Optical instruments

Refraction at plane surfaces

Question: A microscope is focused on an ink mark. A glass plate is placed over spot. In order to focus spot the microscope is raised 1mm. Next time the scratch on upper surface of glass plate is focused by further 2mm raise. Calculate refractive index of glass.   

 Asked by: H. R. Gauli

Answer
 
 

During observation of ink spot,

Real depth = thickness of glass plate= 3 mm

Apparent depth = thickness – raise in sale while observing spot through glass plate

                        = 3-1 = 2 mm

Now, refractive index of glass is given by,

 

Question: An astronomical telescope having magnification, M= 10 has two thin lenses 44cm apart. Find the focal length of the lenses.

 Answer: 

 

In case of Astronomical telescope which forms image of distant objects at infinity,

Distance between two lenses,

f_o + f_e = 44 cm  …………………(i) (given in question)

Also, magnification     M = f_o/f_e = 10

Or,                               f_o = 10 f_e

Substituting value of f_o in equation( i)           

10f_e +f_e = 44

f_e  = 4cm

and      f_o = 44 - 4 = 40 cm