Saturday, June 27, 2009

Lesson: 5. Electromagnetic waves and Wave Optics.

Lesson: 5. Electromagnetic waves and Wave Optics.
1.Whar are Electromagnetic waves?
According to Maxwell, an accelerated charge is a source of electromagnetic radiation. In an electromagnetic wave, electric and magnetic field vectors are at right angles to each other and both are at right angles to the direction of propagation. They possess the wave
character and propagate through free space without any material
medium. These waves are transverse in nature.
2.State the Characteristics of electromagnetic waves.
(i) Electromagnetic waves are produced by accelerated charges.
(ii) They do not require any material medium for propagation.
(iii) In an electromagnetic wave, the electric field vectors are at right angles to each other and to the direction of propagation. Hence electromagnetic waves are transverse in nature.
(iv) Variation of maxima and minima in both electric &magnetic fields occur simultaneously.
(v) They travel in vacuum or free space with a velocity
3 × 108 m s−1 given by the relation C =1/ μo εo
(μo – permeability of free space and εo - permittivity of free space)
(vi) The energy in an electromagnetic wave is equally divided
between electric and magnetic field vectors.
(vii) The electromagnetic waves being chargeless, are not deflected
by electric and magnetic fields.
3.State the uses of radio waves.
These waves are used in radio and televisioncommunication systems.
AM band is from 530 kHz to 1710 kHz.
Higher frequencies upto 54 MHz are used for short waves bands.
Television waves range from 54 MHz to 890 MHz.
FM band is from 88 MHz to 108 MHz.
Cellular phones use radio waves in ultra high frequency (UHF) band.

