**DAWN OF MODERN PHYSICS **

**1. ****A frame of reference is the set of coordinates relative to which measurements are taken.**

**2. ****An inertial frame is the one in which law of inertia is valid and is a un accelerated frame.**

**3. ****An accelerated frame is known as a non inertial frame of reference.**

**4. ****According to special theory of relativity (a) all laws of physics are same in all inertial frames of reference; (b) speed of light in free space has same value for all observers, and is independent of their state of motion.**

**RESULTS of special theory** **of relativity** are that,

TIME DILATION: time is not absolute quantity. If an event takes place at a high speed then time of its completion increases according to equation, t>t0

LENGTH CONTRACTION: distance from earth to a distance star measured by an observer in a moving spaceship ,l, seems smaller than the distance l0 measured by a stationary observer on earth according to equation

Mass variation: mass is varying quantity and depends on speed of object. It increases with increase in speed according to equation,

Mass and energy are inter convertible according to equation E = mc2

**Black body** absorbs all the radiations falling on it (when cold) and emits these radiations when heated.

Nature of radiations emitted depends on temperature.

At a given temperature emitted energy has a maximum value for a certain wavelength λmax. and λmax x T = constant = 2.9x 10-3 mK (wein’s constant)

As temperature is increased intensity of shorter wavelength is increased.

Total energy E radiated at any temperature T is proportional to the T4 i.e.