white light is made up of seven colors VIBGYOR. All the seven colors will show interference, we can treat each color separately as a monochromatic light and study their interference separately. Each light will have their bright and dark fringes separately but at y=0 i.e for a center, each lights bright fringe will coincide. Two or more lights may have their bright and dark Fringe together or bright for one and dark for others may coincide depending upon the situation.

When Young 's double-slit set up for interference is shifted from air to within water then the fringe width decreases because the refractive index of water is more than that of the air.
Originally the fringe width is given by:
β1 ​=λD/2d ​
The new fringe width within water will be given by  
β2 ​= λD ​/2nd
So, β2 ​= β1/n ​​
Here, n is the refractive index of medium.
 

Dark fringes will be produced when there are destructive interference. The condition for that is the two waves should have a phase difference of  an odd integral multiple of π.

Fringe width β=λD/d ​
On d ′=d/2 ​, D ′=2D
New fringe width β′= λD ′​/d ′=4 β

We observe interference pattern when the size of the slits are comparable to the wavelength of light. But here, the size of the bulb is bigger than wavelength of light.

Position of the p^th dark fringe is at a distance of  
y=(2p −1) (λD/2d)​from the centre
where  
λ: Wavelength
D: Distance between slits  
d: slit width

Because Presence of two coherent sources of radiation is one of the PRIME CONDITIONS FOR INTERFERENCE.

When the light is incident on a soap bubble of certain thickness it constructively interferes for wavefronts which are in phase to produce white light and when the wavefronts are out of phase, they undergo destructive interference to produce a series of colors. Thus, interference is the reason.

We know that for two coherent sources, the intensity of their maxima is (I_max +  I_min )² and that of its minima is (I_max +  I_min )² . Thus we get the intensity of maxima = 121k and that of minima = 9k, for some constant k as (I_max = 100k and  I_min = k.
Thus we get the ratio as 121:81.

Interference is a known phenomenon in which two coherent sources of light meet and produce a resultant effect, i.e either a wave of greater or lower amplitude.
The intensity and amplitude changes depending on whether it 's constructive or destructive interference. However, energy is distributed equally and hence remains conserved.