#$&*
course Phy 232
Experiment 28: Image FormationNote: Due to the breakability of the original lenses this experiment has been modified to use a pair of thin plastic lenses. Click here for the instructions for the modified experiment.
Using the lenses from previous expreiments we investigate image formation, image size and object size. Results are analyzed using the lens equation.
For this experiment you will need two small but fairly intense sources of light. The candles included in your kit would work very well. As an alternative you can use two flashlights if you mask the lens of each so that only a circle in the center of about the radius of the bulb remains. The candles are probably much more convenient.
You will also need a dark room and a flat surface such as a tabletop.
In this experiment you will use your convex lenses to
• create images of your light source, with the light source at various distances from your lens
• create images of other objects
• explore the relationship among image distance, object distance, focal length and magnification
• make a spotlight.
Create images of a single light source
You will use both convex lenses. If you have not already done so, determine the focal length of both lenses by the most expedient method possible (recommendation: form a sharp image of a distant candle and measure the focal distance directly).
Focal length 15cm and 30cm respectively
Begin by lighting one candle and placing it on a tabletop, or by turning on a flashlight and placing it on a tabletop aimed at the 4-inch lens.
Position the light source at the far end of the tabletop, at least 1 meter away from the other end (if your tabletop is too small, you might need to support the source on something at the same height as the tabletop).
About 20 cm from the other end of the tabletop, place the lens so it is facing the light source.
Place the screen behind the lens at the edge of the tabletop (about 20 cm from the lens), so that light shines from the source through the lens and onto the screen.
If the lights in the room are on, turn them off.
• Move the screen toward the lens until the image of your light source becomes as sharp as possible.
• Determine the distance from the center of the lens to the screen, and the distance of your light source from the center of the lens.
• Now move the light source to a distance of about 50 cm from the lens, locate the screen to form the sharpest possible image, and repeat your measurements.
Repeat for distances of 40, 30 and 20 cm from the lens. For some of these measurements it might be necessary to change the position of your lens (it might be too close to the tabletop).
Distance of Screen from Lens Distance of Light Source from Screen
19.5cm 69cm
22.5cm 50cm
25cm 40cm
69cm 20cm
Create images of a double light source
Light both candles and place them side by side.
Place the lens about 50 cm away from the two candles. The line from the candles to the lens should be perpendicular to the line connecting the two flames, so that has seen from the lens one candle lies a few centimeters to the right and the other a few centimeters to the left.
• Position the screen to form a sharp image of the two flames.
• Measure the distance from the candles to the center of the lens and from the center of the lens to the image.
• Measure the separation of the two flames, and the separation of their images.
• If you place something in front of the candle on the left, which image should disappear, the one on the left or the one on the right? Why?
At 50cm
Distance from candles to lens is 50cm. Distance from lens to image is 22cm
Separation of flames is 6cm. Separation of image is 2cm
The image on the right would disappear because the images are flipped when they hit the screen.
At 30cm
Distance form candles to lens is 30cm. Distance from lens to image is 30cm
Separation of flames is 6cm. Separation of images is 4.5cm
At 20cm
Distance form candles to lens is 20cm. Distance from lens to image is 46.5cm
Separation of flames is 6cm. Separation of images is 10.5cm
Repeat for distances of 30 and 20 cm. If the images get too far apart, you might have to form one image at a time on the screen.
Now move the candles to within 3 cm of the focal distance and repeat.
At 18cm
Distance form candles to lens is 18cm. Distance from lens to image is 95cm
Separation of flames is 6cm. Separation of images is 18cm
Make a spotlight
Place your lens on the tabletop at least 2 meters from a wall, with the wall behind the lens.
Place your source at a distance in front of the lens equal to the focal distance of the lens.
Look at the image formed on the wall. It should have the same shape and size as the lens, and should be sharply defined.
If this is not so, adjust the position of the lens so that it becomes so.
• Accurately measure the distance from the source to the center of the lens.
16cm
If you have a larger darkened area available, see how sharply you can make the image of the lens at a distance of several meters from a wall.
• Do the sharpness and the size of the sharpest image change with distance?
The sharpness stays about the same, but the object gets much greater with a larger distance.
Create an image
Now you will create an image of another object.
Select an object not more than a few inches high and not less than an inch high, and with a significant amount of white in its background. As an alternative, you could make a cone about 3 inches high out of aluminum foil use it as your object.
Place this object on the tabletop about 40 cm in front of your 4-inch lens.
Use your light source and the 3-inch lens as a spotlight to illuminate the object as brightly as possible.
• Place the screen behind the first lens and determine the distance of the object from the center of the lens, and the distance from the center of the lens at which the sharpest image of your object appears.
• Measure the height of the object and of its image.
Repeat for the same object a distance of 30 cm in front of this lens.
Sharpest image 23.4 cm away
Sharpest image 30 cm away
Height of object 2cm. Height of image 9cm
Height of object 2cm. Height of image 7cm.
Analyze your results
If image distance is i, object distance is o and focal distance is f, then 1/f = 1/i + 1/o.
• Verify this formula for the image and object distances observed in each part of this experiment.
If image distance is i and object distances o, then the magnitude of the magnification is the ratio i / o; this ratio is equal to the magnitude of the ratio of image size to object size.
Sample calculations
Part 1
1/15cm=1/22.5cm+1/50cm
0.06=0.067
Part 2
1/15cm=1/30cm+1/30cm
0.06=0.06
Part 3
n/a because very hard to measure distance to image
Part 4
1/15cm=1/40cm+1/23.4cm
0.06=0.068
For the two-candle images, verify that the distances between the images of the flames are in the same proportion to the actual distance as the image distance i to the object distance o.
2cm/6cm=22cm/50cm
0.33=0.44
This value was a little off most likely due to the limitation of measuring device used.
What is the evidence that the images of the two candles are inverted?
I observed that the flames were upside down.
"
&#Very good data and responses. Let me know if you have questions. &#