4.State the uses of micro waves.
Due to their short wavelengths, they are used in radar communication system. Microwave ovens are an interesting domestic application of these waves.
5.State the uses of infra red waves.
Infrared lamps are used in physiotherapy.
Infrared photographs are used in weather forecasting.
As infrared radiations are not absorbed by air, thick fog, mist etc, they are used to take photograph of long distance objects.
Infra red absorption spectrum is used to study the molecular structure.
6.State the uses Ultra−violet radiations
(i) They are used to destroy the bacteria and for sterilizing
surgical instruments.
(ii) These radiations are used in detection of forged documents,
finger prints in forensic laboratories.
(iii) They are used to preserve the food items.
(iv) They help to find the structure of atoms.
7.State the use X rays & γ−rays
X rays
(i) X rays are used as a diagonistic tool in medicine.
(ii) It is used to study the crystal structure in solids.
γ−rays
Study of γ rays gives useful information about the
nuclear structure and it is used for treatment of cancer.
8.Define:Emission spectra
When the light emitted directly from a source is examined with
a spectrometer, the emission spectrum is obtained. Every source has
its own characteristic emission spectrum.
9. Define Continuous emission spectrum
· It consists of unbroken luminous bands of all wavelengths
containing all the colours from violet to red. These spectra depend only
on the temperature of the source and is independent of the
characteristic of the source.
· Incandescent solids, liquids, Carbon arc, electric filament lamps
etc, give continuous spectra.
10. Define Line emission spectrum
Line spectra are sharp lines of definite wavelengths. It is the
characteristic of the emitting substance. It is used to identify the gas.
Atoms in the gaseous state, i.e. free excited atoms emit line spectrum. The substance in atomic state such as sodium in sodium vapour lamp, mercury in mercury vapour lamp and gases in discharge tube give line spectra
11. Define Band emission Spectrum
It consists of a number of bright bands with a sharp edge at one end but fading out at the other end.
Band spectra are obtained from molecules. It is the characteristic of the molecule. Calcium or Barium salts in a bunsen flame and gases like carbon−di−oxide, ammonia and nitrogen in molecular state in the discharge tube give band spectra.
When the bands are examined with high resolving power spectrometer,each band is found to be made of a large number of fine lines, very close to each other at the sharp edge but spaced out at the other end. Using band spectra the molecular structure of the substance can be studied.
12. Define: Absorption Spectra
When the light emitted from a source is made to pass through an
absorbing material and then examined with a spectrometer, the
obtained spectrum is called absorption spectrum. It is the
characteristic of the absorbing substance.
Absorption spectra is also of three types
1. continuous absorption spectrum
2. line absorption spectrum and
3. band absorption spectrum
13 Define: Continuous absorption spectrum
A pure green glass plate when placed in the path of white light,
absorbs everything except green and gives continuous absorption
spectrum.
14. Define: Line absorption spectrum
When light from the carbon arc is made to pass through sodium
vapour and then examined by a spectrometer, a continuous spectrum
of carbon arc with two dark lines in the yellow region is obtained
15. Define: Band absorption spectrum
If white light is allowed to pass through iodine vapour or dilute
solution of blood or chlorophyll or through certain solutions of organic
and inorganic compounds, dark bands on continuous bright background are obtained. The band absorption spectra are used for making dyes.
16.What are Fraunhofer lines?
If the solar spectrum is closely examined, it is found that it
consists of large number of dark lines. These dark lines in the solar
spectrum are called Fraunhofer lines. Solar spectrum is an example of
line absorption spectrum.
By comparing the absorption spectra of various substances with
the Fraunhofer lines in the solar spectrum, the elements present in the
sun’s atmosphere have been identified.
17.What is Fluorescence?
When an atomic or molecular system is excited into higher energy
state by absorption of energy, it returns back to lower energy state in
a time less than 10−5 second and the system is found to glow brightly
by emitting radiation of longer wavelength.
When ultra violet light is incident on certain substances, they
emit visible light.
It may be noted that fluorescence exists as long as the fluorescing
substance remain exposed to incident ultraviolet light and re-emission
of light stops as soon as incident light is cut off.
18.What is Phosphorescence?
There are some substances in which the molecules are excited by
the absorption of incident ultraviolet light, and they do not return
immediately to their original state. The emission of light continues even
after the exciting radiation is removed. This type of delayed
fluorescence is called phosphorescence.
19.What are two possible modes of propagation of energy?
The two possible modes of propagation of energy from one place to another
(i) by stream of material particles moving with a finite velocity
(ii) by wave motion, wherein the matter through
which the wave propagates does not move along the direction of the
wave.
20.Define :Scattering of light
Lord Rayleigh was the first to deal with scattering of light by air
molecules. The scattering of sunlight by the molecules of the gases in
Earth’s atmosphere is called Rayleigh scattering.
The basic process in scattering is absorption of light by the
molecules followed by its re-radiation in different directions.
21.State Rayleigh scattering law.
The amount of scattering is inversely proportional to the fourth
power of the wavelength. This is known as Rayleigh scattering law.
22.Why sky appears blue at noon?
The shorter wavelengths are scattered much more than the
longer wavelengths. The blue appearance of sky is due to scattering of
sunlight by the atmosphere. According to Rayleigh’s scattering law,
blue light is scattered to a greater extent than red light. This scattered
radiation causes the sky to appear blue.
23.Why sun appears red at sunset/sunrise?
At sunrise and sunset the rays from the sun have to travel a larger part of the atmosphere than at noon. Therefore most of the blue light is scattered away and only the red light which is least scattered reaches the observer. Hence, sun appears reddish at sunrise and sunset.
24.What is Tyndal scattering?
When light passes through a colloidal solution its path is visible inside the solution. This is because, the light is scattered by the particles of solution. The scattering of light by the colloidal particles is called Tyndal scattering.
25.What is Raman effect?
In 1928, Sir C.V. Raman discovered experimentally, that the
monochromatic light is scattered when it is allowed to pass through a
substance. The scattered light contains some additional frequencies
other than that of incident frequency. This is known as Raman effect.
The lines having frequencies lower than the incident frequency are called Stoke’s lines and the lines having frequencies higher than the incident frequency are called Anti−stokes lines.
26.Define :stokes lines.
If a photon strikes an atom or a molecule in a liquid, part of
the energy of the incident photon may be used to excite the atom of the
liquid and the rest is scattered. The spectral line will have lower
frequency and it is called stokes line.
27.Define: Antistokes lines.
If a photon strikes an atom or a molecule in a liquid, which is in
an excited state, the scattered photon gains energy. The spectral line will
have higher frequency and it is called Anti−stoke’s line.
28,State the Applications of Raman Spectrum
(i) It is widely used in almost all branches of science.
(ii) Raman Spectra of different substances enable to classify them
according to their molecular structure.
(iii) In industry, Raman Spectroscopy is being applied to study the
properties of materials.
(iv) It is used to analyse the chemical constitution.
29.Define: Wavefront.
The surface which envelopes the particles that are in the same state of vibration is known as a wave front. The wave front at any instant is defined as the locus of all the particles of the medium which are in the same state of
vibration.
A point source of light at a finite distance in an isotropic medium
emits a spherical wave front. A point source of light in an
isotropic medium at infinite distance will give rise to plane wavefront
A linear source of light such as a slit illuminated by a lamp,
will give rise to cylindrical wavefront
30.State: Huygen’s principle
Huygen’s principle states that, (i) every point on a given wave front
may be considered as a source of secondary wavelets which spread out
with the speed of light in that medium and (ii) the new wavefront is the
forward envelope of the secondary wavelets at that instant.
31.State the conditions for total internal reflection.
For total internal reflection to take place (i) light must travel
from a denser medium to a rarer medium and (ii) the angle of incidence
inside the denser medium must be greater than the critical angle. i.e i > C.
32.State the superposition principle.
When two or more waves simultaneously pass through the same
medium, The resultant displacement of any particle is the vector addition of the displacements due to the individual waves. This is known as principle of superposition. Y = Y + Y
33.What are Coherent sources?
Two sources are said to be coherent if they emit light waves of the
same wave length and start with same phase or have a constant phase
difference.
34.Is two independent sources be coherent?Discuss.
Two independent monochromatic sources, emit waves of same
wave length. But the waves are not in phase. So they are not coherent.
This is because, atoms cannot emit light waves in same phase and
these sources are said to be incoherent sources.
35.Define constructive interference.
At points where the crest of one wave meets the crest of the other wave or the trough of one wave meets the trough of the other wave, the waves are in phase, the displacement is maximum and these points appear bright. This type of interference is said to be constructive interference.
36.Define destructive interference.
At points where the crest of one wave meets the trough of the other wave, the waves are in opposite phase, the displacement is minimum and these points appear dark. This type of interference is said to be destructive interference.

