2. A makeup mirror is a concave mirror. If
you sit near one and look into it, you'll see an enlarged,
upright image of your face behind the mirror. Is that image real
or virtual? How can you tell?
The image is virtual. Virtual images appear to be behind the
mirror, where the light does not truly go.
4. A makeup mirror is a concave mirror. When
you are quite far from it, you see an image of your face that
appears upside down and located between you and the mirror. Is
that image real or virtual? How can you tell?
The image is real. It appears in front of the mirror where the
light rays really go.
10. You can use a concave makeup mirror as a
primitive reflecting telescope. If you hold a small sheet of
paper in front of the mirror, you will be able to find real
images of objects located far behind you. Why won't you be able
to find real images of nearby objects?
The focal length of a makeup mirror is relatively long so that
you will see a virtual image while sitting a comfortable distance
away. For a real image to form, the object must be further away
than a focal length. When the object is nearby and then close to
the mirror focus, the real image is very far away from the
mirror. As the object becomes far away, the real image's
location approaches the mirror focus.
16. Two similar looking magnifying glasses
have different magnifications. One is labeled 2X (two times) and
the other 4X (four times). Which magnifying glass must you hold
closer to the object you're looking at in order to see a virtual
image of that object far in the distance?
You must hold the 4X lens closer to the object. The magnified
virtual image of the object must comfortably fill your field of
view independent of the magnifying glass you are holding. The
lower magnification lens can do so with less bending of the light
from a larger region of the object, so it can be farther from the
object. The higher magnification lens needs to bend the incident
light through larger angles so that a smaller region of the
object will fill your field of view. To do this, it must be
closer.
20. Images in cheap microscopes exhibit
bands of color. Why?
Cheap microscopes use simple lenses in their objective and
eyepiece that suffer from chromatic aberration. This is the
dispersion of the different colors of light passing through the
lenses due to the changes in refractive index of the material
with the wavelengths corresponding to the colors. More expensive
microscopes use compound, achromatic lenses to correct this.
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