ELECTROMAGNETIC WAVES ELECTROMAGNETIC WAVES RADIO WAVES ELECTROMAGNETIC WAVES

ELECTROMAGNETIC WAVES

ELECTROMAGNETIC WAVES RADIO WAVES

ELECTROMAGNETIC WAVES Radio. What waves are radio These are used for radio, Ø Electro magnetic waves of wavelength longer than about 1 m are waves? TV, telephone and called radio waves. satellite communications. Ø These waves are produced as a result of charges accelerating through conducting wires and are generated by electronic devices like LC In nuclear magnetic oscillators. resonance imaging. Ø Since these waves have long wavelengths, they spread round hills and buildings by diffraction.

ELECTROMAGNETIC WAVES Radio Waves Radio waves can be Ø Frequencies from a few hertz upto about 109 hertz. detected by an antenna Ø Corresponding wavelengths are fromsimilar about 10 meters a few to 8 the one to used centimeters. for generation. Ø Usually produced by an AC circuit attached to an antenna. Ø A simple wire can function as an antenna Ø Antennas containing multiple conducting elements are usually more efficient and more common.

ELECTROMAGNETIC WAVES Radio Waves produces a time. As This the current varying magnetic field oscillated with time, Ø Parallel wires can act as an antenna. and theis. EM wave. the charge Ø The AC current in the antennaaccelerated. is produced by time-varying electric fields in the antenna. In general, when an electric charge is accelerated, it produces electromagnetic radiation.

ELECTROMAGNETIC WAVES Radio Waves Ø Radio waves are produced by the accelerated motion of charges in conducting wires. Ø They are used in radio and television communication systems. Ø They are generally in the frequency range from 500 k. Hz to about 1000 MHz. Ø The AM(amplitude modulated) band is from 500 k. Hz to 1710 k. Hz. Ø Higher frequencies upto 54 MHz are used for short wave bands. Ø TV waves range from 54 MHz to 890 MHz.

ELECTROMAGNETIC WAVES Ø The FM (frequency modulated) radio band extends from 88 MHz to 108 MHz. Ø Cellular phones use radio waves to transmit voice communication in the ultrahigh frequency (UHF) band. Low frequency waves – the AM band: Ø Radio waves having wavelengths of 10 m or more (frequency less than 30 MHz) are said to constitute the AM band. Ø The lower atmosphere is transparent to these waves, but the ionosphere reflects them back. Ø A signal transmitted from a certain point can be received at another point in two possible ways directly along the surface of the earth (called ground wave) and after reflection from ionosphere (called sky wave).

ELECTROMAGNETIC WAVES 1. Electromagnetic waves of wavelength longer than about 1 meter are called. . . a) microwaves b) x-rays c) gamma rays d) radio waves MCQ S

ELECTROMAGNETIC WAVES 2. What is frequency range of a micro wave?

ELECTROMAGNETIC WAVES MICROWAVES

ELECTROMAGNETIC WAVES Microwaves: For studying atomic Used in Radar andwavelengths molecular in the approximate range Ø Electric magnetic waves with Telecommunications. properties The of matter. of 1 mm to 0. 3 m are microwaves. frequency range is 1 GHz to 3 GHz. Ø Discovered by Hertz. Ø These are produced by special electronic devices, called klystrons and magnetrons. Ø Because of their short wavelengths they travel as a beam in a signal. Ø These are used in radar systems for aircraft navigation. Ø To analyze the fine details of the molecular structure.

ELECTROMAGNETIC WAVES Microwaves: Ø Based on the microwaves, speed guns are designed which are used to time fast balls, and in Tenni’s serves and automobiles. Ø Microwaves produce heat when absorbed by matter. Ø Frequency of water molecules Water molecules vibrate in Microwave oven is a is 3 GHz. the frequencies of the order domestic appliance tois selected Ø The frequency of microwave to match the resonant of microwave frequencies. cook food items. frequency of water molecule.

ELECTROMAGNETIC WAVES Microwaves: This raises the When a metal Ø When such waves pass through watercontainer the KE is of the waves are temperature of food The food materials it may accumulate some The heat developed efficiently transferred used, to the water molecules. This raises the containing water. are to be placed in and there is amay danger in the container temperature of the water. charges porcelain container. of getting shock when sometime melt it. with much smaller Ø The molecules in the porcelain container vibrate touched. frequencies. Because large molecules vibrate with much smaller frequencies. Ø So it cannot absorb microwaves as they are of smaller frequencies. Hence the container is unaffected and it becomes cool. Ø Basing on the microwaves speed guns are designed which are used to time fast balls and in tennis serves and automobiles.

ELECTROMAGNETIC WAVES Microwaves: Microwave ovens Ø Microwaves have frequencies between 109 Hz - 1012 Hz. generate radiation a frequency Ø Corresponding wavelengths are fromwith a few cm to a few tenths of near 2. 5 x 109 Hz. a mm. The microwave energy is transferred to water molecules in the food and heats the food.

