What features should a light bulb filament material have?

Using Plank’s law and Wien’s law you can predict the wavelength and intensity of light emitted by a filament at different operating temperatures. Since, only light visible to the eye is useful, the greater the fraction of visible light emitted by the filament, the more efficient it will be. The largest fraction of visible light is emitted from a filament operating at ~5,000 K. More light is emitted at higher temperatures, but more of it is out of the range of visible light. Inventors have used these principles to invent and improve incandescent filaments. 

Now take a short quiz on the essential features a filament material should have.
 
Question 1

A filament material should be able to conduct electricity

True
False

 
Evaluation of your response for Question 1

YES!  If the filament can not conduct electricity, it can not be heated electrically.

 
Evaluation of your response for Question 1

Please review the previous pages in the Filament section to select the correct response. 

 
Question 2

A filament material should be able to generate light when heated to a certain point.

True
False 


 
 
Evaluation of your response for Question 2

YES!  This is not difficult. Any material that can be heated to a high enough temperature, will give off light.

 
Evaluation of your response for Question 2

Please review the previous pages in the Filament section to select the correct response.  

 
Question 3

A filament material should be transparent ?

True
False 

 
Evaluation of your response for Question 3

YES! A filament need not be transparent.  The light is given off from the surface of the filament. The light does not have to pass through the filament.

 
Evaluation of your response for Question 3

Please review the previous pages in the Filament section to select the correct response. 

 
Question 4

A filament material should not burn in air.

True
False 


 
 
Evaluation of your response for Question 4

Good! Your filament need not have this property. The filament is sealed in an inert atmosphere of argon and/or nitrogen. Air does not contact the filament.

 
Evaluation of your response for Question 4

Try again! It is not necessary that a filament not burn in air, because the filament is sealed in a glass bulb filled with nitrogen/argon, so air does not come into contact with the filament.

 
Question 5

A filament material should last a long time.

True
False 


 
 
Evaluation of your response for Question 5

Of course, any good filament material should last a long time and the longer the better.  Most filaments slowly erode by a process called sublimation, in which filament atoms evaporate off the surface because of its high temperature.  Slowing down sublimation increases filament life.

 
Evaluation of your response for Question 5

Please review the previous pages in the Filament section to select the correct response. 

 
Question 6

A filament material should have a high melting point.

True
False 


 
 
Evaluation of your response for Question 6

Good! The higher the temperature the better for giving off light. Once the filament melts, it is destroyed. Therefore, a filament should have a high melting point.

You have finished the quiz. Go on to the next section.

 
Evaluation of your response for Question 6

Please review the previous pages in the Filament section to select the correct response.  

SYNOPSIS

1)A filament should conduct electricity

2) A filament when heated to a high enough temperature, should give off light.
3)A filament need not be transparent.
4) A filament need not be stable when heated in air.
5)A filament material should last a long time.