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Rectilinear Propagation, Shadows and Eclipse :
* Light travels in straight line.
* The kind of shadow depends on the size of the source of light
* Shadow obtained is a region of total darkness called umbra
* Shadow obtained partial darkness called penumbra.
* Lunar eclipse - earth comes between the sun and the moon.
* Solar eclipse - moon comes between the sun and the earth.
Reflection of light
* A highly smoothed surface, such as a mirror, reflects most of the light falling on it.
(i) The angle of incidence is equal to the angle of reflection, and
(ii) The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.
An image formed in a plane mirror has the following Characteristics.
* The image is of the equal size as the object.
* It is as behind the mirror as the object is in front of it.
* It is virtual
* It is laterally inverted.
* Light is incident on a rough surface, the reflected rays are scattered in all direction the many surface irregularities. This is called diffuse reflection.
Inclined mirror :
* When an object is placed between two inclined mirrors several images of the object one formed
* Number of images depends on the angle between the mirror
No. of image = 𝟑𝟔𝟎° /𝐚𝐧𝐠𝐥𝐞 𝐛𝐞𝐭𝐰𝐞𝐞𝐧 𝐦𝐢𝐫𝐫𝐨𝐫𝐬
Kaleidoscope :
* It is toy image are formed by two strips of plane mirrors placed at an angle of 60° inside the tube five images are seen.
Refraction :
* Light bends when its travel from one medium to another this called refraction of the lights.
* Different speeds of light in medium different densities.
* Speed of light in vaccum is - 3 x 10^8m/s.
* When light travels from a rarer medium and enters a denser medium it will be deviated towards to the normal line
Ex : From air to glass.
* The light will be deviated away from the normal when it passes from a denser into a rare medium
Ex : Glass to Air.
Atmosphere Refraction:
* The density of the atmosphere surrounding the earth decreases with increasing altitude. Thus if light enters the atmosphere from outside it encounters layers of air increasing density and,
therefore, bends gradually producing a curved path.
Ex - Star twinkling, Mirage
Refractive Index:
𝜇 = 𝑆𝑖𝑛 𝑖/𝑆𝑖𝑛 𝑟
=Velocity of light in air / Velocity of light in medium
It has no unit & no dimension
* Water - 1.33
* Crown glass - 1.52
* Dense Flint glass - 1.62
* Diamond - 2.42
Total Internal Reflection :
* Ray or light passes through the denser medium to the rare medium the refracted Ray is bent away from the normal line.
* Angle of incidence increase the angle of refraction also increase.
* At a certain angle of incidence the angle of reflection becomes90°. This angle is called critical angle (c).
* If the angle of incidence is more than the critical angle the rays curves inside the denser medium itself. This phenomenon is called total internalreflection.
Optical Fibre
* An optical fibre is a device formed on the principle of total internal reflection.
*Optical fibres are thin, flexible and transparent strands of glass which can carry light along them very easily. A boundle of such thin fibres from a light pipe.
Uses of Optical Fibre
* Optical fibres are used to transmit communication signals.
* In medicine, optical fibres are preowned endoscope and laparoscopes.
Dispersion:
* Separation of light into colours is called dispersion.
* Seven colours - Violet, Indigo, Blue,Green, Yellow, Orange and Red.
* Violet Colour has Minimium Wave Length and Maximum Frequency.
* Red Colour has Maximum Wave Length and Minimum Frequency.
* Vaccum all colours are same speed but different medium and different speed.
The Rainbow
* The most spectacular illustration of dispersion.
* Droplets acts as a prism.
* Rainbow is seen in the sky opposite to the sun.
* Each droplets there is dispersion as well as total internal reflection.
Colour of objects
* Leaves reflect the green colour the remaining colour are observed.
Mixing Coloured Light
* All colours can be suitable mixture of these three colour. (Red, Blue, Green) Therefore called primary colour, others secondary colour.
* The colours which give white light when put together, are called complementary colour blue + yellow - complementary colours.
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. The strength of
scattering depends on the wavelength of the light and also the size of the particle which cause scattering. The amount of scattering is inversely proportional to the
fourth power of the wavelength. This is known as Rayleigh scattering law.
* Hence, 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.
* 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
Tyndal scattering
* The scattering of light by the colloidal particles is called Tyndal scattering.
