Page 228 - Physics_Form_2

Physics for Secondary Schools

The angle subtended by the object at Uses of an astronomical telescope
the objective lens is the same as the A telescope is used to view distant objects
angle subtended by the first image at the such as stars and planets. It is one of the
h primary tools in the fields of astronomy
objective lens. Thus, tanβ ≈ β = and astrophysics.
f e
FOR ONLINE READING ONLY
Construction of a simple astronomical
Similarly, the angle subtended by the telescope
final image at the eye is the same as the
angle that a ray coming from the head of One can easily construct a simple
first image and travelling parallel to the astronomical telescope using locally
available materials.
principal axis makes with the principal
axis after it passes through the lens. Then,
h Activity 6.2
tanβ ≈ β = .
f e Aim: Construct a simple
Thus, astronomical telescope
using materials that
h f f are available in your
m   o  o .
f e h f e surroundings

Materials: two convex lenses of
Therefore, the magnification produced different focal lengths, hard
by an astronomical telescope is the ratio manila sheets, scissors,
of the focal length of the objective lens tape or glue
to that of the eyepiece lens. Note that the Procedure
distance between the two lenses,
1. Measure the focal length of each lens

L = f + e f o by focusing sunlight onto a paper
and measuring the distance to the
Example 6.3 sharpest point.
2. Use the black manila sheets to
Find the magnification of a telescope construct two tubes with slightly
with an objective lens whose focal different diameters such that the
length is 100 cm and an eyepiece lens smaller tube can slide into the larger
whose focal length is 2 cm. tube. Make sure each tube is longer

Solution than the focal length of your lens.
f 3. Paint the inside of both tubes black or
m = o line them with black paper to reduce
f e internal reflections of light.

100 cm 4. Fix the objective lens securely at one
m = = 50
2 cm end of the larger tube using tape or
glue.

222
Student’s Book Form Two

Physics Form 2 Final.indd 222 25/10/2025 10:28