Physics 102002 Announcements

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Physics 102-002 Announcements

• WebAssign
• No Chapter 26 Homework
• Chapter 28 due next Wednesday
• Exam 3 Corrections due This Wed, Apr 25
• Final Exam May 7, 530-730, in regular classroom

Picture Image of diffraction of the beam of a
Helium-Neon laser when passed through a single
slit.
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Class Schedule
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Chapter 28Reflection and Refraction

• Reflection and Principle of Least Time
• Law of Reflection
• Plane Mirrors
• Diffuse Reflection
• Refraction
• Cause of Refraction
• Dispersion
• Rainbows
• Total Internal Reflection
• Lenses (and Image Formation by Lenses)
• Lens Defects

Last time
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Cause of Refraction
In the picture, when the light wave crosses the
air-glass boundary there is a change in speed and
wavelength. When passing from air into glass,
both the speed and the wavelength decrease.
Also, the light changes directions as it
crosses the air/glass boundary. This bending of
the path of light is known as refraction. In
other words, it "bends." As seen in the diagram,
the wavefront is bent only along the boundary.
Once the wavefront passes across the boundary, it
travels in a straight line.
You can draw a ray perpendicular to the
wavefront this ray represents the direction
which the light wave is traveling. Notice that
the ray is a straight line inside of each of the
two media, but bends at the boundary.
Remember the analogy with the tractor and how it
starts to turn as a wheel moves from pavement
onto the grass? The light wave bends for the
same reason. In the diagram, as the right edge
of the wave front moves into the slower glass,
the wave slows down and bends.
This bending of light across an interface between
two media is what causes the fish to appear
nearer to the surface than it actually is.
Physics Place figure and video
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Dispersion
Different wavelengths of light travel through
dense media (anything other than a vacuum) at
different speeds. In general, the higher the
frequency, the slower the speed of the light
wave. This means that, for visible light, violet
light travels more slowly through a given medium
than red light. The colors in between (orange,
yellow, green, blue, and indigo) travel at
intermediate speeds. Because different colors of
light travel at different speeds in transparent
materials, they refract by different amounts and
become separated. This property is called
dispersion.
Rainbows
Dispersion is the property of light that makes a
prism separate white light into the colors of the
rainbow.
Descartes, in 1637, was the first to propose an
answer to why rainbows appeared in the sky.
The rainbow is produced because the refraction of
the light by each water droplet in the sky causes
red light to bounce back through 42 degrees and
blue light through 40 degrees from the direction
of the sun.
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Total Internal Reflection
Consider light traveling from a slow medium and
striking an interface with a fast medium (like
from water into air).
Normally there is some reflection and some
refraction observed.
However, if the angle of incidence is gradually
increased, eventually an angle, called the
critical angle is reached beyond which no light
escapes from the slow medium. Beyond this
angle, you get total internal reflection.
For comparison, a good mirror is normally only
about 90 reflective the other 10 being lost.
This picture shows the transition to total
internal reflection as the angle of incidence
approaches the critical angle.
Prisms are used in a variety of optical
instruments because of total (100) internal
reflection.
Physics Place figure
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Lenses
The refractive property of light enables us to
make lenses of different shapes to make light
rays converge (come together) or diverge (spread
apart) to achieve desired effects. A lens is
just a carefully ground or molded piece of
transparent material which refracts light rays in
such a way as to form an image. Lenses can be
thought of as a series of tiny refracting lenses,
each of which refracts light to produce their own
image. When these prisms act together, they
produce a bright enough image to be focused at a
point.
A converging lens is thicker in the middle, and
focuses light at a point. A diverging lens is
thinner in the middle and diverges the light.
Lens terminology . There are important terms
used to describe all lenses.
Center of Curvature
Center of Curvature
Physics Place figure
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Question 1
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Question 1 Answer
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Image Formation by a Lens
Lenses can be used to concentrate light so that
images can be formed or magnified. A single
converging lens can be used to project a REAL
image outside the focal point of the lens.
Upside down REAL image projected on screen
outside the focal point.
Object outside the lens focal point
A single diverging lens can be used to create an
upright virtual image (one that appears to be
located somewhere that it isnt)
For this reason, a single diverging lens is used
as a sighting lens, or a viewfinder.
eye