye is termed the visible region of electromagnetic spectrum. Colour in association the eye makes with selected portions of that visible region ie., particular colours are associated with specific wavelengths of visible light. A nanometer (10 9 m) is the most common unit used for characterising the wavelength of visible light. Using the unit, the visible portion of the electromagnetic spectrum is located between 380nm-750nm and the component of colour regions of the visible spectrum are Red ( nm), Orange ( nm), Yellow ( nm), Green ( nm), Blue ( nm), Indigo ( nm) and Violet ( nm) Red (the longest in wavelength) is the lowest in energy. As wavelengths contract towards the blue and of visible region of the electromagnetic spectrum; the frequencies and energies of colour steadily increase. The colour of light emitted by a particular substance is characteristic of the atoms of the substance. We see different colours produced by Diwali crackers. Similarly, the light emitted from the sun and distant stars gives information about the composition of the matter present in them. Infrared would be located first beyond red side of the rainbow with ultra violet appearing just beyond the violet end. Electromagnetic radiation with a wavelength between 380 nm and 760 nm is detected by human eye and perceived as visible light.

5 Ultra violet radiation: Ultra violet radiation whose wavelength is shorter than the violet end of the visible spectrum and longer than that of an X-ray. Being very energetic, UV can break chemical bonds making molecules unusually reactive or ionizing them in general changing their mutual behaviour. Sun burns is caused by the disruptive effect of UV radiations on skin cells, which is the main cause of skin cancer. The sun emit a large amount of UV radiations, which could quickly turn. Earth in to a barren desert. However most of it is absorbed by the atmosphere ozone layer before reaching the surface. Ultra violet radiation ranging from about 0.4µm to 1 nm. The atmospheric ozone is depleted in recent years as a result of chemical reactions with fluoro carbons released from aerosol sprays, refrigeration equipment and other pollutants. X-rays: After UV come X-rays, which are also ionizing, but due to their higher energies they can also interact with matter by means of Compton effect. Wavelength of X-rays range from nm (0.01A ) to 10 nm (100A ). The spacing between the atoms in a crystalline solid ranges from 0.1 to 10A. Therefore X-rays of shorter wavelengths are used to determine the structure of material. These X-rays are called hard X-rays. X-rays are also used for testing of materials in industry. X-rays of longer wavelengths (10 to 100A ) can easily penetrate through soft tissues a human body but can not pass through the bones and other solid materials. For this reason, these X-rays called soft X-rays, are used in medical diagnosis called "radiography". The method by which some diseases can be curved is "Radiotherapy". Gamma (γ) rays: Gamma rays are electromagnetic radiations with shortest wavelengths ranging from nm to 0.1 mm or from (0.001A ) to 1A. Natural radioactive substance like 235 U emit. These γ-radiations. These are emitted when an excited nucleus comes to its ground state. They are most penetrating electromagnetic radiations. exposure to intense gamma radiation leads to harmful effects on human body. Finally we conclude that, EM radiation can be described in terms of a stream of photons, which are massless particles each travelling in a wave-like pattern and moving at the speed of light. Each photons contains a certain amount (bundle) of energy and all EM radiations contains a certain these photons. The only difference between the various types of EM radiation is the amount of energy found in the photons. Radio waves have photons with low energies, microwave have a little more energy than radio waves infrared has still more than visible light, Ultra violet, X-rays and the most energetic of all is gamma rays. Very Short Answers (1 Mark) 1. What is a spectrum? A. 1. When white light disperses through a prism and falls on a screen we get different coloured bands on it. 2. This pattern is called spectrum. 2. What are electromagnetic radiations? A. Electromagnetic radiations are characterised by oscillating electric and magnetic fields perpendicular to each other and perpendicular to the direction of propagation.

