An astronomical refracting telescope is made from two lenses.The objective lens has a focal length of
And the eyepiece lens has a focal length of
(a)What is the total length of the telescope?
(b)What is the angular magnification of the telescope?
(c)What is the magnification by turning the telescope around and looking through the objective lens first?
A)(a)131 cm (b)6.71 (c)0.15
B)(a)97 cm (b)0.15 (c)6.71
C)(a)262 cm (b)0.87 (c)1.15
D)(a)194 cm (b)1.18 (c)0.85

Question

An astronomical refracting telescope is made from two lenses.The objective lens has a focal length of 
And the eyepiece lens has a focal length of 
(a)What is the total length of the telescope?
(b)What is the angular magnification of the telescope?
(c)What is the magnification by turning the telescope around and looking through the objective lens first?
A)(a)131 cm (b)6.71 (c)0.15 
B)(a)97 cm (b)0.15 (c)6.71 
C)(a)262 cm (b)0.87 (c)1.15 
D)(a)194 cm (b)1.18 (c)0.85

Quizplus · Accepted Answer

(a) The total length of the telescope is given by the sum of the focal lengths of the two lenses. Therefore, Total length = 36 cm + 95 cm = 131 cm.
(b) The angular magnification of the telescope is given by the formula - magnification = -fo/fe, where fo is the focal length of the objective lens and fe is the focal length of the eyepiece lens. Therefore, magnification = -(36 cm)/(5.5 cm) = -6.55 ≈ -6.71.
Note that the negative sign indicates an inverted image.
(c) The magnification by turning the telescope around and looking through the objective lens first is given by the formula - magnification = fo/fe. Therefore, magnification = (36 cm)/(95 cm) ≈ 0.379 ≈ 0.15.
Again, the negative sign indicates an inverted image.

An Astronomical Refracting Telescope Is Made from Two Lenses