Double slit experiment: effect of changing wavelength

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SUMMARY

The discussion centers on the effects of increasing the wavelength (λ) in Young's double slit experiment while keeping the slit width (a) and distance to the screen (D) constant. When the wavelength is increased to 1.5λ, the separation of the fringes increases by 1.5 times. However, the maximum brightness of the fringes decreases, contrary to initial assumptions that it would remain the same. This reduction in brightness is attributed to the sensitivity of the human eye to different wavelengths and the increased distance light travels to reach the screen, affecting the intensity of the fringes.

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Homework Statement



The question is regarding Young's double slit experiment.

"Describe the effect, if any, on the separation and on the maximum brightness of the fringes when the following changes are made.
...
...
2) The wavelength λ is increased to 1.5λ, keeping a and D constant.

Separation:
Maximum brightness:

Homework Equations



λ=ax/D

The Attempt at a Solution



My answer:

Separation: Increases by 1.5 times
Maximum brightness: Same (Wrong, answer: less bright)

Why would the change in wavelength alter the brightness of the fringes?
 
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Sensitivity of the eye to different wavelengths perhaps?
How is brightness defined here?

For a longer wavelength, the brightness of the fringes would probably fall off more with the order of the fringe ... since the light has further to go to reach the screen. But I'd have said the central fringe would have the same brightness for all wavelengths affected - evidence: white light interference: the central fringe is white.
 
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