Phys 212: LENSES I
Part 1 Activities with a single converging lens.
1. Obtain a converging lens (converging lenses are fatter in the center than near the edges). If it
is a sunny day take the lens (plus a sheet of paper and a meter stick) into the sunshine. You
should be able to focus the sunlight down onto a small spot on the paper. Measure the
distance from the center of the lens to the paper, and record. This distance is your focal
length.
__________ cm
(Does it matter if you flip your lens over?)
Next, set up your lens and a screen on an optics bench as shown below
lens
screen
o p tics bench
Orient the bench so the lens is between the screen and the doorway. Request that the
classroom be darkened, and try to make an image of the most distant object you can see
through the doorway. (You may need to move to a new location so you can get a good view
down the hallway or outdoors.)
Measure the distance between the lens and the screen. Is the distance nearly the same as the
focal length? (If it isn’t, you’d better check with your instructor!)
Is the image on the screen inverted (i.e., upside down and flipped left to right)? (Again, if it
isn’t, double check with your instructor!)
The image that the converging lens produces on the screen is called a real image. (It’s real
because it’s really there--you can see this image projected on a screen). Now put your eyes down
at the level of the bench on the opposite side of the screen from the lens (but not too near the
screen!), and lift the screen. Can you still see the image?
screen
lens
o p tics bench
The key concepts from #1 are:
A converging lens produces a real, inverted image of a distant object. This image is
produced very close to the focal point of the lens.
Point your bench at various objects that are several meters away, and for each notice the
creation of the real, inverted image near the focal point.
2. Now we are going to start looking at objects that aren’t as far away. To do this
systematically, we have some handy-dandy light sources that we can image. Set the apparatus
up as shown, making sure that the light source is further from the lens than its focal length.
(The further away the better at this time.)
light
source
screen
lens
optics table
Move the screen around until you obtain a focused image. Measure:
a. The distance from the source to the center of the lens (we’ll call this distance s)
b. The distance from the lens to the screen (we’ll call this distance s’)
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c. The size h of your object (either the arrow itself or the diameter of the white
circle).
d. The size h’ of your image.
How do the ratios h’/h and s’/s compare?
A formula that is supposed to relate the object distance s and the image distance s’ to each
other and to the focal length f of the lens is
1/s +1/s’ = 1/f
How well does this formula work for your measured values of s, s’, and f?
3. Now repeat. Place the object and lens at various positions along the optics table, and for each
of these positions measure the object distance (the distance between the object and the lens),
and the image distance (the distance between the lens and the image formed on the screen).
For all of these measurements, make sure that the object distance is always greater than the
focal length value you measured in question #1. (Also note whether the image is larger or
smaller than the object.)
Object distance s
Image distance s’
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1/s + 1/s’
Any conclusions?
4. For one of the arrangements that gave a focused image on the screen, if you take the screen
away and look through the lens, do you still see the real image?
Replace the screen with a vertical rod (but be sure the rod is in the location where the screen showed
a focused image. If you look from the end of the optics bench, the rod should cut right through the
image. Move your head from side to side, looking at how the rod and the image shift relative
position as you move. Do they move separately or together? (You might want to try this with one
eye closed.) Discuss your results with your instructor.
Now change the position of the rod slightly, so it no longer at the position of the image. Now do
the image and the rod shift relative position differently as you move your head?
Challenge: For a new position of the source, can you find the location of the image without using
the screen?
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5. Now move the object to a location that is half the focal length away from the lens, and look
through the lens at the object. Is the image upright or inverted? On which side of the lens is the
image? Use the rod to find its location as best you can.
If you place a screen where you think the image is, does the image show on the screen? If not, then
your image is said to be virtual, rather than real.
Some physicists claim that the relationship 1/s + 1/s’ = 1/f can still be used if one adopts a
convention of having s’ be a negative number whenever the image is virtual. Does this
relationship still apply?
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6. Repeat for other distances from the lens.
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