How an LDR(light dependent resistor) works!!!!

An LDR is a component that has a resistance that changes with the light intensity that falls upon it. They have a resistance that falls with an increase in the light intensity falling upon the device.   

An LDR is made of a high resistance semiconductor. In the dark, an LDR can have a resistance as high as a few megaohms (MΩ), while in the light, an LDR can have a resistance as low as a few hundred ohms. If incident light on an LDR exceeds a certain frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electrons (and their hole partners) conduct electricity, thereby lowering resistance. The resistance range and sensitivity of an LDR can substantially differ among dissimilar devices. Moreover, unique LDR may react substantially differently to photons within certain wavelength bands.

The resistance of an LDR may typically have the following resistances. 
Daylight =5000Ω  Dark =20,000,000Ω  

You can therefore see that there is a large variation between these figures. If you plot this variation on a graph you would get something similar to that shown by the graph below. 

Applications 

There are many applications for Light Dependent Resistors. These include:  

Lighting switch  The most obvious application for an LDR is to automatically turn on a light at certain light level.

 An example of this could be a street light.  

Camera shutter control LDRs can be used to control the shutter speed on a camera. The LDR would be used the measure the light intensity and the set the camera shutter speed to the appropriate level.