Review
2.3 PROPERTIES OF ELECTROMAGNETIC WAVES
Note:-
In addition to the above two processes there is another process. An electron in a higher energy level may remain in that level for sometime. If another photon of energy hv=(E2 – E1) is incident on it, then the electron in the higher energy level is made to jump to the lower energy level emitting a photon of exactly the same frequency as the incident photon (figure). This type of emission is called stimulated emission. Incident photon is the stimulating photon and the emitted one due to this, is the stimulated photon. The emitted photon is exactly in phase with the stimulating photon. This makes the laser action possible. Thus, two identical photons are produced in stimulated emission. This process is called light amplification by stimulated emission and laser works on this principle.
You have learnt in your previous classes about energy, heat, waves and light. Let us recall some of those concepts.
Energy is the capacity to do work.
Energy exists in various forms.
Energy can be transformed from one form to another.
Wave is a disturbance setup in a medium.
Energy can be transmitted by wave motion. Wave velocity is the product of frequency and wave length.
Radiation is a mode of emission of energy.
Some waves like sound waves require a material medium for their propagation.
Some waves like light waves do not require any material medium for their propagation.
2.1 INTRODUCTION
The term radiation refers to energy in motion also. The type of radiation for which light, heat, X-rays are examples, is characterized by velocity equal to that of light. This type of radiation is referred to as electromagnetic radiation. It is different from a-rays and b-rays which are material particles. These have speeds lesser than the velocity of light. We will study about electromagnetic radiations.
2.2 ELECTROMAGNETIC RADIATION
You know that a current (change in motion) produces a magnetic field and a changing magnetic field produces current.
A changing electric field can produce magnetic field and a changing magnetic field can produce electric field. Therefore it should be possible to sustain the electric and magnetic fields jointly.
An alternating electric field produces an alternating magnetic field; this alternating magnetic field would produce and electric field and so on.
We may say that, once started, the two fields sustain each other and a disturbance, that is a wave propagates through the medium. The wave is called electro-magnetic wave. Electromagnetic wave consists of varying electric and magnetic fields in mutually perpendicular planes. The direction of wave propagation is perpendicular to the planes of electric and magnetic fields. Hence electromagnetic wave is a transverse wave (electromagnetic waves can also be considered as waves produced by oscillating or accelerating charges). As characteristic of wave motion, electromagnetic energy called electromagnetic radiation is transmitted in the direction of wave propagation of an electromagnetic wave.
Note
Just as an magnet produces a magnetic field, an electric charge produces an electric field. Motion of charge, that is current, results in a changing electric field and this gives rise to a magnetic field.
James Clerk Maxwell, a Scottish physicist, summed up the researches of Michael Faraday in electricity and magnetism, in a set of equations called Maxwell’s equations. Maxwell showed that these equations represent the propagation of transverse electromagnetic waves and these waves travel with the speed of light.
Heinrich Hertz(1857-1994), a German physicist, succeeded in 1887, in producing electromagnetic wave (wavelength about 10 meters). He showed that these waves travel with the velocity of light.
Varying electric and magnetic fields are represented in the XY plane and XZ plane respectively. X axis represents direction of propagation.
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| Electromagnetic Wave |
Electromagnetic waves are transverse waves.
They have electric and magnetic fields as components which vary periodically with time and in space.
The fields are perpendicular to each other and to the direction of propagation.
Electromagnetic waves exhibit properties such as reflection, refraction interference and diffraction in common with other forms of waves.
they have a wide range of frequencies or wavelengths (The velocity c of the waves is given by c= fl, where f= frequency and l= wavelength)
Electromagnetic radiation has particle like properties in addition to those associated with wave motion.
Radiations of different frequencies react differently with matter.
Vacuum is the only perfect transparent medium for electromagnetic waves and all other materials media absorb energy strongly from some regions of the entire range of electromagnetic radiations.
The differences in the properties of the different electromagnetic radiations, are a consequence of their different frequencies.
2.4 ELECTROMAGNETIC SPECTRUM
Light is not he only kind of electromagnetic radiation. Ultra violet rays, infrared rays, X-rays, g-rays and radio waves are also electromagnetic radiations. We are surrounded by a sea of electromagnetic radiations. The entire group is designated as electromagnetic spectrum . They travel through space with the velocity of light(3 X 108 m/s).
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| Electromagnetic Spectrum |
The range of wavelengths of electromagnetic waves varies from about 10-15m to about 100km. the entire range of electromagnetic spectrum is divided into different regions depending on their effects. Each region of the spectrum overlaps the adjoining regions at both ends.
