Why is light from a single source incoherent?

[zmike]

Just a quick q,

My book says that to make the light source young used to be coherent, he passed it thru a SINGLE slit THEN passed it thru a double slit. Why would he do that? couldn't he just put the light source directly to the double slit? I thought all light from a single source was coherent?

thanks

 

[Delphi51]

The degree of coherence is very low for most light sources. The single slit would improve the level of coherence.

Only laser light is highly coherent.

 

[zmike]

The degree of coherence is very low for most light sources. The single slit would improve the level of coherence.

Only laser light is highly coherent.

why would a single slit improve coherence?

why is a single light source incoherent? doesn't light from a single source have a frequency and intensity that is identical?

 

[rl.bhat]

Any light source can be treated as the collection of large number of point sources. According to the Hygiene's wave theory, each point source forms a wavefront which propagates in the forward direction. So at any point from the source several wave front can reach there producing in coherent light. Due to diffraction, a narrow single slit can behave like a point source.

 

[buffordboy23]

why is a single light source incoherent? doesn't light from a single source have a frequency and intensity that is identical?

Incoherence is related to the type of light source. The design of a laser generally permits the produced light to be in phase (coherent) and of the same wavelength (monochromatic). Compare that with an incandescent light bulb. The light bulb has a filament that gets hot, which results in the production of light. However, the light emitted ranges over a variety wavelengths--about 10% of the emitted light is in the visible spectrum and the rest is in the infrared spectrum--so it is polychromatic. Light from a light bulb is incoherent because the light production is heat driven and the temperature variations cause the electrons to jump from orbital to orbital in a random manner.