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Reflected image
• Draw one ray from the object
that enters the eye after
reflecting from the mirror. Is
this one ray sufficient to tell
you eye/brain where the
image is located?
• Draw another ray to locate
and label the image point.
• Do any of the rays that enter
the eye actually pass through
the image point?
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Constructing the image from a plane mirror II
• Images from a plane mirror show left/right reversal.
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Reflection and refraction
• Whenever a wave changes medium it will experience both reflection and
refraction.
• The path a light ray takes when refracting is the path that takes the
shortest amount of time.
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Q33.2
Light passes from vacuum (index of refraction n = 1) into
water (n = 1.333).
If the incident angle qa is in the range 0° < qa < 90°,
A. the refracted angle is greater than the incident angle.
B. the refracted angle is equal to the incident angle.
C. the refracted angle is less than the incident angle.
D. the answer depends on the specific value of qa .
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
A33.2
Light passes from vacuum (index of refraction n = 1) into
water (n = 1.333).
If the incident angle qa is in the range 0° < qa < 90°,
A. the refracted angle is greater than the incident angle.
B. the refracted angle is equal to the incident angle.
C. the refracted angle is less than the incident angle.
D. the answer depends on the specific value of qa .
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Q33.3
Light passes from a medium of index of refraction na into a
second medium of index of refraction nb. The angles of
incidence and refraction are qa and qb respectively.
If na < nb,
Aqa > qb and the light speeds up as it enters the second medium.
B. qa > qb and the light slows down as it enters the second medium.
C. qa < qb and the light speeds up as it enters the second medium.
D. qa < qb and the light slows down as it enters the second medium.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
A33.3
Light passes from a medium of index of refraction na into a
second medium of index of refraction nb. The angles of
incidence and refraction are qa and qb respectively.
If na < nb,
Aqa > qb and the light speeds up as it enters the second medium.
B. qa > qb and the light slows down as it enters the second medium.
C. qa < qb and the light speeds up as it enters the second medium.
D. qa < qb and the light slows down as it enters the second medium.
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Prism
Complete rays through the two prisms shown.
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Why should the ruler appear to be bent?
•
The difference in index of refraction for air and water causes your
eye to be deceived. Your brain follows rays back to the origin they
would have had if not bent.
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Goldfish
You are looking at a goldfish in a fish tank from
the top. The fish appears to be at an angle of 30
degrees below the horizontal from your
perspective and the fish appears to be 10 cm
below the water surface. Where is the actual
fish?
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Q33.4
Light passes from a medium of index of refraction na into a
second medium of index of refraction nb. The critical angle for
total internal reflection is qcrit.
In order for total internal reflection to occur, what must be true
about na, nb, and the incident angle qa?
A. na > nb and qa > qcrit
B. na > nb and qa < qcrit
C. na < nb and qa > qcrit
D. na < nb and qa < qcrit
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
A33.4
Light passes from a medium of index of refraction na into a
second medium of index of refraction nb. The critical angle for
total internal reflection is qcrit.
In order for total internal reflection to occur, what must be true
about na, nb, and the incident angle qa?
A. na > nb and qa > qcrit
B. na > nb and qa < qcrit
C. na < nb and qa > qcrit
D. na < nb and qa < qcrit
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Goldfish 2
•Describe what a goldfish sees if another fish
swims directly overhead and moves past the
angle of total internal reflection. Where is this
point of total internal reflection?
•Is there a spot where you cannot see the goldfish
looking down from the top of the tank?
•Is there a situation where you can see the
goldfish, but the fish can’t see you?
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Sunsets
•
The light rays from the sun are refracted the most at sunset and sunrise.
•
The light path is longest through atmosphere at sunrise and set, hence
more scattering.
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Dispersion
• Index of refraction is higher
(waves travel slower) for
light waves with short
wavelengths.
• At shorter wavelengths than
visible light, there is
ultraviolet or UV light and
the index of refraction
approaches infinity! What
does this mean?!
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A DOUBLE rainbow!!!
•
Rainbows are from
dispersion of light
refracting within
water droplets
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The focal point and focal length of a spherical mirror
• The focal point is at half of the mirror’s radius of curvature.
• All incoming rays will converge at the focal point.
