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Figure 8.9 Double slit interference pattern.

very poor estimate of wavelength. If the light is monochromatic (has one wavelength) there is a single set of well-defined bands. The same is true of a conventional lamp if most of the light is emitted at one wavelength. Laser light is as monochromatic as you can get and produces very clear interference patterns.

What happens if the separation changed?

The condition for constructive interference is dQ = mX. When d is increased, the angle Q between adjacent bright bands must decrease if mX is to remain constant. This means that as slit separation increases, the bands become closer together. The light intensity diminishes uniformly with increasing lateral distance from the central bright band. This is addressed in the next section.

From the middle of the 19th century, the wave theory of light was generally accepted.

The stature of Isaac Newton was one, if not the only, factor which delayed the widespread adoption of the wave theory for almost a century from the time Huygens' wavelets were used to show that light travels more slowly in a denser medium to the time Thomas Young demonstrated the interference of light.

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