Transcript Physics 102 Introduction to Physics

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, 5:30-7:30, in regular
classroom
Picture: Image of diffraction of the beam of a
Helium-Neon laser when passed through a
single slit.
Class Schedule
4/9
Midterm Exam #3
4/11
Chapter 24
Magnetism, (Pg 458-470)
4/16
Chapter 26
Properties of Light
4/18
No class
4/23
Chapter 28
Reflection and Refraction, Part 1 (Pg 530-540)
4/25
Chapter 28
Reflection and Refraction, Part 2 (Pg 540-551)
4/30
Midterm Exam #4
5/2
Review and hand back Exam #4
5/7
Final Exam, 5:30 – 7:30 PM
Chapter 28
Reflection 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
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
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.
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
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
Physics Place figure
Center of
Curvature
Question 1
Question 1 Answer
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.
Object outside
the lens’ focal
point
Upside down
REAL image
projected on
screen outside
the focal point.
A single diverging lens can be used to create an upright virtual image (one that appears to be
located somewhere that it isn’t):
For this reason, a single diverging lens is
used as a “sighting” lens, or a
“viewfinder”.
eye