Build a Telescope

Galileo and Kepler made telescopes from just two lenses. Pick your lenses, slide them until the picture is sharp, and see how much bigger the Moon looks.

Look at
Magnification
×10.0
Focus
Blurry: move apart
Separation
52.0 cm
Best separation
55.0 cm

The maths, live

Magnification M = fo ÷ feM = 50 ÷ 5 = 10.0×
In focus when d = fo + fe50 + 5 = 55.0 cm (yours: 52.0 cm)
Moon looks 0.5° × M across0.5° × 10.0 = 5.0°

Slide the lenses apart until the view is sharp. Then swap in a shorter eyepiece. Why does the tree look upside down?

How it works

Light from something very far away arrives as parallel rays. The big objective lens brings them together to make a small, upside-down image at its focal point. The eyepiece then works like a magnifying glass to look at that image.

The view is sharpest when the lenses are the two focal lengths apart: d = fo + fe. The angular magnification is M = fo ÷ fe, so a long objective and a short eyepiece give the biggest view. This kind of telescope shows everything upside down, which doesn't matter for the Moon.

Try this

  1. Find the separation that makes the picture sharp. Does it match fo + fe?
  2. Which eyepiece gives the biggest magnification?
  3. Look at the tree. Why is it upside down?