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Thin Lenses & Their Applications Questions
A converging lens is sometimes called a "burning glass". Does the sunlight burn because the lens
magnifies? Where, relative to the lens, must the object to be burned be placed?
A lens can be formed by a bubble of air in water. Is such a lens a converging, or a diverging one?
Use a diagram in your answer.
Explain briefly how contact lenses correct eye defects, and how they remain in the correct
position for many hours.
Using one or two converging lenses, an optical bench, and a candle as an object, arrange the
lens(es) and the candle to simulate the following optical instruments: a camera, a microscope, a
photo copier, a refracting telescope.
Why do swimmers with normal eyesight see distant objects as blurry when swimming
underwater? How do swimming goggles or a face mask correct this problem?
A converging lens of diamond and a lens of crown glass have the same shape. Which lens will
have the larger focal length? Explain your answer.
A copy machine has a converging lens with a focal length of 40 cm. How far from the lens must
documents be placed if the copies are to be exactly the same size as the original?
In cameras the number affects the depth of field (resolution) of a particular photograph. What
is the relationship between the f number and the resolution of the lens in a camera? Some
research will be necessary to answer this question.
1
A properly exposed photograph was taken at 100
s with an aperature of f/8. What lens opening
would be required if the shutter speed is changed to (a)
1
50
s, (b)
1
200
s?
10. A person's vision in one eye is corrected using a lens with a power of 5.2 d. Is the eye
nearsighted or farsighted?
11. To direct parallel light rays through the slide, the light source of a projector is located at the
centre of curvature of the converging mirror and at the principal focus of the converging lens in
the condenser, as illustrated. Why?
12. An object 8.0 cm high is placed 80 cm in front of a converging lens of focal length 25 cm. By
means of a scale ray diagram locate the image and determine its height.
13. Use accurate scale drawings to locate the images in each of the following situations, and state
whether the image is real or virtual, erect or inverted, and smaller, larger, or the same size.
(a) A converging lens has a focal length of 20 cm. A 5.0 cm object is placed (i) 10 cm from the
lens, (ii) 30 cm from the lens, and (Hi) 40 cm from the lens.
(b) A diverging lens has a focal length of 20 cm. A 5.0 cm object is placed (i) 10 cm from the
lens, and (ii) 30 cm from the lens.
14. Using a scale ray diagram, locate the position of the image of a candle 10 cm high, placed 20 cm
in front of a converging lens of focal length 25 cm.
15. Using a scale ray diagram, locate the image of an object 5.0 cm high that is 15 cm in front of a
diverging lens of focal length 25 cm.
16. A lamp 10 cm high is placed 60 cm in front of a diverging lens of focal length 20 cm. By means of
a scale ray diagram, locate the image and determine its height.
17. For questions 14, 15, and 16, check your answer using the lens equation.
18. A candle is placed 36 cm from a screen. Where between the candle and the screen should a
converging lens with a focal length of 8.0 cm be placed to produce a sharp image on the screen?
19. An object 5.0 cm high is placed at the 20 cm mark on a metre stick optical bench. A converging
lens with a focal length of 20 cm is mounted at the 50 cm mark. What are the position and size
of the image?
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20. A camera lens has a focal length of 6.0 cm and is located 7.0 cm from the film. How far from
the lens is the object positioned if a clear image has been produced on the film?
21. A lens with a focal length of 20 cm is held 12 cm from a grasshopper 7.0 mm high. What is the
size of the image of the grasshopper? State its position and type.
22. A projector is required to make a real image, 0.5 m tall, of a 5.0 cm object placed on a slide.
Within the projector, the object is to be placed 10.0 cm from the lens. What must be the focal
length of the lens?
23. A 3.0 cm flower is placed 40 cm from a lens with a focal length of 10 cm. What are the position,
size, and type of image?
24. An amateur photographer takes a series of photographs of a lunar eclipse using a camera with a
focal length of 6.0 cm. What will be the size of the moon's image on the film? (diameter of
moon = 3.5 x 106 m, distance to the moon = 3.7 x 108 m)
25. A philatelist (stamp collector) uses a magnifying glass with a focal length of 10 cm. In order to
examine a rare postage stamp, he holds the lens 4.0 cm from his eyes. If the distance for the
most comfortable viewing of the image is 30 cm from his eyes, how far from the lens must the
postage stamp be placed? What will the magnification be?
26. A lens has a focal length of +20 cm and a magnification of 4. How far apart are the object and
the image?
27. (a) What is the focal length of a lens with a power of 7.5 d?
(b) What is the power of a lens with a focal length of -15 cm?
(c) Are the lenses converging or diverging? Justify your answers.
28. A book is placed 25 cm from an eyeglass lens with a power of 2.5 d. Where is the image formed?
29. A lens with a power of +5.0 d produces a clear image on a screen 2.0 m away. What is the
magnification of the object?
30. In a compound microscope, the image is formed by two converging lenses. The real image,
formed by the first lens (called the objective), is then used as the object by the second lens,
called the eyepiece, to create a larger, virtual image. Use the following information to
determine the position and size of the final image created by the two lenses: the object is 1.0
cm in height and is placed 2.5 cm from the objective lens (f = 2.0 cm); the eyepiece is 12 cm
from the objective lens and has a focal length of 2.3 cm.
Answers
Lenses and their Applications
17. -100 cm, -9.4 cm, -15 cm
18. 12 cm and 24 cm, or 24 cm and 12 cm
19. 60 cm, -10 cm
20. 42 cm
21. 17.5 mm, -30 cm, virtual
22. 9.1 cm
23. 13.3 cm, 1.0 cm, real
24. -0.57 mm
25. 3.6
26. 75 cm
27. (a) 0.13 m
(b) -6.7 d
28. -67 cm
29. -0.11
30. -15.3 cm from the eyepiece, -31 cm
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