Two lenses, of focal lengths 3.0 cm and 30.0 cm, are used to build a small telescope. (a) Whic

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Two lenses, of focal lengths 3.0 cm and 30.0 cm, are used...

Two lenses, of focal lengths $3.0 \mathrm{~cm}$ and $30.0 \mathrm{~cm}$, are used to build a small telescope. (a) Which lens should be the objective? (b) What is the angular magnification? (c) How far apart are the two lenses in the telescope?

Key Concepts

Objective Lens

The objective lens is the primary element in a telescope that gathers the incoming light and forms a real image. Its focal length determines the light gathering power and resolution of the telescope; hence, in a telescope, the lens with the longer focal length is typically used as the objective because it collects more light from distant objects.

Eyepiece Lens

The eyepiece lens, often referred to as the ocular, is the secondary lens that magnifies the image formed by the objective. Its focal length is usually shorter than that of the objective lens, allowing for a high magnification of the final image observed through the telescope.

Angular Magnification

Angular magnification in telescopes is defined as the ratio of the focal length of the objective lens to that of the eyepiece lens. It quantifies how much larger a telescope makes an object appear compared to viewing it with the naked eye, thereby providing a direct measure of the telescope's ability to enlarge images.

Telescope Tube Length

The distance between the objective and the eyepiece in a simple astronomical telescope, often referred to as the tube length, is determined by the sum of the focal lengths of the objective and the eyepiece. This configuration ensures that the intermediate image formed by the objective lens is located at the focal point of the eyepiece, allowing for proper image formation and magnification.

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