# Introduction to Modern Physics

## Introduction to Modern Physics for Class 11 -12

**Frame of Reference**

The coordinate system in a three dimensional space with respect to which we can describe the motion of an object is called frame of reference.

**Inertial Frame of Reference**

The frame of references moving with constant velocity to each other in which Newton's law of motion can be applied are called inertial frame of reference.

**Galilean Transformation**

Taking time as absolute and ignoring postulates of special theory of relativity, the equations obtained which establish relations between two sets of coordinates of two fame of references moving with velocity constant relative to each other and time are called Galilean transformation.

**Lorentz Transformation**

Taking time as non absolute and applying postulates of special theory of relativity, the equations obtained which establish relations two sets of coordinates two frames of reference moving with constant velocity relative to each other and time are called Lorentz transformation.

**Time Dilation**

Time duration of an event at motion with respect to an observer is greater than the time duration of the same event at rest. This is called time dilation.

**Length contraction**

With respect to an observer, the length of an object at motion seems shorter than that the same object at rest. This effect is called length contraction.

**Photo Electric Effect**

If light rays of high frequency incident on a metal plate, electrons emit. This effect is called photo electric effect.

**Compton Effect**

Wavelength of scattered radiation of monochromatic x-ray changes due to scattering. This effect is called Compton Effect.