A light ray is incident perpendicularly to one face of a $${90^ \circ }$$ prism and is totally internally reflected at the glass-air interface. If the angle of reflection is $${45^ \circ }$$, we conclude that the refractive index $$n$$

The image formed by an objective of a compound microscope is

To get three images of a single object, one should have two plane mirrors at an angle of

If two mirrors are kept at $${60^ \circ }$$ to each other, then the number of images formed by them is