Name: ____________________________________
Lenses
A convex lens is thicker in the middle of the lens than at the edge. Convex lenses converge light rays or
bend light inward.
The focal point of the lens depends on the shape of the lens and the index of refraction of the material
making up the lens. Twice the focal point (2F) is equivalent to the center of curvature in mirrors.
A concave lens is thinner in the middle than at the edge. Concave lenses diverge light rays or bend light
outward.
Procedure
1.
Find the focal length of your convex lens. Using your lens focus a distance object onto an index
card. Measure the distance between your lens and the index card. This is your focal length.
2. Set the lens at the middle of the meter stick and mark the focal point and 2F on both sides of
the lens.
3. Move the candle to the positions indicated in table 1 and find the image of the candle on the
index card.
Table 1
Position of Candle
Large
Distance
Beyond 2F
2F
Between 2F
and F
Between F
and the Lens
Position of Image
Type of Image (Real or
virtual)
Image Size
(Larger/Same/Smaller
than object)
Direction of Image
(Upright or Inverted)
4. Concave Lens. Repeat procedure 1-3 for convex lens using a concave lens. Record your
observations in Table 2.
Table 2
Position of Candle
Position of Image
Type of Image (Real or
virtual)
Image Size
(Larger/Same/Smaller
than object)
Direction of Image
(Upright or Inverted)
Summary
Large
Distance
Beyond 2F
2F
Between 2F
and F
Between F
and the Lens
Summarize the characteristics of images formed by convex lenses in each of the following
situations. In each case, using the previous diagram as a guide, draw a ray diagram to locate the
image.
1.
The object is located beyond 2F.
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Ray Diagram
2. The object is located at 2F
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Ray Diagram
3. The object is located between 2F and F
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Ray Diagram
4.
The object is located between the focal point and the lens
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Ray Diagram
5. Why were you never asked to put the candle at the focal point? Support your answer with a
ray diagram.
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Ray Diagram
6. Place and measure the candle at three separate locations from the lens. Locate and
measure the corresponding image for each of the three locations. Using these
measurements and the mirrors/lens equation calculate the focal length. Compare the
calculated focal lengths to the measured focal length of your lens.