Double slit experiment: effect of changing wavelength

[coconut62]

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?

 

[Simon Bridge]

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.