2. PURPOSE LEARNING
After learnt optical instruments,
student able to identify the
principle application of the
magnifying glass,
The students can use the
magnifying glass well
3. MAGNIFYING GLASS
Magnifying glasses normally are used to
produce images that are larger than the
related objects, but they also can produce
images that are smaller than the related
objects
4. A converging lens can be used to ignite paper (a).
Light entering parallel to the
principal axis converges at the focal point of the
lens, concentrating solar energy
5. The two principal rays show that a convex lens
forms a virtual image that is upright and larger
compared to the object when the object is
located between the lens and the focal point.
Because the principal rays are simply part of a
model to help locate an image, they do not have to
pass through the picture of the lens in a diagram.
In reality, the image is formed only by the light
that passes through the actual lens.
7. Magnification of the magnifying
glasses
Observe with the accommodation eyes
𝑆𝑛
M=
+1
𝑓
Observe with non accommodation eyes
𝑆𝑛
M=
𝑓
8. Example problem
A magnifier with a focal length
of 30 cm is used to view a 1-cm-
tall object. Use ray tracing to
determine the location and size
of the image when the
magnifier is positioned 10 cm
from the object.
9. THE ANSWER
Analyze and Sketch the
Problem
• Sketch the situation,
locating the object and the
lens.
• Draw the two principal rays.
Known: Unknown:
do 10.0 cm di ?
ho 1.0 cm hi ?
f 30.0 cm
10. a. Use the thin lens to determine d1
1
𝑓
=
1
𝑑1
do =
=
+
1
𝑑0
𝑓𝑑 𝑜
𝑑 𝑜−𝑓
(30 𝑐𝑚)(10 𝑐𝑚)
(10 𝑐𝑚) −(30 𝑐𝑚)
= - 15 cm( 15 cm away from the lens on
the side opposite the object)
11. Use the magnification equation and solve for
image height.
M=
h1 =
ℎ1 −𝑑1
=
ℎ0
𝑑0
−𝑑1.ℎ0
𝑑𝑜
=
−(−15 𝑐𝑚)(2 𝑐𝑚)
10 𝑐𝑚
= 3 cm (3 cm tall)
The positive means that the image is erect.