6 3. Define the light ray? A. The path of a light wave or light particle is called light ray. 4. What is thermopile? A. A device which is used to detect or measure the intensity of heat radiation or temperature is called thermopile. 5. What are the various types of electromagnetic radiations? A. Electromagnetic radiations are seven types they are: 1) Visible 2) IR spectrum 3) Microwaves 4) Radio waves 5) UV spectrum 6) X-rays 7) γ-rays 6. Which electromagnetic radiations are having larger wavelength? A. Radio waves 7. Which electromagnetic radiations are having shorter wavelength? A. Gamma rays 8. What is meant by Radiography? A. Diagnosis of a disease by the use of soft X-ray is called radiography. 9. What is meant by Radiotherapy? A. Cure of disease by the use of soft X-rays is called radiotherapy. Short Answer Questions (2 Marks) 1. What are various types of electromagnetic radiations? A. The various types of electromagnetic radiations are 1) visible radiations 2) infrared radiations 3) Microwaves 4) Radio waves 5) Ultraviolet radiations 6) X-rays and 7) γ-rays. 2. Mention the uses of infrared radiations. A. Infrared radiations are used in i) physiotherapy ii) to take photographs of objects in darkness 3. What are the applications of microwaves? A. Microwaves are often used in

7 1) Radar 2) Telemetry 3) Microwave ovens and 4) in satellite communications 4. Distinguish between soft X-rays and hard X-rays. A. Soft X-rays 1. X-rays of longer wavelengths are called soft X-rays. 2. Their wavelength is 10A to 100A. 3. They have low penetrating nature. 4. They are used in radiography and radiotherapy. Hard X-rays 1. X-rays of shorter wavelength are called X-ray. 2. Their wavelength is 0.01A to 10A. 3. They have high penetrating nature. 4. These rays are used in the determination of structure of materials and for testing the materials in industries. 5. What are the common features among all the electromagnetic radiations? A. The common features among all the electromagnetic radiations are 1. They all can be described in terms of oscillating electric E and magnetic () B fields perpendicular to each other and hence are called electromagnetic (e.m) radiations. 2. These electric and magnetic fields are found to oscillate perpendicular to the direction of propagation of radiations. 3. All electromagnetic waves are transverse in nature. 4. All these radiations travel with the same speed that is the speed of light (C) in vacuum. Long Answer Questions (4 Marks) 1. Discuss the different types of electromagnetic radiations. Mention their wave length ranges. How are they produced? A. 1. Visible Spectrum: The wavelength of the visible spectrum extends from about 0.4 µm to 0.7 µm, that is, from violet colour to red colour. Visible spectrum is emitted when the excited valence electrons in atom jump back to their normal states. 2. Infrared (IR) Spectrum: Infrared radiations have wavelengths larger than those of visible light. That is from 0.7µm to about 100µm. They are emitted by ()

8 molecules when they changes their states of rotational or vibrational motion. 3. Microwaves: Microwaves are electromagnetic waves with wavelengths in the range 10 µm to 10m. These waves are commonly produced by electromagnetic oscillators with high frequency ( c/s) in electric circuits. 4. Radio waves: Radio waves have wavelengths from 1m to about 100 km. They are produced by the electromagnetic oscillators of low frequency. 5. Ultra Violet (UV) Spectrum: The radiations of wavelength shorter than the visible light ranging from about 0.4 µm to 1 nm are called ultraviolet (UV) radiations. These radiation are produced by the transition of electrons in atoms. 6. X-rays: Wavelengths of X-rays range from nm to 10 nm X-rays are produced when incident electrons are decelerated inside the target atoms. 7. Gamma rays: Gamma rays wavelengths ranges from nm to 0.1 nm. Natural radioactive substances like 235 U emits gamma rays. 2. In what way various types of electromagnetic spectrum differ from one another? How are they produced? A. The various types of electromagnetic spectrum differ from one another in the following ways: 1. Wavelength or frequency range of the spectrum. 2. The mode of production of different types of radiations. Production of electromagnetic waves or spectrum: 1. Visible spectrum is produced when the electrons of excited atoms jump down from the upper energy level to their normal energy levels (lower energy levels) to fill the vacancy of electron. 2. Infrared spectrum is produced by the molecules of hot bodies when they change their rotational or vibrational state. 3. Microwaves are produced in electric circuit by oscillating electric and magnetic fields of high frequency.

