Q: In my iPhone the auto brightness slider not appear.

Hey there arvind93!
 

I like how my screen will adjust based on light brightness or my own setting.  We can certainly check things out about the missing slider. 

First off, the Brightness slider should be available in 2 places on the iPhone.  Settings > Display and Brightness at the top or by swiping up from the bottom of the screen to open Control Center.  It should appear below the top row of buttons. 

Use Control Center on your iPhone, iPad, and iPod touch - Apple Support

Adjust the display settings on your iPhone, iPad, or iPod touch - Apple Support

The second article mentions having an ambient light sensor and the iPhone 5 does have one included: iPhone 5 - Technical Specifications

Let us know how that goes and if needed add screenshots of what you see on those menus: How to take a screenshot on your iPhone, iPad, and iPod touch - Apple Support

Cheers!