ELECTROMAGNETIC WAVES Microwave oven: Ø Micro wave oven is a domestic appliance to cook the food items. Ø All food items contain water as a constituent. Ø When the temperature of a body rises, the energy of the random motion of atoms and molecules increases and the molecules travel or vibrate or rotate with higher energies. Ø Frequency of rotation of water molecules is 3 GHz. Ø The frequency of microwaves is selected to match the resonant frequency of water molecule.

ELECTROMAGNETIC WAVES Ø When such waves pass through water, the KE of the waves are efficiently transferred to the water molecules. This raises the temperature of the water. Ø The food materials are to be placed in porcelain container. Ø The molecules in the porcelain vibrate with much smaller frequencies because its large molecules vibrate and rotate with much smaller frequencies. Ø So, it can not absorb microwaves as they are of smaller frequencies. Hence the container is unaffected and it is cool.

ELECTROMAGNETIC WAVES Ø When a metal container is used, it may accumulate some charges and there is a danger of getting shock when touched. Ø The heat developed in the container may some times melt it. Ø The heat is directly delivered to water molecules which is shared by the entire food unlike in the conventional heating. Ø In the conventional heating the vessel on the burner gets heated first, and then the food inside because of transfer of energy from the vessel.

ELECTROMAGNETIC WAVES MCQ S 1. Electromagnetic wave with wavelength in approximate range of 1 mm to 0. 3 m are. . . a) microwaves b) x-rays c) gamma rays d) radio waves

ELECTROMAGNETIC WAVES 2. Microwaves are produced by special electronic devices called. . . a) klystrons b) magnetrons c) cyclotrons d) both (a) and (b)

ELECTROMAGNETIC WAVES INFRA RED RAYS

ELECTROMAGNETIC WAVES INFRA-RED RAYS: What are Infrared Ø Electromagnetic wavesrays? ranging all the wavelengths from the short -wavelength of microwaves (far infrared) to the red limit of visible light (near infrared) are called IR waves. Ø They have a wavelength range from 0. 7 m to about 1 cm. Ø To detect infra-red rays, thermocouples, thermopiles, bolometers, photo conducting cells are used.

ELECTROMAGNETIC WAVES They are scattered When allowed to fallless on as compared to rays thered visible light matter, infra rays Infra-red affect Infra-red rays are electromagnetic waves and travel with the by atmosphere. Hence, produce an increase in a the photographic plate. speed of 3 108 ms-1. infratemperature. red rays can travel Infra-red rays obey laws of reflection and refraction. through longer distance Infra-red rays can produce interference and can be polarised. through atmosphere under the conditions of smoke, fog, etc. Nitrogen andby oxygen gases are When absorbed molecules, the energy of infra red rays gets found be transparent converted intoto molecular vibrations. medium to all the wavelengths of infra red rays. Properties: Ø Ø

ELECTROMAGNETIC WAVES Applications: Infra-red rays are used solarwarm waterand hence help Ø Infra-red rays from the sun keep thein earth heaters and cookers. to sustain life on earth. Ø The coal deposits in the interior of earth are the result of conversion of forest wood into coal due to infra-red rays. Ø Infra-red rays are used for taking photographs during the conditions of fog, smoke, etc.

ELECTROMAGNETIC WAVES Applications: Ø Infra-red absorption spectra is used in the study of molecular structure and then to check the purity of the chemicals. Ø Infra-red rays are used for producing dehydrated fruits. Ø Infra-red rays are used to provide electrical energy to satellites by using solar cells. Ø Infra-red rays are used to treat muscular strains.

ELECTROMAGNETIC WAVES Infra-red Ø Infra-red radiation has frequencies from 1012 Hz to 4 x 1014 Hz. Ø Wavelengths from a few tenths of a mm to a few microns. We sense this radiation as Blackbodyheat. radiation from objects near room temperature fall into this range. Also useful for monitoring the earth’s atmosphere.

ELECTROMAGNETIC WAVES 1. Electromagnetic waves ranging from the short wavelength and of the microwaves to the red limit of visible light are called. . . a) ultra violet waves b) infrared waves c) gamma waves d) microwaves MCQ S

ELECTROMAGNETIC WAVES 2. Infra-red waves obey laws of. . . a) reflection b) refraction c) interference d) all the above

ELECTROMAGNETIC WAVES ULTRA VIOLET RAYS

ELECTROMAGNETIC WAVES ULTRA VIOLET RAYS: Ø UV is an electromagnetic radiation beyond the violet and of the visible spectrums. Ø Its wavelength ranges from approximately 380 nm to 0. 6 nm. What are ultra violet rays?

ELECTROMAGNETIC WAVES PROPERTIES: Ultra violet rays are electromagnetic waves and travel with the speed of 3 108 ms-1. They can also undergo interference and can be polarised. They also obey the laws of reflection and refraction.

