# Reflection off of reflections in silvered glass slabs

The back of the glass slab is silvered, and M' is the front-shifted image of the silvered surface. While explaining reflection off of this, he stated the object distance to be; $$u = x + t/n$$ Apparently, light reflects off the image of the silvered surface, forming an image equally spaced behind M'.

How is this possible? I have heard of virtual objects, but can they serve as mirrors?

• Refer shift due to glass slab/Apparent depth. Anyway, I don't know why we considered shift of mirror, the incident rays of object is the one that undergoes shift. So the object distance would be (x+t)-(t/n) from the position of the old mirror, it would reflect back and the image would undergo a shift of t/n again in the direction of incident rays. – user600016 Apr 27 at 7:47
As you probably might know that the shift caused by a slab of refractive index $$n$$ of thickness $$t$$ is:
$$s=t\left(1-\frac 1 n\right)$$
What this means is, the effect of the slab is same as shifting the object towards the mirror by an amount of $$s$$ and then removing the slab. Or in other words, shifting either the mirror or the object by $$s$$ after removing the slab has the same effect of the presence of the slab.