# Calculating Telescope Length: Refracting Telescopes

In summary, the length of a refracting telescope is calculated by multiplying the focal length of the objective lens by the magnification of the eyepiece. The focal length of an objective lens is the distance between the lens and the focal point, typically measured in millimeters. The magnification of an eyepiece can be found by dividing the focal length of the objective lens by the focal length of the eyepiece. The length of a refracting telescope can be adjusted by changing the eyepiece or using a Barlow lens. Other factors that can affect the calculation of telescope length include the type and quality of lenses, the curvature and thickness of lenses, and the type of mount used.

in a refracting telescope, the lens's diameter is 1.02 m and its focal length is 19.4 m.
how long must the telescope be to get an image, 40 times bigger?

i would also like to get a link to a site explaining this kind of elementary optics.

just saw the warning, please don't delete my posts :S they are in a close enough format i think..

I can provide a response to the question about calculating the length of a refracting telescope. To determine the length of the telescope, we need to use the formula for magnification, which is given as M = f/fe, where M is the magnification, f is the focal length of the lens, and fe is the effective focal length of the telescope. To achieve a magnification of 40 times, we can rearrange the formula to solve for fe, which gives us fe = f/M. Plugging in the values given in the question, we get fe = 19.4 m / 40 = 0.485 m.

This means that the effective focal length of the telescope should be 0.485 m. To determine the length of the telescope, we need to add the focal length of the lens (19.4 m) to the effective focal length of the telescope (0.485 m), which gives us a total length of 19.885 m. Therefore, the telescope must be at least 19.885 m long to achieve an image that is 40 times bigger.

For further information on elementary optics, I recommend checking out this website from the University of Colorado Boulder: https://phet.colorado.edu/sims/html/geometric-optics/latest/geometric-optics_en.html. It provides interactive simulations and explanations on topics such as lenses, mirrors, and telescopes.

## 1. How is the length of a refracting telescope calculated?

The length of a refracting telescope is calculated by determining the focal length of the objective lens and multiplying it by the magnification of the eyepiece. This will give you the total length of the telescope.

## 2. What is the focal length of an objective lens?

The focal length of an objective lens is the distance between the lens and the point at which it focuses incoming light rays. It is typically measured in millimeters.

## 3. How do you determine the magnification of an eyepiece?

To determine the magnification of an eyepiece, divide the focal length of the objective lens by the focal length of the eyepiece. For example, if the objective lens has a focal length of 1000mm and the eyepiece has a focal length of 20mm, the magnification would be 50x (1000/20=50).

## 4. Can the length of a refracting telescope be adjusted?

Yes, the length of a refracting telescope can be adjusted by changing the eyepiece or by using a Barlow lens, which is an additional lens that can increase the focal length and magnification of the telescope.

## 5. Are there any other factors that affect the calculation of telescope length?

Yes, other factors that can affect the calculation of telescope length include the type and quality of the lenses, the curvature and thickness of the lenses, and the type of mount used for the telescope.