Why does t = $\frac{1}{4}$ for destructive interference by parallel-sided thin films?

I knew for destructive interference for reflected light in thin films: 2t = mλ

Where t is the thickness of the thin film, m is an integer (0,1,2...) and λ is the wavelength of light in the thin film.

However, on some websites and books t = $$\frac{λ}{4}$$

My questions are:

1. Why does t = $$\frac{λ}{4}$$?
2. Does it have something to do with a phase change on reflection of π?
3. Does a phase change on reflection happen when light reflects off from less dense → more dense or vice versa?

Destructive interference for reflection would occur for $$t=\frac{\lambda}{4}$$ when the film has an index of refraction between that of the two other media. For example an anti reflective coating on a lens would have a value less than that of the glass (and more than that of air). In that way there is a phase inversion at both interfaces for light starting in the air, or neither for light originating in the glass, either case resulting in an effective path difference for reflection of half a wavelength.