A astronomical telescope has objective and eyepiece of focal

A astronomical telescope has objective and eyepiece of focal lengths 40 cm and 4 cm respectively. To view an object 200 cm away from the objective, the lenses must be separated by a distance :

Given: Focal length of objective, f₀ = 40cm
Focal length of eye – piece f_e = 4 cm
image distance, v₀ = 200 cm
Using lens formula for objective lens

	1	-	1	=	1	⇒	1	=	1	-	1
	v₀		u₀		ƒ₀		v₀		ƒ₀		u₀

⇒	1	=	1	+	1	=	+ 5 - 1
	v₀		40		- 200		200

⇒v₀ = 50 cm
Tube length ℓ = |v₀| + f_e = 50 + 4 = 54 cm.