the power of a lens is the reciprocal of its focal length

the unit of power of the lens is diopter

the power of two or more thin lenses in contact is the power of the combination of the lenses

All of these

declination

azimuth

dip

bearing.

bench mark

datum point

reduced level

reference point.

in the objective glass

at the centre of the telescope

at the optical centre of the eye piece

in front of the eye piece.

10 m and 20 m length

15 m and 20 m length

20 m and 30 m length

25 m and 100 m length