Luminous bodies

Luminous bodies: Those objects which emit light by themselves are called luminous bodies.

e.g. - sun, stars, electric bulb etc.

Non- luminous bodies: those objects which do not emit light by themselves but are visible by the light falling on them emitted by self luminous bodies are called non-luminous bodies.

A material can be classified as:

(i)                  Transparent: the substances which allow most of the incident light to pass through them are called transparent e.g. glass, water.

(ii)                Translucent: The substances which allow a part of incident light to pass through them are called translucent bodies e.g. oiled paper.

(iii)               Opaque: The substances which do not allow the incident light to pass through them are called opaque bodies. E.g. mirror, metal, wood etc.

Reflection of light: Light moving in one medium when falls at the surface of another medium, part of light returns back to the same medium. This phenomenon of returning back of light in the first medium at the interface of two media is known as reflection of light.

Laws of reflection

(i)                  The incident ray, reflected ray and normal to the reflecting surface at the incident point all lie in the same plane.

(ii)                The angle of reflection is equal to the angle of incidence.

Reflection from plane mirror

(i)                  The image is virtual, laterally inverted.

(ii)                The size of image is equal to that of object.

(iii)               The distance of image from the mirror is equal to distance of object from the mirror.

(iv)              If an object moves towards (or away from) a plane mirror with speed v, relative to the object the image moves towards (or away) with a speed 2v.

(v)                If a plane mirror is rotated by an angle θ, keeping the incident ray fixed, the reflected ray is rotated by an angle 2θ.

(vi)              To see his full image in a plane mirror, a person requires a mirror of at least half of his height.

(vii)             If two plane mirrors are inclined to each other at an angle θ the number of images (n) of a point object formed are determined as follows:

(a)    If 360  /θ  is even integer, then n= 360 /θ = 1

(b)   If  360/ θ is odd integer,

Then n=360/ θ -1 for the object is symmetrically placed and

N = 360 /θ for the objects is not symmetrically placed.

(c)    If 360 /θ are a fraction then n is equal to integral part.

Range of different thermometers

Mercury Thermometer: from -30˚C to 350˚C

Constant volume gas thermometer: from -200˚C to 500˚C (with H₂), below -200˚C upto -268˚C (with He) above 1000˚C upto 1600˚C (with N₂ gas and bulb of glazed porcelain)

Platinum resistance thermometer: form -200˚C to 1200˚C

Thermocouple thermometer: form -200˚C to 1600˚C

Total Radiation Pyrometer

When a body is at high temperature, it glows brightly and the radiation emitted by the body is directly proportional to the fourth power of absolute temperature of the body. Radiation pyrometer measures the temperature of a body by measuring the radiation emitted by the body.

This thermometer is not put in contact with the body. But it cannot measure temperature below 800˚C because at low temperature emission of radiation is very small and cannot be detected. 

Specific Heat Capacity: Specific heat capacity of a material is the amount of heat required to raise the temperature of unit mass of substance through 1˚. Its SI unit is Joule/ gram˚C.

1 calorie / gram˚C = 4200 Joule / kg Kelvin.

Thermal Expansion

When a body is heated its length, surface area and volume increase. The increase in length , area and volume with the increase in temperature are measured in terms of coefficient of linear expansion or linear expansively (α) coefficient of superficial expansion or superficial expansively (β) and coefficient of cubical expansion or cubical expansively (γ).

Specific Heat Capacities of different materials (J/kg K)

Water                             4200

Ice                                  2100

Iron                                460

K. Oil                             210

Mercury                         140

Lead                               130    

Relation between α, β and γ.

α: β :γ =1:2:3                   or,    β=2   α and  γ =3α

Properties of electromagnetic waves

 Properties of electromagnetic waves

(i)                  They are neutral.

(ii)                They propagate as transverse wave.

(iii)               They propagate with the velocity of light.

(iv)              They contain energy and momentum.

(v)                Their concept was introduced by Maxwell.

Speed of sound

Speed of sound:

Speed of sound is different in different mediums. In a medium, the speed of sound basically depends upon elasticity and density of medium.

Speed of sound is maximum in solids and minimum in gases.

When sound enters from one medium to another medium, its speed and wavelength changes but frequency remains unchanged.

In a medium, the speed of sound is independent of frequency.

Effect of pressure on speed of sound: The speed of sound is independent of pressure i.e. speed remains unchanged by the increase or decrease of pressure.

Sound Wave

Sound waves are longitudinal mechanical waves.

According to their frequency range, longitudinal mechanical waves are divided into the following categories:

       Audible or Sound waves: The longitudinal mechanical waves which lie in the frequency range 20 Hz to 20000 Hz are called audible or sound waves. These waves are sensitive to human ear. These are generated by the vibrating bodies such as tuning fork, vocal cords etc.   


 Infrasonic waves: the longitudinal mechanical waves having frequencies of bigger size such as earth quakes, volcanic eruptions, ocean waves and by elephants and whales.  

   Ultrasonic Waves: The longitudinal mechanical waves having frequencies greater than 20000 Hz are called ultrasonic waves. Human ear cannot detect these waves. But certain creatures like dog, cat, bat, and mosquito can detect these waves. Bat not only detect but also produce ultrasonic.

Ultrasonic waves can be produced by Galton’s whistle or Hartman’s generator or by the high frequency vibrations of a quartz crystal under an alternating electric field (Piezo- electric effect) or by the vibrations of a ferromagnetic rod under an alternating magnetic field (Magnetostriction).

Applications of Ultrasonic Waves

1.       For sending signals.

2.       For measuring the depth of sea.

3.       For cleaning cloths, aero planes and machinery parts of clocks.

4.       For removing lamp-shoot from the chimney of factories.

5.       In sterilizing of a liquid.

6.       In Ultrasonography.

Derived Units

Derived Units :

          All the units which are defined / expressed in terms of fundamental units are called derived units.

Some important derived units.

         

Physical Quantity	cgs units	SI unit	Relation
Force	Dyne	Newton	1 newton =10^5 dyne
Work	erg	Joule	1 joule=10^7 erg

SI Units

SI Units :

          There are seven fundamental units given in the following table:

Physical Quantity	SI Unit	Symbol
Length	Metre	m
Mass	Kilogram	Kg
Time	Second	s
Electric Current	Ampere	A
Temperature	Kelvin	K
Luminous intensity	Candela	Cd
Amount of substance	Mole	mol

          Besides these seven fundamental units, two supplementary units are also defined, viz, radian [rad] for plane angle and steradian (sr) for solid angle.

Units

Units :

          The Chosen standard used for measuring a physical quantity is called unit.

          Unit should be :

·        Well defined,

·        Easy to reproduce,

·        Easy to compare,

·        Internationally accepted,

·        Independent of changes in physical conditions

Units are two types :

·        Fundamental unit, and

·        Derived unit.