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
one dimension
two dimensions
three dimensions
four dimensions.
intersection
traversing
radiation
none of these.
2.900 m
3.030 m
0.770 m
0.785 m