Question

why the filament of bulb becomes white hot and emit light

Answer 1

You will have noticed that the element in an oven or toaster turns red when it is heating. Nearly all metals begin to emit red light when heated. As the temperature increases, the color works its way through the spectrum up to orange and yellow until they full white.

All of these metals emit the same color for the same temperature (whether molten or still solid), which is how we have developed a “temperature scale” for color. Aluminium is a notable exception that stays grayish silver regardless of temperature, even if the steel crucible is glowing red hot.

Answer 2

Halogen lamps are similar to normal incandescent light bulbs— only hotter and brighter. When electricity flows through the filament, the lamp gives off light and gets hot in the usual way. In a normal incandescent lamp, the filament is made of tungsten metal and surrounded by a nonreactive ("inert") gas such as argon.

Tungent has a very very high melting point and when electricity is passed it becomes red and hotter to yellow by the VIBGYOR process. Argon just influences the light emission.