9 4. Radio waves are produced by an oscillating electromagnetic field of low frequencies. 5. Ultraviolet waves are produced by jumping down of electrons from high energy levels to the lower energy levels in an atom. 6. X-rays are produced either by transition of inner electrons from the outer most or free electrons or by slowing down of a beam of electrons from hard metallic target. 7. γ-rays are emitted out by the radioactive nucleus when they come from excited state to the normal by emitting some sub atomic particles or only γ-rays. I. Multiple Choice Questions PART - B Objective Type Questions 1. Velocity of electromagnetic radiation ( ) a) ms 1 b) ms 1 c) kms 1 d) cms 1 2. What type of waves does light constitute? ( ) a) mechanical waves b) magnetic waves c) electromagnetic waves d) none of these 3. The properties of different electromagnetic radiations are identified by using a) their color ( ) b) their use c) their frequencies only d) their frequencies and wavelength 4. Electromagnetic waves are in nature. ( ) a) longitudinal b) transverse c) both longitudinal and transverse d) none of these 5. Which of the following has shortest wavelength? ( ) a) γ-rays b) radio waves c) visible spectrum d) microwaves 6. Which of the following has longest wavelength? ( ) a) visible spectrum b) γ-rays c) radio waves d) all of these three

10 7. Frequency and energy of an electromagnetic radiation are ( ) a) directly related b) inversely related c) unrelated d) related as E ν 2 8. Electromagnetic waves transport ( ) a) charge b) frequency c) wavelength d) energy 9. 1A = cm ( ) a) 10 8 b) c) 10 9 d) The energy of electromagnetic radiation depends on ( ) a) both its amplitude and wavelength b) its wavelength c) its amplitude d) temperature of the medium through which it passes nm = A ( ) a) 100 b) 1000 c) 10 d) Infrared radiations are nothing but a form of ( ) a) sound energy b) light energy c) heat energy d) none of these 13. Electromagnetic radiation with minimum wavelength is ( ) a) ultraviolet b) radiowave c) infrared d) X-rays 14. are used in telemetry ( ) a) infrared spectrum b) microwaves c) radio waves d) ultraviolet spectrum 15. Mapping of the radio emissions from extra terrestrial sources is known as a) asteroids b) radio astronomy c) radio mapping d) radio photography

11 II. Fill in the Blanks 1. The emitted by a particular substance is characteristic of the atoms of the substance. 2. are emitted by molecules when they change their states of rotational or vibrational motion. 3. bodies are the sources of I.R. radiations. 4. are used to take photograph of objects in darkness. 5. are commonly produced by electronic oscillators with high frequency. 6. are produced by the electromagnetic oscillation of low frequency. 7. radiations are produced by the transitions of the electrons in atoms. 8. are used to determine the structure of materials. 9. The method of curing diseases with the help of X-rays is called 10. γ-rays are emitted in 11. A is a group of wavelengths of frequencies. 12. Visible spectrum is emitted when in atoms jump to their normal states. 13. glass absorbs infrared radiations. 14. Prism made of are used for observing infrared radiations. 15. The velocity of UV radiation is 16. In electromagnetic wave electric and magnetic fields are mutually 17. The electric and magnetic fields are found to oscillate perpendicular to the direction of 18. C = λ 19. The region of the electromagnetic spectrum is most familiar to us. 20. The colour of light emitted by a particular substance is of the atoms of the substance. 21. radiations are sued to take photographs of objects in darkness. 22. Ozone layer is depleted in recent years due to the chemical reactions with 23. waves are used in Radar. 24. Most penetrating radiations are III. Matching. Group-A Group-B 1. Infrared spectrum a) Light emitted by sun and stars 2. Microwaves b) Hot bodies 3. Radio waves c) They are commonly produced by electromagnetic oscillator with high frequency cs 1 4. Ultraviolet spectrum d) When electrons are accelerated in an aerial 5. Gamma rays e) Produced by transitions of the electrons in atoms. f) Produced when electrons are deaccelerated g) They also produce in the decay of certain elementary particle.

12 I. Multiple Choice Questions 1. b 2. c 3. d 4. b 5. a 6. c 7. a 8. d 9. a 10. b 11. c 12. c 13. d 14. b 15. b II. Fill in the Blanks 1. colour of light 2. infrared radiation 3. hot 4. infrared radiations 5. microwaves 6. radio waves 7. ultraviolet 8. X-rays 9. radiotherapy 10. radioactivity 11. spectrum 12. excited electrons 13. soda 14. rock salt cms perpendicular 17. propagation of radiation 18. ν 19. visible 20. characteristic 21. infrared 22. fluoro carbons 23. micro 24. γ-rays Answers III. Matching 1. b 2. c 3. d 4. e 5. g