1. Purpose of this Minilab
Use lens formula to determine focal length of a lens.
Learn about image magnification in magnifying glasses, microscopes, and telescopes.

2. Activity 1: Focal Length of a Lens
Method 1: f
Flashlight or table lamp at the
end of classroom (long distance
compared to focal length).
Light rays enter lens
approximately parallel.
Lens
Screen or sheet of
paper to see image.
Move sheet until image is in focus.
Then measure f.

3. Activity 1: Focal Length of a Lens
Method 2: so si
Object (illuminated cross
on the light source)
Screen or sheet of
paper to see image.
Move sheet until image is in focus.
Then measure so and si and calculate f with:

4. The Imaging Equation for Lenses
so: object distance
si: image distance
f : focal length

5. Sign Rules For Lenses Convex lenses: f is positive
Concave lenses: f is negative
Most objects are real.
Real objects: so is positive
Virtual objects: so is negative
Real images: si is positive
Virtual images: si is negative
Virtual images cannot be
picked up with a screen.

6. Virtual or Real Image?
In Activity 1.2 (using a converging lens) place the object at a
distance larger than f away from the lens to get a real image.
Hint: To answer Q1, do a similar analysis for the concave lens (f < 0).

7. Activity 2: Magnification
An inverted image
means that h’and h have
opposite sign.  M < 0 h’ so a si a h

8. 2.2 Virtual image magnification (magnifying glass)
Without the magnifying glass:
eye
25 cm (typical nearest distance a human can focus on)
With the magnifying glass:
virtual image
eye f

9. Verifying this magnification of a magnifying glass
viewing screen
your
eye (close
to lens)
linear graph
paper
Optical Bench
 25 cm
lens
f = +100mm
Tape linear graph paper on viewing screen.
Place lens about 25cm away from screen.
Hold a second piece of graph paper close to lens.
Move your eye close to the lens.
Move the second piece of graph paper so it is in focus.
Compare the size of graph paper seen through the lens
with the size of the graph paper on the screen (seen not
through the lens). See next page for illustration.
hand held linear graph
paper (close to lens)

10. …then check whether this agrees with
What you should see ….
viewing screen
lens
Compare:
3.5 divisions
on the graph paper
taped to the screen
= 1 division on the
hand held graph
paper seen
through the lens.
M=3.5
(in this example)
hand held graph paper
seen through lens
hand held
graph paper
graph paper on
viewing screen
…then check whether this agrees with

11. Remarks to formula for magnifying glass….
The actual magnification depends on exactly where the object is placed:
If the object you magnify is placed exactly at the focal point of the
magnifying glass, then
If you move the object even closer to the lens, the magnification can get
as high as
You could get a theoretical value anywhere between those two
magnifications, depending on where exactly you hold the paper.

12. Activity 3: Microscope si so eye objective fo=200mm eyepiece fe=100mm
virtual image
need so > fo
real image
between lenses

13. Microscope: Building Instructions
Step 1: Install light source and objective lens.
handheld piece of paper:
move so that the image
of the arrow is in focus.
illuminated arrow
on this side
200mm lens
(objective)
light source
optical bench
so  30cm
measure si
record so and si

14. Microscope: Building Instructions
Step 2: Install eyepiece lens.
200mm lens
(objective)
100mm lens
(eyepiece)
optical bench
so  30cm Si
(as previously determined)
some small
distance further

15. Microscope: Building Instructions
Step 3: Replace light source with white viewing screen
linear graph
paper
white
viewing
screen
200mm lens
(objective)
100mm lens
(eyepiece)
optical bench
so  30cm Si
Viewing screen must be placed where the arrow used to be.
Cover the viewing screen with linear graph paper.

16. Microscope: Building Instructions
Step 3: Look through eyepiece and adjust it’s position.
linear graph
paper
white
viewing
screen
200mm lens
(objective)
100mm lens
(eyepiece)
eye
optical bench
so  30cm Si
Adjust eyepiece position
so that the image of the
graph paper is in focus.

17. Microscope: Measuring the magnification
linear graph
paper
hand held
linear graph
paper
200mm lens
(objective)
100mm lens
(eyepiece)
eye
optical bench
25cm
Step 1: Hold a second piece of graph paper approximately 25cm from your eye.
That extra graph paper should be a bit to the side so you can still see the image
of the graph paper that is on the viewing screen.

18. Step 2: What you should see ….and measure
viewing screen
hand held
graph paper
(25cm from
eye)
image of graph paper
on viewing screen
graph paper on
viewing screen
Compare:
2.8 divisions
on the hand held graph paper
= 1 division on the image of
Graph paper taped to the screen.
M=2.8
(in this example)
…then check whether this agrees with

19. Activity 3: Telescope  fo + fe si  fo  fe
virtual image
eye
si  fo
 fe
so   (looking at far away objects)
eyepiece
fe=100mm
objective
fo=350mm

20. Telescope: Building Instructions
Install objective and eyepiece
100mm lens
(eyepiece)
350mm lens
(objective)
optical bench
Separate objective and eyepiece
by fo+fe (=450mm)

21. Telescope: Measuring the Magnification
350mm lens
(objective)
100mm lens
(eyepiece)
lamp
eye
optical bench
View white board through
telescope from the back of
the room.
White board in the front of the room.
Draw a thick scale on the white board.
Illuminate the scale with a lamp.

22. What you should see … white board telescope eyepiece
Compare scale seen through
telescope with scale seen
directly to determine M.
Here: Magnification looks like M  (negative because inverted)

23. Using the Desk Lamp On/Off switch of lamp
Lamp Plug (black) must be plugged
into dimmer plug.
Dimmer plug (white) must be plugged
into power outlet.
Dimmer
On/Off
switch
of lamp