ELECTROMAGNETIC WAVES Ø When allowed to fall on metals they cause the emission of photo electrons. Ø They can affect a photographic plate. Ø Ultra violet rays can cause fluorescence in certain metals. Ø Ultra violet rays cannot pass through glass. But quartz, flurite and rock salt are transparent to them. Ø Ultra violet rays possess the property of synthesizing vitamin D, when the skin is exposed to the sunlight.

ELECTROMAGNETIC WAVES APPLICATIONS: Ø Ultra violet rays are used for checking the mineral samples by making use of its property of causing fluorescence. Ø Ultra violet absorption spectra is used in the study of molecular structure. Ø Ultra violet rays destroy bacteria and hence it is used for sterilizing surgical instruments. Ø As ultra violet rays can cause photoelectric effect, they are used in burglars alarm.

ELECTROMAGNETIC WAVES Ultraviolet UV-B: 280 nm UV-A: 200 315 nm nm UV-C: 14 Hz Ø Ultraviolet (UV) light has frequencies from about 8 × 10 to 315 nm. to 400 nm. to 280 nm. to 1017 Hz. Ø Corresponding wavelengths are about 3 nm to 400 nm. Ø The UV portion of the spectrum is commonly subdivided into several regions Ø Greatest potential for damaging tissue.

ELECTROMAGNETIC WAVES 1. Ultra violet rays can cause. . . . which are used in burglar alarms. a) interference b) reflection c) refraction d) photoelectric effect MCQ S

ELECTROMAGNETIC WAVES 2. The wave length range of UV rays is approximately. . a) 380 nm to 0. 6 nm b) 0. 7 m to 1 cm c) 1 GHz to 30 GHz d) none of these

ELECTROMAGNETIC WAVES X-RAYS AND GAMMA RAYS

ELECTROMAGNETIC WAVES X-RAYS: What are X -rays? Ø X-rays are electromagnetic waves with wavelengths in the range of approximately 10 nm to 10 -4 nm. Ø These are produced when high-energy electrons bombarding a metal target. Ø These are very penetrating and dangerous. Ø These are used to study crystal structure. Ø In the treatment of certain forms of cancer.

ELECTROMAGNETIC WAVES Gamma Whatrays: are These are used to find These are used to kill flaws in matels and Ø The electromagnetic waves of uppermost frequency range of the Gamma rays? cancerous growths. sterilize equipment. known electromagnetic spectrum are called gamma rays. Ø Their wavelengths ranges from approximately 10 -1 nm to less than 10 -5 nm. Ø These are produced in certain nuclear reactions. Ø Cosmic rays from outer space is also a major source of -rays. Ø They have same properties as that of X-rays.

ELECTROMAGNETIC WAVES 1. X- rays are produced when. . . electrons are bombarded with a metal target. a) low energy b) highly energetic c) low momentum d) none of these MCQ S

ELECTROMAGNETIC WAVES 2. The electromagnetic waves of. . . frequency range of electromagnetic spectrum are called gamma rays. a) lower most b) upper most c) visible d) none of these

ELECTROMAGNETIC WAVES PROPAGATION OF RADIO WAVES

ELECTROMAGNETIC WAVES PROPAGATION OF RADIO WAVES i) Low frequency waves – the AM band How does the of 10 m or more (frequency less than Ø Radio waves having wavelengths a AM band. 30 MHz) are saidpropagation to constituteofthe radio wave take Ø The lower atmosphere is transparent to these waves, but the place? ionosphere reflects them back.

ELECTROMAGNETIC WAVES Ø A signal transmitted from a certain point can be received at another point in two possible ways-directly along the surface of the earth (called ground wave) and after reflection from ionosphere (called sky wave). These two regions of the AM 1500 bandk. Hz are (wave lengths above Ø Waves having frequencies upto about called medium wave and short wave ground bands because low 200 m) are mainly transmitted through Therefore, higher frequencies are respectively. frequency skymainly waves transmitted lose their energy very through thequickly sky. than the ground waves.

ELECTROMAGNETIC WAVES ii) High frequency waves – Television transmission Ø Above a frequency of about 30 MHz, the ionosphere does not reflect the wave towards the earth. Ø The television signals have frequencies in the range 100 -200 MHz. Therefore, TV transmission via sky is not possible.

ELECTROMAGNETIC WAVES Ø Only direct reception via the ground is possible. Ø Therefore, in order to have larger coverage, the transmission has to be done through very tall antennas.

ELECTROMAGNETIC WAVES 1. The television signals have frequencies in the range of. . a) 200 to 300 MHz b) 300 to 400 MHz c) 100 to 200 MHz d) none of these MCQ S

ELECTROMAGNETIC WAVES 2. Waves having frequencies of about 1500 k. Hz are mainly transmitted through ground because low frequency sky waves. . . very quickly than the ground waves. a) gain their energy b) lose their energy c) conserve their energy d) none of these

ELECTROMAGNETIC WAVES Thank you…