The range of visible light is very narrow, extending from about 400 nm to about 750 nm (4 X 10-7 to 7.5 X 10-7 m)
Beyond the red on the longer wavelength side, are the infrared rays extending from 750 nm to about 0.4 nm
The waves having wavelengths from about 0.4 nm to about 10 cm are called microwaves. The waves with wave lengths greater than 0.1 m are called radio waves and they are classified as short, medium and long waves.
Just beyond the violet on the shorter wavelength side, there are ultraviolet rays, extending upto 4nm. X-rays occur beyond the ultraviolet and extend upto 0.1 Å . waves in the region with wavelengths from 0.1Å to 10-2 Å are called g - rays
Remember
Wavelengths are expressed in submultiples and multiples of meter.
1 nanometer = 1nm = 10-9 m
1 mm = 1 micrometer = 10-6 m
1Å = 10-8 cm = 10-10 m Å is read as angstrom
Frequencies (cycles/s) are expressed in hertz(Hz)
The approximate range of wavelength and order of frequency of different regions of electromagnetic spectrum are given in the following table.
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| Wavelength Range |
2.5 USES OF ELECTROMAGNETIC RADIATIONS
Light :
Light plays a major role in life. It is very difficult to imagine life and other activities in the absence of light. Although light has a very narrow range of wave length its importance is very high. Almost every one is familiar with the uses of light.
Infrared radiations :
infrared radiation was first detected in 1800 by W. Herschel, by the heating effect of the radiation.
Activity
List the uses of light in daily life and in various devices.
Infrared rays find various applications.
Infrared spectrum of a compound may be used for identification and in the determination of molecular structure.
They are found to be most suitable for long distance photography.
Infrared photography is used in detecting enemy concentration, in examining old paintings and in the detection of forgery of old paintings.
Infrared search lights and telescopes were used during the second world war.
In medical field, they are useful in the diagnosis of superficial tumors, dislocations of bones and in the treatment of sprains.
It stimulates blood circulation.
The remote and set of a Tv uses infrared radiation to control different settings.
Infrared radiations from the sun are used in solar energy devices.
Ultraviolet radiation :
Ultraviolet radiation was discovered by J. W. Ritter, in 1801, by its photographic action. Short ultraviolet radiations are harmful to living tissues; these harmful radiations are absorbed by the ozone layer surrounding the earth.
Ultraviolet rays have a verity of applications.
Ultraviolet radiations activate some chemical reactions.
They excite fluorescence in many substances which led to the development of fluorescent tubes.
Uv-radiations are used to distinguish between real gems and artificial gems.
They are used in the treatment of rickets, diseases of the bone and skin diseases.
Uv-radiations of lower frequencies are useful in the synthesis of vitamin D in our bodies.
They are useful in the operation of photoelectric alarms.
X–rays :
X-rays were discovered by a German scientist William Rontgen in 1985. He as awarded Nobel prize in 1901. X-rays find extensive applications in medicine, industry and scientific research.
Fracture of bones can easily be located by X-ray photograph.
X-rays are used to locate foreign bodies such as bullets, coins, pins etc., in human body.
X-rays are used for the treatment of cancer and some skin diseases.
They are used to detects like cavities in castings and cracks in welding and in locating flaws in the parts of machines and all kinds of transport vehicles. This technique is called radiography.
They are used in the study of crystal structure.
Gamma rays (g-rays) :
Gamma rays were first discovered among the radiations emitted by radioactive nuclei. g-rays like X-rays, find extensive applications.
Gamma rays are used in the treatment of cancer.
They act as catalyst in the manufacture of some chemicals.
They are used in g -ray microscopes.
Gamma rays are used to produce photoelectric effect.
They are used in radiography.
Microwaves :
Microwaves find applications in Radar, Satellite communication and microwave ovens. They are used for experimental purposes.
Radio waves :
As these waves are used in radio and television, they are called radio waves. Radio waves of short wavelength are used in communication systems including satellite systems, in Radars and TV broadcasting. Radio waves of longer wavelengths are used in radio broadcasting.
Do you know
Dr. J.C. Bose (1858-1937) an Indian physicist, had demonstrated the possibility of radio communication. In 1901 Guglielmo Marconi, succeeded in long distance communication across the Atlantic ocean.
2.6 PHOTOELECTRIC EFFECT
Introduction :-
Max Plank’s quantum theory of black body radiation - 1900
The atoms of the black body absorb or emit radiation.
the atoms absorb or emit radiation as if they are oscillators.
(oscillator is a particle which is oscillation)
Oscillators cannot have any arbitrary energy.
They can have energies which are integral multiples of hv where v is the natural frequency of the oscillator, h is Planck’s constant, h= 6.63 X 10-27 erg s or 6.63 X 10-34 Js
i.e.,E = nhv where n = { 1,2,3,4……….}
Note this
A black body is one which absorbs all the radiation falling on it; radiation emitted by a black body, when hot is called black body radiation.
