Dispersion, Colours, Scattering, Diffraction of Light, Magnetism & Electricity | Science & Technology for UPSC CSE PDF Download

In vacuum all the seven colours of white light travel with the same speed but in a transparent glass medium they have different speeds. Violet travels the slowest through glass and red travels the fastest. The refractive index is more for violet and least for red. Due to different speeds, the colours are refracted through different angles and, therefore, when a narrow beam of white light passes through a glass prism, it is split up into its constituent colours. This results spectrum. The colour in the spectrum do not have any sharp boundaries, rather colours overlap. A pure spectrum is obtained by spectrometer or spectroscope. Light of the sun, when observed through spectrometer, is crossed by hundreds of dark lines. These dark lines are called Fraunhoffer’s lines.

Emission Spectra When a substance is heated to a high temperature the atoms collide with each other violently. Due to this,electrons in the atoms are raised to higher energy orbits.
When one such electron returns back, light of definite wavelength is emitted. Each wavelength corresponds to the change in energy between two orbits. Such a spectra is called emission spectra. It is of three types:

(i) Line spectra obtained from atoms.

(ii) Band spectra obtained from molecules.

(iii) Continuous spectra given by solids and liquids

where atoms and molecules are very close to each other. In the formation of rainbow, the droplets act like prisms. In each droplet there is dispersion as well as total internal reflection.

Colours

A plant leaf appears green in white light, because it reflect the green colour and absorbs all the rest component colours of the white light.

Red, Blue and Green are the primary colours. All other colours are called the secondary colours which can be obtained by mixing of primary colours.

Red + Green = Yellow

Red + Blue = Magenta

Green + Blue = Cyan

Also,

Green + Magenta = White

Red + Cyan = White

Blue + Yellow = White

But the mixing of coloured pigments does not follow the same formula. For example, when blue and yellow paints are mixed they produce green pigment but not white. It happens because the pigments in common use are of impure colours.

Formula for paint mixing is:

VIBGYOR

Blue + Yellow = Green

or I, B G + G Y O = Green

If there is no common colour component resulting colour of the mixture well the black.
Scattering of light is due to dust particles and even the molecules of atmospheric gases. Violet light is scattered most, blue somewaht less and in that order Red is affected least of all. The sky generally looks blue because the short blue waves are scattered more than the longer wave of red light. Deep water appears blue for the same reason.
Scattering

Although scattering of violet is the most, the sky does not appear violet, because the sun is relatively weak in violet light.

Diffraction of Light

Ordinarily, light travels in a straight line. If an obstacle of large size is placed in its path, it gives a distinct geometrical shadow on the screen. However, if the size of the obstacle is made small, then light enters the geometrically prohibited region showing that the light bends round the corners of the obstacle. A similar result is found if we use a slit of small size. This phenomenon of bending of light round the corners and spreading into the regions of the geometrical shadow is called diffraction.

Holography: It is the technique of recording and reproducing three-dimensional images. A laser beam partly reflected from an object and partly from a mirror produces interference fringes on a photographic plate, which then becomes a hologram. When laser light is transmitted through the hologram, one can see a three-dimensional virtual image of the object.

Laser (Light Amplification by Stimulated Emission of Radiation) A laser is an optical device which produces an intense beam of coherent monochromatic light. In addition to holography, lasers are finding applications in surgery, computers, industry, controlled thermonuclear reactions, etc. Even in the proposed star wars programme of the USA, laser beams will be used for destroying enemy missiles.

Refraction, Interference and Diffraction all serve to establish the wave theory of light. Light is a transverse wave. This proof comes from the Polarisation of light.Diffraction of sound is more pronounced in daily life than that of light waves. Because wavelength of sound is much greater than that of the light waves and is comparable with the size of the obstacle which is the essential condition for diffraction of waves.

