The image formed by an objective of a compound microscope is

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

Wavelength of light used in an optical instrument are $${\lambda _1} = 4000\mathop A\limits^ \circ $$ and $${\lambda _2} = 5000\mathop A\limits^ \circ ,$$ then ratio of their respective resolving powers (corresponding to $${\lambda _1}$$ and $${\lambda _2}$$ ) is :

Which of the following is used in optical fibres?