Dual nature
Radiation sometimes manifests itself as particles and sometimes as waves. This holds good for matter also.
Note:-
According to Planck, radiation is ‘wave like’ in nature & the black body can have only energies that are integral multiples of hv & not fractions of it.
According to Niels Bhor, a system (oscillator) emits energy when it falls from the higher energy state to the lower energy state & absorbs energy when it goes from the lower energy state to the higher energy state. This process of absorption of emission will be through electromagnetic waves or electromagnetic radiation.
Do you know
Photoelectric effect was discovered by H. Hhertz in 1887. Detailed experimental investigation was done by German physicists W Hallwachs and P. Lenard.
Einstein’s quantum theory of light- 1905
(Albert Einstein 14-3-1879 to 12-4-1955; German physicist; Nobel prize 1921)
Light behaves like a stream of particles.
These particles are called QUQNTA.
Each quantum of light has an energy given by E=Cv,
where C is a constant & v is the frequency of the radiation.
If Planck’s theory is invoked, then the constant C turns out to be equal to the Planck’s constant h.
Thus Einstein quantized light.
In 1926, American physicist Lewis Gilbert named the ‘light quanta’ as photons.
Properties of Photons
Photons travel at the speed of light in vacuum. i.e., 3 X 108 ms-1.
Photons travel in straight lines(only in a homogeneous medium)
Energy of a photon depends on its frequency. Therefore energy of the photons does not change when it travels from one medium to another.
Photons do not have any change. Therefore they are electrically neutral.
Phenomenon :
Several experiments towards the end of the last century indicated that certain materials(usually metallic surfaces) emit electrons, when exposed to light. This phenomenon of emission of electrons by materials under the action of light is called photoelectric effect. The electrons so emitted are called photoelectrons. Ultraviolet rays, X-rays and g-rays also produce this effect to certain materials.
Following are the experimental facts regarding photoelectric effect.
The phenomenon is instantaneous.
There is a certain frequency called threshold frequency for radiation, below which no photoelectric effect takes place. Threshold frequency is different for different materials.
For radiation of a given frequency, the number of photoelectrons released in proportional to the intensity of the radiation. But the kinetic energy of photoelectrons remains the same.
When radiations of different frequencies are used, the velocity (energy) of photoelectron increases with increasing frequency.
EINSTEIN’S EXPLANATION OF PHOTOELECTRIC EFFECT.
According to Einstein, each quantum of light, i.e., a photon has an energy equal to hv.
It penetrates the metal surface & transfers all its energy to an electron.
Electrons are held inside the metal by internal attractions.
He made the reasonable assumption that the electron must perform some work W to come out of the metal.
If the transferred energy is more than the internal attractions, the electron gets liberated.
Thus one electron is released per photon.
The kinetic energy of the liberated electron is given by
mv2 = hv - W, where m is the mass of the electron, v is the velocity of the liberated electron, W is the constant for a given metal. This is the famous Einstein's equation for the photoelectric effect.
We can see clearly that when the frequency v increases, the kinetic energy of the electron increases.
The above equation implies that when hv<W, the kinetic energy of the electron becomes negative which is not physically possible. For frequencies below a threshold equal to there will be no photoelectric effect.
In the Einstein’s quantum theory, the intensity of a light beam is equal to the number of photons in it. Thus , as the intensity of light increases, the number of photons increases & this in turn increases the number of liberated electrons.
These features are borne out by experiments.
Quantum : Latin – Quantus
Discrete amount of energy.
Hypothesis : It is an educated guess based on observation.
It is a rational explanation of a single event or phenomenon based on what is observed, but which has not been proved.
Law : It is a statement of fact to explain in concise terms, an action or set of actions.
Theory : It is an explanation of set of related observations or events based upon proven hypothesis & verified multiple times by detached groups of researchers.
Arbitrary : Random, any value, not fixed.
n : Greek alphabet, to be pronounced as nv.
Applications :
The principle of photoelectric effect is used in photoelectric cells to convert light energy into electrical energy. The number of applications is enormous.
They are used in reproduction of sound in cinematography.
They are used in exposure meters.
Photoelectric cells are used for automatic switching on and off of street lights.
They are used in automatic control of traffic signals.
They are used in counting machines.
Photoelectric cells are used in the operation of burglar alarm.
They are used in television transmission.
2.7 LASER
The acronym LASER stands for Light Amplification by Stimulated Emission of Radiation. Laser is a device for producing a highly intense narrow beam of nearly monochromatic light. Laser light can travel large distances without spreading and is capable of being focused to give enormous power density as high as 108 Watt/cm2. power density is the energy incident on unit area in one second.