Diffraction
* Sound is propagated in the form of waves. Sound created in an adjoining room reaches us after bending round the edges of the walls. Similarly, waves on the
surface of water also bend round the edges of an hurdle and spread into the region behind it. This bending of waves around the edges of an hurdle is called diffraction. Diffraction is a characteristic
property of waves. The waves are diffracted, only when the size of the obstacle is comparable to the wavelength of the wave.
* Fresnel showed that the amount of bending produced at an barrier depends upon the wavelength of the incident wave. Since the sound waves have a longer
wavelength, the diffraction effects are pronounced. As the wavelength of light is very tiny, compared to that of sound wave and even tiny barrier have large size, compared to the wavelength of
light waves, diffraction effects of light are very less.
Fresnel and Fraunhofer diffraction
Diffraction phenomenon can be
classified under two groups (i) Fresnel
diffraction and (ii) Fraunhofer diffraction
Polarisation
The phenomena of reflection,
refraction, interference, diffraction are
common to both transverse waves and
longitudinal waves. But the transverse
nature of light waves is demonstrated
only by the phenomenon of polarization
The phenomenon of restricting the vibrations into a particular plane is known as polarization (for glass it is 57.5o)
Types of crystals
Crystals like ice, calcite, quartz 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
Polaroids
A Polaroid is a material which polarises light. The phenomenon of selective absorption is made use of in the building of polariods
Uses of Polaroid
1.Polaroids are used in the laboratory to produce and verify plane polarised light.
2.Polaroids are widely used as polarising sun glasses.
3.They are used to remove the head light glare in motor cars.
4.They are used to increase 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 fitted outside the window while the other is fitted inside which can be rotated. The intensity of light can be altered 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 watches and calculatord, numbers and letters are formed by liquid crystal display (LCD) through polarisation of light.
9.Polarisation is also used to study size and shape of molecules
Theories of light
* Corpuscular theory
According to Newton, a source of light or a luminous body repeatedly emits tiny, massless (negligibly small mass) and
perfectly elastic particles called corpuscles. They travel in straight lines in a analogous medium in all directions with the speed of light
* Wave theory
According to Huygens, light is increased in the form of waves, through a continuous medium. Huygens assumed the existence
of an invisible, elastic medium called ether, which permeate all space
* Electromagnetic theory
Maxwell showed that light was an electromagnetic wave, conveying electromagnetic energy and not mechanical energy as believed by Huygens He also showed that no medium
was important for the propagation of electromagnetic waves.
* Quantum theory
1900, Planck had proposed that energy was emitted and absorbed, not continuously but in multiples of discrete pockets of energy called Quantum which
could not be subdivided into smaller parts. In 1905, Einstein expanded this idea and suggestedthat light waves consist of small pockets of energy called photons. The energy related with each photon is E = hν, where h is Planck’s constant (h = 6.626 × 10–34Js) and ν is the frequency of the electromagnetic radiation. It is now established that photon seems to have a dual character. It acts as particles in the region of higher energy and as
waves in the region of lower energy
Mirror and Lens
Focal Length : - The distance between the pole and the principal focus of a special mirror is called the focal length. (F)
Pole : - A point in the centre of the reflecting surface of a spherical mirror is
called the pole. It is represented by the
letter P.
Radius of Curvature
The radius of the sphere of which the
reflecting surface of a spherical mirror
creates a part is called the radius of
curvature of the mirror (R). Spherical mirrors of small apertures the radius of curvature is found to be equal to twice the focal length.
R=2f
Principal Axis :
Assume a straight line passing through the pole and the centre of curvature of a spherical mirror. This line is called the principal axis.
Uses of Convex Mirrors
1. Rear view mirrors in vehicles, CCTV Camera
Uses of Concave mirrors
1. Tourch Light
2. Street Light
3. Vehicles head lights
4. Shaving mirrors
5. Dentists use to see large images
of the teeth of patients.
6. Used to concentrate sun light to reproduce heat in solar turnaces.
Mirror Formula
(1/f)=(1/v)+(1/u)
* f = focal length
* u = object distance
* v = image distance
Lens formula
(1/f)=(1/v)-(1/u)
Magnification
The magnification created by a lens is defined as the ratio of the height of the image to the height of the object.
M=v/u
Power of Lens
The power of lens is described as the reciprocal of its focal length.
P=1/f
The S.I unit power of a lens is dioptre. It is denote by the letter D.