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Q34.2
A concave mirror with a radius of curvature of 20 cm
has a focal length of
A. 40 cm.
B. 20 cm.
C. 10 cm.
D. 5 cm.
E. answer depends on the index of refraction
of the air around the mirror
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
A34.2
A concave mirror with a radius of curvature of 20 cm
has a focal length of
A. 40 cm.
B. 20 cm.
C. 10 cm.
D. 5 cm.
E. answer depends on the index of refraction
of the air around the mirror
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Thin lenses and focal point
•Continue the rays through lens and out other side.
•What is the point where the rays converge?
•Place a point source at the place where the rays converged. Draw
several rays heading left through the lens. Do these rays converge?
Would an image form?
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Q34.1
Which of the following changes its focal length
when it is immersed in water?
A. a concave mirror
B. a convex mirror
C. A converging lens
D. all of the above
E. none of the above
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
A34.1
Which of the following changes its focal length
when it is immersed in water?
A. a concave mirror
B. a convex mirror
C. A converging lens
D. all of the above
E. none of the above
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Thin lenses I
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Q34.8
An object PQ is placed
in front of a converging
lens, forming a real
image P´Q´. If you use
black paint to cover the
lower half of the lens,
A. only the object’s upper half will be visible in the image.
B. only the object’s lower half will be visible in the image.
C. only the object’s left-hand half will be visible in the image.
D. only the object’s right-hand half will be visible in the image.
E. the entire object will be visible in the image.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Thin lenses and images
f1
f2
•An object is placed to the left of the focal point, f1. Where is its
image and is the image inverted or upright? Label s, s’. Are they
positive or negative?
•An object is placed between the lens and the focal point, at a distance
s = f1/2. Where is its image and is the image inverted or upright?
Label s, s’. Are they positive or negative?
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Magnifying lens
A magnifying lens with focal length 15 cm is used
to magnify an ant which is 10 cm away.
• Draw a ray diagram of this situation
• Calculate the distance the image is away from
the lens
• How big does the 2 mm long ant appear?
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Magnifying glass
You found two lenses in your physics lab with
focal lengths fA = 10 cm and fB = 20 cm. You
want to use each lens as a magnifying glass so
you can magnify a tiny insect you found.
• Where should you place the insect relative to
lens A and lens B to get the biggest image?
• Which lens will make the insect look the largest
when you look through the lens? Explain why.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
A camera
• If you want to use a
lens to make a large
real image in your
camera of an object,
what focal length
should you use?
• A) Large f
• B) Small f
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Other lenses
•
Diverging lenses have f < 0
•
You can design your own lens using the lensmaker’s equation.
Which of the
following
describes the
quantity?
A) > 0
B) < 0
C) = 0
D) = ∞
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Graphical methods for lenses
•
Ray-tracing for converging and diverging lenses.
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Two lenses
•
We will use two lenses, both with f = 15 cm to make an image. Place
your object, a 2 mm long ant, 20 cm away from first lens. Then place
the second lens 70 cm away from the first lens. Where is the image, is
it real or virtual, and what is the total magnification?
•
Use a diagram to locate the object, images and lenses, but don’t draw
the rays.
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Selecting one orientation of the EM wave—the Polaroid
•
A Polaroid filter is a polymer array that can be thought of like teeth in
a comb. Hold the comb at arm’s length with the teeth pointing down.
Continue the mental cartoon and imagine waves oscillating straight up
and down passing without resistance. Any “side-to-side” component
and they would be blocked.
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Polarization I
• Light is polarized using reflection off a surface
• Polarized sunglasses use polarization to eliminate glare from the
sun off of water or other surfaces.
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Polarization II
• Consider Figure 33.28.
• Follow Example 33.6, illustrated by Figure 33.29.
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The astronomical telescope
•
Optical elements are arranged to magnify distant objects for visual
inspection.
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The reflecting telescope
•
Optical elements are arranged to reflect collected light back to an
eyepiece or detector. This design eliminates aberrations more likely
when using lenses. It also allows for greater magnification.
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The eye—vision problems
•
When the lens of the eye
allows incoming light to
focus in front of or behind
the plane of the retina, a
person’s vision will not be
sharp.
•
Figure 34.45 (at right) shows
normal, myopic, and
hyperopic eyesight.
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Vision correction—examples
•
Follow Example 34.13, illustrated in Figure 34.49 in the middle of the
page.
•
Follow Example 34.14, illustrated in Figure 34.50 at the bottom of the
page.
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