When an electron from an orbit of higher energy (E2) jumps to an orbit of lower energy (E1), a photon of energy = (E2 – E1)= hv is emitted, where v is the frequency of the photon emitted. As this takes place spontaneously, it is called spontaneous emission.
If a photon of proper energy falls on an atom, it may be absorbed completely and an electron of the atom in a lower energy state may be raised to a higher energy state. This process is called excitation.
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| Electron at initial energy level |
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| Photon |
Usually, most of the atoms in a system remain in the lowest energy state. If light amplification has to take place, it is necessary to raise a larger proportion of the atoms to higher energy levels. The process of raising atoms from lower energy levels to higher energy levels is called population inversion. Population inversion is achieved by supplying energy from external sources. The process of supplying energy from an external source, to achieve population inversion in a system, is called optical pumping.
Once population inversion is attained to sufficient extent, laser process is started by a stray photon of suitable energy. Photon amplification takes place by stimulated emission. These photons are made to come out in the same direction as a narrow beam.
A mixture of helium and neon in a definite proportion is filled in a narrow cylindrical glass tube. Two mirrors, one perfectly reflecting and the other partially reflecting, are fixed one at each end of the tube such that their surfaces are perfectly parallel to one another. The mixture of helium-neon in the cylinder is ionized by passing direct current. This provides the energy required for optical pumping and the resulting population inversion.(Figure)
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Helium-Neon Gas Laser |
During stimulated emissions. Photons which travel perpendicular to the mirror surfaces, move to and fro after being reflected by the mirrors. Due to multiple reflections, intensity of light increases. When it reaches a certain level, light comes out continuously through the partially reflecting mirror as a strong narrow beam of monochromatic light.
Characteristics of laser radiation
Laser light is highly directional. It is emitted in a single direction as a narrow beam where as ordinary light spreads.
It is highly monochromatic(It has almost a single wavelength) whereas ordinary light is polychromatic.
Laser light is coherent. The emitted photons are in phase with each other in other words the photons are identical. Ordinary light is incoherent.
Laser light has high intensity than ordinary light.
Uses :
Lasers have a very large number of applications. Some of them are given below.
(i) The distance between two objects can be found accurately using laser reflectors. This technique is known as ‘laser ranging’.
(ii) Laser is used in laser Raman spectroscopy to understand the molecular structure of a material.
(iii) It is used in laser optical surgery in welding back detached retina into proper position and save eye sight.
(iv) It is used in the treatment of dental decay and skin diseases.
(v) Laser cutting, drilling and welding have wide ranging industrial applications.
(vi) One of the most useful applications of laser is optical communication using optical fibers. This technology has revolutionized the modern communication system.
(vii) Laser is used in the measurement of pollutants in the atmosphere.
(viii) Laser are extensively used in holography and its applications.
Do you know
The distance between the earth and the moon has been calculated using laser ranging technique. In 1969 astronauts of Apollo-11 had left behind laser reflector on moon. A laser light is sent from the earth to the moon. Light reflected by the reflector on the moon is received. Distance to moon can we calculated knowing the time taken for to and fro journey. The distance can be measured to an accuracy of 5 cm in 4 lakh km.
Holography
Holography is a technique which helps in taking complete three dimensional images of a given object or scene. The word ‘holography’ is derived from the Greek words ‘holos’ (the whole or complete) and graphos (writing).
POINTS TO REMEMBER
A varying electric field produces a varying magnetic field and vice-versa. Such a disturbance propagates in a medium in the form of electromagnetic wave.
Electric field and magnetic field are perpendicular to each other and together perpendicular to the direction of propagation.
Electromagnetic wave is transverse in nature.
Electromagnetic radiation is electromagnetic energy.
Electromagnetic spectrum is an arrangement of electromagnetic radiations.
Electromagnetic radiation are used in molecular study, radiography, treatment of cancer etc.,
The phenomenon of emission of electrons by materials under the action of light is called photo electric effect.
Number of photo electrons is directly proportional to the intensity of the radiation.
Velocity of photo electrons increases with increasing frequency.
Photo electric effect was explained by Einstein on the basis of Planck’s quantum hypothesis.
LASER – Light Amplification of Stimulated Emission of Radiation
Laser production involves the following stages.
(a) Optical pumping
(b) Population inversion
(c) Electron cascade process
(d) Stimulated emission
(e) Amplification
Laser is used in laser Raman Spectroscopy, optical surgery, optical fibers.
Tag: Electromagnetic, Radiation, Waves, Spectrum, IInfrared, Ultraviolet, Photoelectric, X-rays, Gama-rays, Photons, Laser,
More Details: Magnetism and Electricity for standard 10 GSEB Course
More Details: Magnetism and Electricity for standard 10 GSEB Course
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