Magnetism and Electricity

A magnet can be de-magnetised by rough treatment, heating until red hot and then cooling, or by electricity. When a long rod of iron is magnetised by means of an electric current, it is found to increase in length slightly. This is due to the arrangement of molecules parallel to the length of the rod. This phenomenon is known as magnetostriction. A bar magnet tends to become weaker with age, owing to self demagnetisation.(a) If a magnet is suspended freely, it always rests pointing nearly in the geographic north-south direction. It is evident that the north pole of a magnet will point towards the geographic north only when there is a south pole of the earth’s magnet at the geographic north so as to attract the north pole of the magnet. Thus, in the geographic north, there must be the south pole of the earth’s magnet and in the geographic south, there must be the north pole of the earth’s magnet.

If we bury a soft iron bar in north-south direction under the earth’s surface, it is found that after some days it acquires the properties of a magnet.

Cause of the earth’s magnetism

It is presumably due to circulating electric current due to molten charged metallic fluid deep within the interior of the earth’s core of radius 3500 km (compared to the earth’s radius 6400 km). The strength of the earth’s field is typical about 10^-4 Wbm^-2 (1 Oersted).If we picture this field as due to a fictitious magnetic dipole deep inside the earth, its axis does not coincide with the axis of rotation of the earth; Both are “off” by an angle of about 20.

Magnetic induction is the process by which a piece of iron or steel placed in contact with or near a permanent magnet becomes a magnet. The amount of charge passing per unit time is called current. An electromotive force (e.m.f.) provided by a cell or, a generator is essential to maintain a flow of current. If current flow in a circuit is in the same direction it is called direct current; if the electrons flow alternately backward and forward, it is alternating current.

The quantity of current passed is measured by Ammeter. Unit of current is ampere, that of potential difference is volt and of resistance is ohm. Resistance of a wire increase with the length of wire and decreases with the area of cross section of the wire. The reciprocal of resistance is called conductance and its unit is mho. When a high resistance is required e.g. to reduce the current in a circuit, special alloys are used. The most common of these are Constantan (60% copper 40% nickel), manganin (84% copper, 12% manganese, 4% nickel) and nichrome. Constantan is used for general purposes, while manganin is used for making high quality standard resistors.

Rheostat. It increases or decreases the current flowing in a circuit by changing the length of the resistance wire inserted into it. Electric power is the rate of doing work and is measured in watts.

1 kilowatt hour = 1000 watt * 1 hour.

Transformer is a device for converting a large alternating current at low voltage into a small alternating current into high voltage and vice versa. If the voltage is stepped up and the current is decreased, it is called a step up transformer. If the voltage is stepped down and the current is decreased, the transformer is called step down transformer.
It is based on the principle of mutual induction.

Cyclotron. The cyclotron is a machine used to accelerate the charged particles. Its principles is based upon the motion of charged particles in a magnetic field when a moving charged particle enters a magnetic field which is perpendicular to both its velocity and the field. Hence, it is deflected from its path. If the field is uniform and the particle move in a plane normal to its path, become circular. The radius of the path depends upon the velocity of the particle i.e., the radius of the circle being larger for higher energies.

Van Allen radiation belt. A vast zone in the equatorial regions, high up in the atmosphere, where charged particles are trapped by the earth’s magnetic field is called Van Allen radiation belt.

D.C. Generator

D.C. dynamo converts mechanical energy into electrical energy. It produces unidirectional current and e.m.f. It is based on the principle of electromagnetic induction in which e.m.f. is induced in a coil rotating in a magnetic field due to change in the number of lines of force linked with the coil. The direction of the induced current is given by Fleming’s right hand rule.

A.C. Generator

Generator or dynamo is a device of coverting mechanical energy into electrical energy. It is based on the principle of electromagnetic induction in which e.m.f. is induced in a coil rotating in a magnetic field due to change in the number of lines of force linked with the coil. The direction of the induced current is given by Fleming’s right hand rule.

Electric Motor

An electric motor is a device of converting electrical energy into mechanical energy. It is based on the principle of magnetic effect of current i.e., when a current carrying conductor is placed in a magnetic field, it experiences a mechanical force and can move.