# How does LLoyd's mirror works?

1. Oct 8, 2009

### KFC

I wonder how does Lloyd's mirror works, how does it produce the interference pattern? I look for many text book, but all of them only tells the result and the setup without telling how to get the fringe and how to get the fringe separation. Anyone show me some hint or recommend me a book about this?

Thanks.

2. Oct 8, 2009

### Andy Resnick

From what I saw online, it looks like a shear plate interferometer- I make them with optical flats in the lab to observe the wavefront.

Basically, it's just like a michaelson interferometer- on my setup, one arm is formed by the reflection off the first surface, and the second arm is formed by the reflection off the back surface of the flat (or glass slide, which is fairly flat).

3. Oct 8, 2009

It's a plane mirror illuminated, usually, by a slit source.The direct waves and the reflected waves overlap to produce a Youngs type interference pattern.The reflected waves undergo a phase change of Pi.It's like having two sources the slit itself and the virtual image
formed by reflection and to get observeable fringes the sources have to be close enough,this will require near grazing incidence and a mirror silvered on its front surface or something equivelent.The slit needs to be parallel to the mirror and the screen at 90 degrees to the mirror and placed at a suitable distance.

Last edited: Oct 8, 2009
4. Oct 8, 2009

### KFC

Yeah, I feel a bit clear now. But I have two question in your reply. Why we need near grazing incidence? And if this is true, does it mean we can only produce a fringes close to the center of the screen? and why we need the front surface of the mirror be silvered, for providing a pi phase shift? And I wonder if the mirror will always produce a pi phase shift no matter what's the incidence angle is?

I am quite confusing the last statement, what does it mean by "the slit needs to be parallel to the mirror and the screen at 90 degrees to the mirror" ?

5. Oct 8, 2009