UniversityPhysicsVolume3-Ch02.  Collin County Community College District GOVT 2306

University Physics Volume III
Unit 1: Optics
Chapter 2: Geometric Optics and Image Formation
Conceptual Questions
1. What are the differences between real and virtual images? How can you tell (by looking)
whether an image formed by a single lens or mirror is real or virtual?
Solution
Virtual image cannot be projected on a screen. You cannot distinguish a real image from a virtual
image simply by judging from the image perceived with your eye.
3. Can you photograph a virtual image?
Solution
Yes, you can photograph a virtual image. For example, if you photograph your reflection from a
plane mirror, you get a photograph of a virtual image. The camera focuses the light that enters its
lens to form an image; whether the source of the light is a real object or a reflection from mirror
(i.e., a virtual image) does not matter.
5. Is it necessary to project a real image onto a screen to see it?
Solution
No, you can see the real image the same way you can see the virtual image. The retina of your
eye effectively serves as a screen.
7. If you wish to see your entire body in a flat mirror (from head to toe), how tall should the
mirror be? Does its size depend upon your distance away from the mirror? Provide a sketch.
Solution
The mirror should be half your size and its top edge should be at the level of your eyes. The size
does not depend on your distance from the mirror.
9. Under what circumstances will an image be located at the focal point of a spherical lens or
mirror?
Solution
when the object is at infinity; see the mirror equation
11. Can an image be larger than the object even though its magnification is negative? Explain.
Solution
Yes, negative magnification simply means that the image is upside down; this does not prevent
the image from being larger than the object. For instance, for a concave mirror, if distance to the
object is larger than one focal distance but smaller than two focal distances the image will be
inverted and magnified.
13. Use a ruler and a protractor to find the image by refraction in the following cases. Assume an
air-glass interface. Use a refractive index of 1 for air and of 1.5 for glass. (Hint: Use Snell’s law
at the interface.)
(a) A point object located on the axis of a concave interface located at a point within the focal
length from the vertex.
(b) A point object located on the axis of a concave interface located at a point farther than the
focal length from the vertex.
(c) A point object located on the axis of a convex interface located at a point within the focal
length from the vertex.
(d) A point object located on the axis of a convex interface located at a point farther than the
focal length from the vertex.
(e) Repeat (a)–(d) for a point object off the axis.
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