The eyepiece of a refracting astronomical telescope has a focal length of F_2 =

Question

The eyepiece of a refracting astronomical telescope has a focal length of F_2 = 9.00 cm. The distance between the objective and the eyepiece is 1.80 m. The final image is essentially at infinity. Use the information given and a diagram you mightwant to draw to get your answer to this part. What is the focal length F_1 of the objective? What is the angular magnification M of the telescope? Draw a clear ray tracing diagram showing both lenses, objective and eye piece. As in the book, show the path of rays of light traveling rightward, say, from an object at infinity (i.e., a star) at far left. Indicate the final large image at infinity also at left.

Explanation / Answer

Hi,

(a) In a refracting telescope the distance between the eye piece and the objective is equal to the sum of the focal distances of the lenses, therefore:

d = f1 + f2 ; where d is the distance between the lenses, f1 is the focal length of the objective and f2 is the focal length of the eye piece.

f1 = d - f2 = 180 cm - 9 cm = 171 cm

As you can see, the focal distance of the objective is much bigger than the one of the eye piece. That is something expected, as the rays of light must travel a certain distance to form the inverted image of the object between the lenses.

(b) The angular magnification of the telescope can be found through the following relation:

M = - f1/f2 ; where the negative sign indicates that the image between the lenses is inverted.

As we are interested in the magnitude of M, we have the following:

M = (171 cm)/(9 cm) = 19

(c) Unfortunately I can't draw a ray diagram for this case, but I can give you a generic image that shows similar features:

http://www.a-levelphysicstutor.com/images/optics/tels-astro-refract.jpg

I hope it helps.

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.