Solve Antique Telescope Homework: Find Fe Focal Length

• xinlan
In summary, the antique telescope in the physics lab has a magnification of 20 and consists of two converging lenses, the objective and the eyepiece. To find the focal length of the eyepiece, the equation M=Fo/Fe can be used, where M is the magnification, Fo is the focal length of the objective lens, and Fe is the focal length of the eyepiece. By setting the angular magnification to -20, which means the final image is inverted when viewed through the eyepiece, and solving for Fe, we get a positive focal length of 2.857 cm.
xinlan

Homework Statement

On one of the shelves in your physics lab is displayed an antique telescope. A sign underneath the instrument says that the telescope has a magnification of 20 and consists of two converging lenses, the objective and the eyepiece, fixed at either end of a tube 60.0 cm long. Assuming that this telescope would allow an observer to view a lunar crater in focus with a completely relaxed eye, what is the focal length Fe of the eyepiece?

Homework Equations

Length (L) = Fo+Fe
Which Fo = Focal length of the objective lens
Fe = Focal length of the eyepiece

M = -Fo/Fe

The Attempt at a Solution

L = Fo+Fe
60 = Fo+Fe
Fe = 60-Fo

M = -Fo/Fe
20 = -(60-Fe) / Fe
Which Fe would be -3.16

But I got wrong..

thank you..

xinlan said:
A sign underneath the instrument says that the telescope has a magnification of 20 and consists of two converging lenses

M = -Fo/Fe

Hi xinlan!

I don't know much about optics …

Why have you put a minus there?

Make the angular magnification -20, that is the final image is inverted when viewed throught the eyepiece. Which would give you a positive focal length for the eyepiece as required for a convex lens.

Last edited:
andrevdh said:
Make the angular magnification -20, that is the final image is inverted when viewed throught the eyepiece. Which would give you a positive focal length for the eyepiece as required for a convex lens.

But the question says "two converging lenses" - wouldn't that make both negative?

andrevdh said:
Make the angular magnification -20, that is the final image is inverted when viewed throught the eyepiece. Which would give you a positive focal length for the eyepiece as required for a convex lens.

Even though the M is -20, but the answer would still be the same..
I put the positive focal length and I still got wrong

I got it..
thanks..

How'd you find it?

Here's How:

M=Fo/Fe
20= (60-Fe)/Fe
Fe = (60/21)

Fe= 2.857

Last edited:

1. What is a focal length?

A focal length is the distance between the lens and the focal point of a lens or mirror. It is an important measurement in optics and determines the magnification and field of view of a telescope.

2. How do I determine the focal length of an antique telescope?

To determine the focal length of an antique telescope, you will need to measure the distance between the lens and the focal point. This can be done by using a ruler or a measuring tape. It is also important to make sure that the telescope is properly focused and the measurement is accurate.

3. Why is it important to know the focal length of a telescope?

Knowing the focal length of a telescope is important because it allows you to calculate the magnification and field of view of the telescope. This information is crucial for astronomers and stargazers who want to know what objects they can observe and how much they will be magnified.

4. Can the focal length of an antique telescope be changed?

No, the focal length of a telescope is a fixed measurement and cannot be changed. However, you can use different eyepieces to achieve different magnifications and fields of view.

5. Are there any other factors that can affect the focal length of a telescope?

Yes, the focal length of a telescope can also be affected by the curvature of the lens or mirror, the quality of the glass, and the overall design and construction of the telescope. These factors can all impact the performance of the telescope and should be taken into consideration when determining the focal length.

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