Q: when browsing internet or watching videos my iphone5 become hot

Hello there, Pijush Roy.

It seems you are concerned with the temperature increasing on your phone when in use or watching videos. The following Knowledge Base article offers up some great information on what expectations to have in regards to the temperature of your iPhone when in usE:

Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures

When using the device or charging the battery, it is normal for it to get warm. The exterior of the device functions as a cooling surface that transfers heat from inside the device to the cooler air outside.

Higher ambient temperature conditions and activities that may cause the device to alter performance and behavior include:

• Leaving the device in a car on a hot day.
• Leaving the device in direct sunlight for an extended period of time.
• Using certain features in hot conditions or direct sunlight for an extended period of time, such as GPS tracking or navigation in a car, or playing a graphics-intensive game.

If the interior temperature of the device exceeds the normal operating range, the device will protect its internal components by attempting to regulate its temperature. If this occurs, you may notice the following:

If the device exceeds a certain temperature threshold, it will present a temperature warning screen similar to this:

• The device stops charging.
• The display dims or goes black.
• In navigation: 
   
  • The device will present this alert and turn off the display: "Temperature: iPhone needs to cool down."
   
   
  • Navigation will continue to provide audible turn-by-turn directions. When approaching a turn, the display will illuminate to guide you through the turn.
   
  • To return the device to normal operation, press the Home Button and slide to unlock. If the device has cooled down enough, you can continue normal usage.
• Cellular radios will enter a low-power state. The signal may weaken during this time.
• The camera flash is temporarily disabled.

To resume use of your device as quickly as possible, turn it off, move it to a cooler environment, and allow it to cool down.

Note (iPhone only): When this message appears, the device might still be able to make emergency calls.

If you still feel the temperature is an issue, you may find further guidance here:

Get help with hardware features on your iPhone, iPad, and iPod touch

If you still have an issue with your device, contact Apple Support.

Thanks for being a part of Apple Support Communities.

Cheers.