Question

We can understand the origin of the dark fringe at the point of contact by recalling that ray 1 undergoes no phase change on reflection, whereas ray 2 experiences a 180° phase change due to reflection. It follows, then, that when the path difference, 2d, approaches zero, rays 1 and 2 will cancel with destructive interference

Can somebody rephrase this into simple words?

It is about dark and bright fringe formula.

What does no phase mean? What does phase change mean regarding to light beam?

Answer 1

No phase change means that before reflection and after reflection the waves having same frequency does not have any change in degrees or radian.That is none of two waves lag Or lead, they wove with each other. If no phase difference then two waves are line up every where. Each particle of one wave is in phase with respect to another.Phase Change : Two waves have phase change it mean one wave lag and another lead by some degree or radian. At each time their is lagging or leading in waves. Waves with phase change are not lined up.  

Before ReJlection T/2

Two waves having same frequency but phase difference can not be lined up.

In above question ray 2 undergoes 180 degree phase change after reflection is means that reflected ray lead by 180 degree.

Clearly two waves, that is reflected and incidence ray have phase diffrence of 180 degree.  

• In interference two light source having same frequency and same phase yields to constructive interference.
• Same frequency and out of phase yield destructive interference.