Transcript Introduction to Light PPT

BHS Physics - Chapter 15 – May 2016
NASA
EM wave
video
LONGER WAVES
SHORTER WAVES
A ray is a straight line
that represents the linear
path of a narrow beam of
light (a). A light ray can
change direction if it is
reflected (b) or refracted
(c).
REFLECTION
REFRACTION
The Sun acts as a luminous source to Earth and
the Moon. The Moon acts as an illuminated source
to Earth. (Illustration not to scale)
The transparent glass allows objects to be seen
through it (a). The translucent lamp shade allows light
to pass through, although the lightbulb source itself
is not visible (b). The opaque tarp covers the statue,
preventing the statue from being seen (c).
Luminous flux is the
rate at which light is
emitted from a
luminous source,
whereas illuminance
is the rate at which
light falls on a
surface at a
specified distance.
The illuminance,
E, produced by a
point source of
light varies
inversely as the
square of the
distance from
the light source.
1
E~ 2
r
Gravity force
Diffraction and the Wave Model of Light
In 1665, Italian scientist Francesco Maria Grimaldi
observed that the edges of shadows are not perfectly
sharp. He introduced a narrow beam of light into a dark
room and held a rod in front of the light such that it cast a
shadow on a white surface. The shadow cast by the rod on
the white surface was wider than the shadow should have
been if light traveled in a straight line past the edges of
the rod.
Grimaldi also noted that the shadow was bordered by
colored bands. Grimaldi recognized this phenomenon as
diffraction, which is the bending of light around a barrier.
Diffraction Applet
In 1678, Dutch scientist Christiaan Huygens argued
in favor of a wave model to explain diffraction.
According to Huygens’ principle, all the points of a
wave front of light can be thought of as new sources
of smaller waves. These wavelets expand in every
direction and are in step with one another.
White light, when passed through a prism, is
separated into a spectrum of colors. This is
caused by different colors of light (which have
different wavelengths), being REFRACTED
slightly differently…
“Additive” Colors Different
combinations of
blue, green, and red
light can produce
yellow, cyan,
magenta, or white
light.
Additive Colors Applet (RGB)
Polarization of Light
In this rope model of light, light is a single wave
oriented in relation to the vertical plane and thus
passes through a vertical polarizer (a). It cannot pass
through a horizontal polarizer (b).
Polarization applet
This photo of a music store, taken without a polarizing
filter, contains the glare of light off of the surface
of the window (a). This photo of the same scene was
taken with a polarizing filter (b).
When two polarizing filters are arranged with their polarizing
axes in parallel, a maximum amount of light passes through
(a). When two polarizing filters are arranged with
perpendicular axes, no light passes through (b).
Red Shift/Blue Shift
“Doppler” effect
Objects moving toward or away from a stationary
observer have their wavelengths “shifted” to a shorter
or longer value. In sound we call this the Doppler
Effect. ( another one! )
To study the Doppler effect for light, the problem can
be simplified by considering axial relative speeds that
are much less than the speed of light (v << c). This
simplification is used to develop the equation for the
observed light frequency, fobs, which is the frequency
of light as seen by an observer.
Doppler Effect Applet
Three emission lines of hydrogen are visibly redshifted
in the spectrum of quasar 3C 273, as indicated by the
taglines outside the spectra. Their wavelengths are
shifted approximately 16% of what they are in a
laboratory setting.
Light and Lenses
Water
Corn Syrup
Vegetable Oil
Water
Water
What happens when you shine a narrow beam of light at
the surface of a piece of glass? As you can see below, it
bends as it crosses the boundary from air to glass. The
bending of light, called refraction, was first studied by
René Descartes and Willebrord Snell around the time of
Kepler and Galileo.
Light bends toward the normal as it moves from air to
glass and bends away from the normal as it moves from
glass to air (a). The bending of light makes objects appear
to be shifted from their actual locations (b).
“Normal”
Lower Index of
refraction
Higher Index
of refraction
Lower Index of
refraction
Light moves from air to glass to air again (a). Light
slows down and bends toward the normal when it
enters a region of a higher index of refraction (b).
Critical Angle of Refraction
Ray A is partially refracted and partially reflected
(a). Ray B is refracted along the boundary of the
medium and forms the critical angle (b). An angle of
incidence greater than the critical angle results in the
total internal reflection of Ray C, which follows the
law of reflection (c).
Fiber Optic
Cables
Light impulses from a source enter one end of the
optical fiber. Each time the light strikes the surface,
the angle of incidence is larger than the critical angle,
and, therefore, the light is kept within the fiber.
A mirage is seen
on the surface
of a road (a).
Light from the
car bends
upward into the
eye of the
observer (b).
The bottom of
the wave front
moves faster
than the top (c).
More Examples of Refraction and it’s Wavelength
dependence
White light directed through a
prism is dispersed into bands of
different colors (a). Different
colors of light bend different
amounts when they enter a
medium (b).
Rainbows form because white light is dispersed as it
enters, reflects at the inside boundary, and exits the
raindrops (a). Because of dispersion, only one color
from each raindrop reaches an observer (b).
(Illustration not to scale)
Secondary
Rainbow
Primary Rainbow
Two Refractions and Two reflections (and
thus a dimmer rainbow) make a Secondary
Rainbow
The refraction of light in nature that forms rainbows and
red lunar eclipses is beautiful, but refraction also is
useful. In 1303, French physician Bernard of Gordon
wrote of the use of lenses to correct eyesight. Around
1610, Galileo used two lenses to make a telescope, with
which he discovered the moons of Jupiter. Since Galileo’s
time, lenses have been used in many instruments, such as
microscopes and cameras.
Lenses are probably the most useful of all optical devices.
This is a convex lens because it
is thicker at the center than at
the edges. A convex lens often is
called a converging lens because
when surrounded by material
with a lower index of refraction
it refracts parallel light rays so
that the rays meet at a point.
This is a concave lens because
it is thinner in the middle than
at the edges. A concave lens
often is called a diverging lens
because when surrounded by
material with a lower index of
refraction rays passing through
it spread out.
Another way to think of a lens …. As a group
of small prisms!!
Lens Geometry
This distance – twice the focal length – is also
the radius of curvature of the lens!
Ray Diagrams
Converging Lens Applet
Lens (and mirror!) Equations
Using the equations for lenses - It is important that you use the
proper sign conventions when using these equations. Table 18-2
shows a comparison of the image position, magnification, and type of
image formed by single convex and concave lenses when an object is
placed at various object positions, do, relative to the lens.
A converging lens can
focus the parallel rays
from the Sun into a single
VERY HOT point!
Concave lenses produce only virtual images that are
upright and smaller compared to their objects.
Concave Lens Applet
The human eye is complex and has many
components that must work together.
A nearsighted person cannot see distant objects clearly because
images are focused in front of the retina (a). A concave lens corrects
this defect (c). A farsighted person cannot see close objects clearly
because images are focused behind the retina (b). A convex lens
corrects this defect (d).
LASIK Eye Surgery…
short for Laser-Assisted
in-Situ Keratomileusis
“Apparent Size”
of distant object
An astronomical refracting
telescope creates a virtual
image that is inverted
compared to the object.
(Illustration not to scale)