PHYSICS 203
Introduction to Astronomy Lab
Lab 7: Optics and Telescopes
Name________________
The focal length of a convex lens determines the position where an
image is formed. In astronomical telescopes, the object to be viewed
(star, planet, etc) is very far from the lens; roughly at infinity. In this
case the image is formed at the focal point of the objective lens. As with
all converging lenses when the object is far away, the image is smaller
than the object and inverted. A second convex lens (the eyepiece) is
positioned such that its focal point is just beyond the focal point of the
objective lens. This lens takes the image from the objective and
produces a virtual image that is larger than the image produced by the
objective (see diagram). Thus, the focal length of the objective in a
refracting telescope is of importance.
1. We will determine the focal length of a convex lens using the equation
1 =
f
1 +
d0
1
di
where f is the focal length
do is the distance from the lens to the object
di is the distance between the lens and the image
(note that if the object is at infinity the image
occurs at the focal length)
Set the lens some distance from the light source and move the screen
until the image is as sharp as possible. Measure d0 and di and record
those values. Repeat the process for two different distances between the
light source and the lens. Note whether the image is larger or smaller
than the arrow of the light source. For each set of numbers calculate the
focal length using the equation above.
do
di
focal length
trial 1
trial 2
trial 3
ave. focal length ___________
2. Now, look at the telescope. The length of the telescope will be the focal
length. Magnification in a refracting telescope can be found as the ratio
of the focal length of the objective to the focal length of the eyepiece
(M = fo/fe). The maximum useful magnification depends of the aperture
of the telescope and can be estimated by 2x per mm of aperture. (Note
that shorter focal length eyepieces result in higher magnification)
Focal length of telescope ______________cm
Aperture of telescope _______________cm
Maximum useful magnification _______________x
Minimum useful focal length for eyepiece ___________cm
Ray diagram for an astronomical telescope.