Worksheet: Telescopes

In this worksheet, we will practice calculating the imaging properties of reflecting, refracting, and compound optical systems used in telescopes.


What is the angular magnification of a telescope that has a 100 cm focal length objective and a 2.50 cm focal length eyepiece?

  • A48x
  • B50x
  • C44x
  • D36x
  • E40x


A 7.5x pair of binoculars produces an angular magnification of 7.50, acting like a telescope (where mirrors are used to make the image upright). If the binoculars have objective lenses with a focal length of 75.0 cm, what is the focal length of the eyepiece lenses?

  • A+10.0 cm
  • B+9.67 cm
  • C+10.3 cm
  • D+10.8 cm
  • E+10.5 cm


A large reflecting telescope has an objective mirror with a radius of curvature of 10.0 m. What angular magnification does it produce when an eyepiece of focal length 3.00 m is used?

  • A133x
  • B175x
  • C150x
  • D188x
  • E167x


A planet at a distance of 10 m from Earth is observed by a telescope that has a focal length of the eyepiece of 1 cm and a focal length of the objective of 1 cm. The planet is seen to subtend an angle of 10 rad at the eyepiece. What is the size of the planet?

  • A9×10 m
  • B1×10 m
  • C2×10 m
  • D6×10 m
  • E4×10 m


Two stars that are 10 km apart are viewed by a telescope and found to be separated by an angle of 10 radians . If the eyepiece of the telescope has a focal length of 1.5 cm and the objective has a focal length of 3 meters, how far away are the stars from the observer?

  • A1×10 km
  • B1×10 km
  • C4×10 km
  • D6×10 km
  • E2×10 km


A telescope can be used to enlarge the diameter of a laser beam and limit diffraction spreading. The laser beam is sent through the telescope in the direction opposite to the normal one and can then be projected onto a satellite or the Moon. This is done with a telescope, producing a 3.50 m diameter beam of 630 nm wavelength light.

What is the minimum angular spread of the beam produced?

  • A2.36×10 rad
  • B4.78×10 rad
  • C3.35×10 rad
  • D1.98×10 rad
  • E2.20×10 rad

Neglecting atmospheric effects, find the size of the spot this beam would make on the Moon. Use a value of 4.00×10 m for the distance to the Moon.


In a reflecting telescope, the objective is a concave mirror of radius of curvature 4.0 m and the eyepiece is a convex lens of focal length 6.0 cm. Find the apparent size of a 5.0 m tree at 20 km that you would perceive when looking through the telescope.


What is the distance between the objective lens and eyepiece in a telescope that has a 100 cm focal length objective and a 4 cm focal length eyepiece needed to produce a final image very far from the observer, where vision is most relaxed?

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