Young's Double slit exp

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

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

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.