Lenses and Telescopes
Objectives
Ÿ Students will be able to construct a telescope using two or more magnifying
lenses.
Ÿ Students will be able to visually recognize the difference between concave and
convex lenses.
Suggested Grade level
7th grade
Subject Area(s)
Physics
Industrial Technology
Timeline
Inquiry and discussion, one 45-minute class period
Hands-on activities, two to five 45 minute class periods
Writing assessment, to be completed as homework
Background Knowledge
Teacher
Refraction is the bending of light as it moves from one substance to another. An
optical lens is made from a material which light can pass through that can be
machined to refract the light in a specific manner. There are two different types of
lenses, convex and concave. A convex lens is a lens which is thicker in the middle
than on the ends. A concave lens is a lens which is thinner in the middle than on the
ends. Convex lenses refract parallel light rays so they come together at a single
point. This is known as convergence. The point where the rays meet is known as
the focal point. The distance from center of lens to the focal point is the focal length.
The thicker the middle of the lens the shorter the focal length. The Image that can be
seen without actually looking through the lens is the real image. When the real
image is held close to objects, inside of its focal length, it produces an image known
as a virtual image. To see a virtual image you must look through the lens. Concave
lenses refracts light rays so they come apart. This is known as divergence. It
always forms a virtual image.
Student
Students need to have a basic understanding of light and have basic investigative
science skills. Students also need to have basic measurement skills.
Materials
10 – 20 different concaved lens, magnifying glasses.
10 – 20 different convex lens magnifying glasses.
Tape measures
Incandescent light sources
Sectional drawing of a concave lens (transparency)
Sectional drawing of a convex lens (transparency)
Overhead projector
Manila folders
Tape
Overhead projector
Lesson
Day 1
Discussion and notes
1. When you see something, what is it that you are actually seeing? What you see is
your visual impression of a reflection of light. (Note use addendums when necessary
to illustrate content). Then ask students what they think “Refraction” means?
Refraction is the bending of light as it moves from one substance to another.
2. Ask students for any more examples they can think of where refraction takes
place? One very usable place that refraction takes place is in an optical lens. An
optical lens is made from a material which light can pass through that can be
machined to refract the light in a specific manner.
3. There are two different types of lenses, convex and concave.
Ask the students if they know the difference between a convex and a concave lens?
A convex lens is a lens which is thicker in the middle than on the ends.
A concave lens is a lens which is thinner in the middle than on the ends.
Way to remember the difference: A concave lens looks like the opening to a cave;
therefore you can remember that it curves inward.
4. Convex lenses refract parallel light rays so they come together at a single point.
(see addendum 4) Does anyone know what it is called when the light rays are
made to come together? This is known as convergence.
The point where the rays meet is known as the focal point. (see addendum 5) The
distance from center of lens to the focal point is the focal length.
The thicker the middle of the lens the shorter the focal length.
An example of a convex lens is a magnifying lens.
5. Show overhead showing parts of convex lens explained above.
The Image that can be seen without actually looking through the lens is the real
image. When the real image is held close to objects, inside of its focal length, it
produces an image known as a virtual image. To see a virtual image you must look
through the lens.
6. Concave lenses refract the rays so they come apart. . Does anyone know what it
is called when the light rays are made to come apart from each other?
This is known as divergence, it always forms a virtual image.
7. Show overhead of concave lens.
8. Practical Uses of Lenses
Who knows of some uses of lenses?
Cameras (normally convex), glasses (near= concave; far= convex); telescopes (at
least 2 convex), microscopes (at least 2 convex).
Ask for questions on material covered and check for understanding.
Day two
Refer to addendum 1. Students will attempt to measure the focal length of various
lenses. Have students form small groups (2-4). Then pass out magnifying glasses
tape measures, incandescent light sources and paper. Student should set up the
light so that they can manipulate the location of the magnifying glasses to focus the
light onto the paper. Once the light has been focused hold the magnifying glass still
and measure the distance. Then record this distance as the focal length of that
individual magnifying glass. Repeat this procedure with all lenses, and record the
information. Once all groups have finished their testing discuss the findings as a
whole class. (note sometimes the light source may have to be natural sunlight,
however when using sunlight exposure time must be limited to prevent paper from
igniting).
Day three and four
Refer to addendum 2 and 6. Students will attempt to construct a telescope using 2
or more magnifying glass lenses. Have students reform into their small groups.
Using manila folders and tape, have students roll tubes and combine them to form
their own functional telescopes. Students should experiment with different lenses
and distances apart to optimize their telescope for the desired function. After the
groups have finished their telescopes they will present them as will as their specific
function to the rest of the class.
Extensions
Students will do research on the making of the first telescope and create an
informative poster about their research.
Evaluations/Assessment
This is to be completed at home. Students will write 3 to 5 paragraphs, describing
in detail how a telescope uses lenses to magnify far away objects.
Resources
http://science.howstuffworks.com/telescope1.htm
Addendum 1
Addendum 2
Slide of simple refractor telescope
This is the simplest telescope design you could have. A big lens gathers the light and directs it to a
focal point and a small lens brings the image to your eye.
Addendum 3
The surface of a lens is curved, but the law of refraction stated above still holds at
every point on the curved surface.
Addendum 4
The figure below shows the path of some light rays from a point object through the a
positive lens. You see that the lens refracts the rays passing through the edge of the
lens while not refracting the ray that passes through the center of the lens. All of the
rays from a point object converge at the same spot, producing a point image.
Addendum 5
When the rays coming from the object are parallel, as occurs for very distant objects
(farther away than about 50 focal lengths), an image forms at a distance from the
lens called the focal length.
Addendum 6
In the figure of a positive eyepiece telescope below, F is the focal length of the
objective lens and f is the focal length of the eye lens. The telescope has a
theoretical magnifying power given by
M = -F/f
There are two kinds of images formed by lenses, real and virtual. A real image can
be projected onto a screen. A virtual image appears between the lens and the
object and cannot be projected on a screen.
Virtual images can be formed by both converging and diverging lenses. Real
images, however, can only be formed by converging lenses.