Double-Split experiment with light

[1way]

Young's Double-Split experiment shows two waves of light interfering with each other.

However I have a question I was hoping someone can help me with.

Why is it we can see the Interference Pattern with large bands of light and dark?

Would not the waves be so small that the interference pattern would be also very small and not detectable by eye?

I can only guess its something to do with the size of the slits and the wave length of visible light.

 

[Born2bwire]

If we have narrow slits, then each slit will act like a point source for a new wave. So instead of a plane wavefront, the point source will be spherical (or cylindrical in 2D) wavefront. So the interference results in the differing path lengths that the two point sources experience when the wavefronts combine. For small angles, the change in the path length for the first few fringes is very slight and so we can get interference bands that are much larger than the actual wavelength.

Wikipedia has a good explanation in their article: http://en.wikipedia.org/wiki/Double-slit_experiment#Classical_wave-optics_formulation