CHAPTER
13
Self-Quiz
SUGGESTED ANSWERS
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(d); Choice (d) is correct. A ray passing through the optical centre of a converging lens does not undergo
refraction. It continues on a straight path through the lens. Choice (a) is incorrect. A ray passing through
the secondary principal focus is refracted by the lens so that it emerges parallel to the principal axis.
Choice (b) is also incorrect. A ray parallel to the principal axis is refracted by the lens and passes through
the principal focus. Choice (c) is incorrect. A light ray that strikes a retro-reflector is reflected back in the
same direction from which it came.
(b); Choice (b) is correct. A compound microscope produces two images. An inverted, real image that is
larger than the object is produced inside the body of the microscope. A larger, inverted, virtual image is the
image you actually see when you look at an object through a compound microscope. Choices (a), (c), and
(d) are all incorrect. They all describe images that are not produced by a compound microscope.
(b); Choice (b) is correct. An object placed at 2F´ produces an inverted, real image that is the same size as
the object. Choice (a) is incorrect. An object placed at F´ does not produce an image because the refracted
rays are parallel to one another. Choice (c) is incorrect. An object placed beyond 2F´ produces an inverted,
real image that is smaller than the object. Choice (d) is incorrect. An object placed between 2F´ and F´
produces an inverted, real image that is larger than the object.
(d); Choice (d) is correct. If you substitute 1 for M into the magnification equation, M 5 (hi / ho), you
will see that the values for hi and ho must be equal to each other. Choice (a) is incorrect. If a lens has a
magnification of 1.0, the lens will still produce an image, but the image will be the same size as the original
object. Choices (b) and (c) are also incorrect. According to the magnification equation, M 5 (hi / ho),
if M 5 1, the height of the object, ho, and the height of the image, hi, must be the same.
False. When light rays strike a converging lens, they are refracted by the lens. The lens then brings these
refracted rays together to form an image. A true version of the statement would be: A converging lens produces
an image by refracting light rays that strike the lens, then bringing the refracted rays together at a single point.
True. To use the thin lens equation correctly, you should always express the distance from the object to the
optical centre of the lens as a positive number.
diverging; When light rays from an object strike a diverging lens, they are refracted by the lens and spread
apart. The viewer’s brain extends the diverging rays backward to form a virtual image that is on the same
side of the lens as the object.
virtual; A magnifying glass is a converging lens that does not produce a real image. Light rays from an
object strike the lens and diverge, but the human brain extends the rays backward to produce an enlarged,
virtual image on the same side of the lens as the object.
accommodation; The muscles in a healthy eye can change the shape of the eye lens, allowing the eye to
focus on distant and nearby objects.
(a)(iv) Light rays parallel to the principal axis are refracted by the lens and converge at a point on the principal
axis on the opposite side of the lens; this point of convergence is the principal focus; (b)(i) The optical centre
is the centre of the lens; (c)(iii) The principal axis is a horizontal line that passes through the optical centre and
is perpendicular to the vertical line that passes through the centre of the lens; (d)(ii) The secondary principal
focus is a point on the principal axis located on the same side of the lens relative to the incident ray.
It is possible to position an object in front of a converging lens so that no image is produced. An object
placed at the secondary principal focus will not produce a real image because the emergent rays are parallel.
However, placing an object between the lens and the secondary principal focus will produce a virtual
image—this is another example where no real image is produced.
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Sample answer: The image of the ring will be about 28 cm from the lens. Using the thin lens equation,
1/do 1 1/di 5 1/f ; 1/di 5 1/f 2 1/do; 1/di 5 (1/15 2 1/32) cm21; 1/di 5 0.0354 cm21; di 5 28 cm
(a) The image is a virtual image, therefore it is located on the same side of the lens as the figurine.
(b) Using the magnification equation, M 5 2(di / do); 2Mdo 5 di ; di 5 2(2.9)(6.5 cm); di 5 218.9 cm;
The image is located 18.9 cm from the lens.
Sample answer: Using the magnification equation, M 5 (hi / ho); M 5 (4.1 cm/10.4 cm); M 5 0.39;
The lens has a magnification of 0.39 at the location of the flower.
Sample answer: No, the distance between the object’s image and the lens will not double. Using the thin
lens equation, 1/do 1 1/di 5 1/f ; 1/di 5 1/f 2 1/do; 1/di 5 (1/22 2 1/126) cm21; 1/di 5 0.0375 cm21;
di 5 27 cm; The new image distance of 27 cm is not double the initial image distance of 34 cm.
(a) The film is loaded upside down.
(b) A movie projector produces an inverted image. Therefore, when the film in the projector is loaded
upside down, the audience will see the movie right side up on the screen.
(a) People with presbyopia have trouble seeing nearby objects because light from nearby objects focuses
behind the retina. By holding a book farther away from their eyes, light from the book focuses on the
retina, allowing them to read the print.
(b) A person with presbyopia would need glasses with converging lenses.
Sample answer: I would tell my friend to think of the principal axis of a lens diagram as the x-axis of a line
graph and to think of the dashed vertical line through the centre of the lens as the y-axis. When an object
is placed to the left of the lens, any real image will appear on the opposite side of the lens, in other words,
to the right of the y-axis. Only positive x-values are located to the right of the y-axis. Any virtual image will
appear on the same side of the lens as the object, in other words, to the left of the y-axis. Any x-values to
the left of the y-axis are negative values.
When you move the camera away from the group, you are increasing the distance between the camera lens
and the object. The farther an object is located from the lens, the smaller its image appears. In this case,
the people appear smaller, but the whole group of people fits into the photo.
Sample answer: Today Antonie van Leeuwenhoek demonstrated his new invention, called the microscope.
He uses this incredible device to view things we cannot see with an unaided eye. The microscope is made
of a small glass lens held by a metal frame, and the specimen to be viewed is mounted at the tip of a holder
in front of the lens. During his demonstration, Mr. van Leeuwenhoek placed a drop of blood on the tip of
the specimen holder and allowed us to look through the microscope lens. What a fantastic sight! It appears
that blood is not just a red liquid. Instead, the blood drop appears to contain many extremely small, round
objects that are far too small to see without the help of the microscope.
Sample answer: The magnification of a lens is the ratio of the height of the image to the height of the
object, or the ratio of the distance of the image from the optical centre to the distance of the object from
the optical centre.
(a) A terrestrial telescope would be best because it contains a third lens that corrects the inverted image
formed by the other two lenses and produces an upright image. An upright image of an object is easier
to recognize than an inverted image.
(b) A refracting telescope would be appropriate, because a star appears as a point of light whether it is
inverted or not.
The film in a camera has a function similar to that of the human retina when you see an object. In a camera,
the image is captured and focused on the film. In the human eye, the image is focused on the retina.
Light rays parallel to the principal axis that pass through a converging lens come together at a single
point, called the principal focus, on the opposite side of the lens. However, light rays can strike the lens
from either side, assuming that the converging lens is symmetrical with equal curvature on both sides.
Consequently, a converging lens has two foci, one on each side of the lens, at equal distances from the
optical centre. To distinguish between these two points, the focus on the opposite side of the lens from the
incident rays is called the principal focus, and the focus on the same side of the lens as the incident rays is
called the secondary principal focus.
Unit E: Light and Geometric Optics
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