Astronomical telescope
Intermediate
Refractor with large objective.
M = f_o/f_eObjective lens (large f) collects light; eyepiece magnifies. In normal adjustment M = f_o/f_e and the tube length is f_o + f_e.
f_o = 100 cm, f_e = 2 cm: M = 50.
Image is inverted.
In more detail
An astronomical telescope uses an objective lens to form an image of a distant object, and an eyepiece to magnify it. The final image is inverted, which is acceptable for viewing stars.
Real-world uses
- Amateur astronomers use refracting telescopes to observe planets.
- Kepler described this two-convex-lens arrangement in 1611, and it became the standard design for refracting telescopes.
- Amateur astronomers use refracting telescopes with a large objective lens to gather light from faint objects.
Open the interactive lesson, with simulation and quiz →
Frequently asked questions
Why is the image inverted?
Because the objective forms an inverted real image, and the eyepiece does not correct it.
What is the magnification?
The ratio of the objective focal length to the eyepiece focal length.
Why does magnification depend on focal lengths?
The angular magnification is the focal length of the objective divided by the focal length of the eyepiece.
Related topics in Optics
Defects of vision · Simple microscope · Compound microscope · Reflecting telescope · Camera and f-number · Wave theory of light