A microscope is focused on a mark on a piece of paper and then a slab of glass of thickness 3 cm and refractive index 1.5 is placed over the mark. How should the microscope be moved to get the mark in focus again?

Apparent depth of mark as seen through a glass slab of thickness x and refractive index µ isApparent  depth=Real  depthRefractive  indexor   x'=xμ=31.5=2  cmAs  image  appears  to  be  raised  by  1  cm,  therefore  microscope  must  be  moved  upward  by  1  cm.