37.State the conditions for the formation of sustained interference.
The conditions for the formation of sustained interference may be stated as :
(i) The two sources should be coherent
(ii) Two sources should be very narrow
(iii) The sources should lie very close to each other to form
distinct and broad fringes.
38.Define: Band width.
The distance between any two consecutive bright or dark bands
is called bandwidth.
Bandwitdth, β =D λ/d
39.State the Condition for obtaining clear and broad interference bands
Condition for obtaining clear and broad interference bands
(i) The screen should be as far away from the source as possible.
(ii) The wavelength of light used must be larger.
(iii) The two coherent sources must be as close as possible.
40.What are Newton’s rings?
When a plano convex lens of long focal length is placed over an optically plane glass plate, a thin air film with varying thickness is enclosed between them. The thickness of the air film is zero at the point of contact and gradually increases outwards from the point of contact. When the air film is illuminated by
monochromatic light normally, alternate bright and dark concentric
circular rings are formed with dark spot at the centre. These rings are
known as Newton’s rings.
41.Why the central ring of Newton’s rings appears dark?
The thickness of the air film at the point of contact of lens L with
glass plate P is zero. Hence, there is no path difference between the
interfering waves. So, it should appear bright. But the wave reflected
from the denser glass plate has suffered a phase change of π while the
wave reflected at the spherical surface of the lens has not suffered any
phase change. Hence the point O appears dark. Around the point of
contact alternate bright and dark rings are formed.
42.Define diffraction:Why sound is more pronounced than light?
The bending of waves around the edges of an obstacle is called diffraction.
The sound waves have a greater wavelength, the diffraction effects are
pronounced. As the wavelength of light is very small, compared to that
of sound wave and even tiny obstacles have large size, compared to the
wavelength of light waves, diffraction effects of light are very small.
43.Differentiate: Fresnel and Fraunhofer diffraction
In the Fresnel diffraction, the source and the screen are at finite distances from the obstacle producing diffraction. In such a case the wave front
undergoing diffraction is either spherical or cylindrical.No convex lens used.
In the Fraunhofer diffraction, the source and the screen are at infinite
distances from the obstacle producing diffraction. Hence in this case
the wavefront undergoing diffraction is plane. The diffracted rays are brought to focus with the help of a convex lens.
44.State the difference between interference and diffraction

Interference

Diffraction

1. It is due to the superposition of
secondary wavelets from two
different wavefronts produced
by two coherent sources
It is due to the superposition
of secondary wavelets emitted
from various points of the
same wave front.

