If this telescope is used to view a 100 m tall tower 3 km away, what is the height of the image of the tower formed by the objective lens?
Height of the tower, h1 = 100 m
Distance of the tower (object) from the telescope, u = 3 km = 3000 m
The angle subtended by the tower at the telescope is given as:
`theta = h_1/u`
`= 100/3000 = 1/30 rad`
The angle subtended by the image produced by the objective lens is given as:
`theta = h_2/f_o = h_2/140 rad`
h2 = Height of the image of the tower formed by the objective lens
`1/30 = h_2/140`
`:. h_2 = 140/30 = 4.7`
Therefore, the objective lens forms a 4.7 cm tall image of the tower.