2. Fringes are equally spaced
Fringes are unequally spaced
3. Bright fringes are of same
intensity

Intensity falls rapidly

4. Comparing with diffraction, it
has large number of fringes

It has less number of fringes.

45.Define: Polarisation.
The vibrations of unpolarised ordinary light are restricted to only one plane parallel to the axis of the crystal, the phenomenon of restricting the
vibrations into a particular plane is known as polarisation.
46. Plane of vibration and plane of polarisation
The plane containing the optic axis in which the vibrations occur
is known as plane of vibration. The plane which is at right angles to
the plane of vibration and which contains the direction of propagation
of the polarised light is known as the plane of polarisation. Plane of
polarisation does not contain vibrations in it.
47.What is a polarizer and analyzer?
A device which produces plane polarised light is called a polariser.
A device which is used to examine, whether light is plane polarised or not
is an analyser. A polariser can serve as an analyser and vice versa.
48.What is the test for polarized and unpolarised light?
If the intensity of light from polarizer varies between maximum and zero, when the analyser is rotated through 90o, then the incident light is plane polarised;
If the intensity of light varies between maximum and minimum (not zero), when the analyser is rotated through 90o, then the incident light is partially
plane polarised.
49.State Brewsters law.
The tangent of the polarising angle is numerically equal to the refractive index of the medium. tan ip = μ
50.What is Double refraction.
Bartholinus discovered that when a ray of unpolarised light is
incident on a calcite crystal, two refracted rays are produced. This
phenomenon is called double refraction.
51.What is optic axis?state its types.
Inside the crystal there is a particular direction in which both the
rays travel with same velocity. This direction is called optic axis.
Crystals like calcite, quartz, ice and tourmaline having only one
optic axis are called uniaxial crystals.
Crystals like mica, topaz, selenite and aragonite having two optic
axes are called biaxial crystals.
53.State the Uses of Polaroid.
1. Polaroids are used in the laboratory to produce and analyse plane
polarised light.
2. Polaroids are widely used as polarising sun glasses.
3. They are used to eliminate the head light glare in motor cars.
4. They are used to improve colour contrasts in old oil paintings.
5. Polaroid films are used to produce three – dimensional moving
pictures.
6. They are used as glass windows in trains and aeroplanes to control
the intensity of light. In aeroplane one polaroid is fixed outside the
window while the other is fitted inside which can be rotated. The
intensity of light can be adjusted by rotating the inner polaroid.
7. Aerial pictures may be taken from slightly different angles and
when viewed through polaroids give a better perception of depth.
8. In calculators and watches, letters and numbers are formed by
liquid crystal display (LCD) through polarisation of light.
9. Polarisation is also used to study size and shape of molecules.
54.Define: Optical activity
When a plane polarised light is made to pass through certain
substances, the plane of polarisation of the emergent light is not the
same as that of incident light, but it has been rotated through some
angle. This phenomenon is known as optical activity. The substances
which rotate the plane of polarisation are said to be optically active.
Examples : quartz, sugar crystals, turpentine oil, sodium chloride etc.
55.Define the types of optically active substances.
Optically active substances are of two types, (i) Dextro−rotatory
(right handed) which rotate the plane of polarisation in the clock wise
direction on looking towards the source. (ii) Laevo – rotatory (left
handed) which rotate the plane of polarisation in the anti clockwise
direction on looking towards the source.
56.State the factors of optical rotation.
The amount of optical rotation depends on :
(i) thickness of crystal
(ii) density of the crystal or concentration in the case of solutions.
(iii) wavelength of light used
(iv) the temperature of the solutions.


57.Define :Specific rotation
Specific rotation for a given wavelength of light at a given
temperature is defined as the rotation produced by one-decimeter
length of the liquid column containing 1 gram of the active material in
1cc of the solution.
If θ is the angle of rotation produced by l decimeter length of a
solution of concentration C in gram per cc, then the specific rotation
S at a given wavelength λ for a given temperature t is given by
S = θ/ l c .

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