Which of these lamps is emitting EM radiation?

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Which of these lamps is emitting EM radiation? 1.
Lamp A 2. Lamp B 3. Both 4. Neither
Answer 3 All bodies with any temperature at all
continually emit EM waves. The frequency of these
waves varies with temperature. Lamp B is hot
enough to emit visible light. Lamp A is cooler,
and the radiation it emits is too low in
frequency to be visibleit emits infrared waves,
which arent seen with the eye. You emit waves as
well. Even in a completely dark room your waves
are there. Your friends may not be able to see
you, but a rattlesnake can!
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Note that EM waves are everywhere! Not just in
air, but in interplanetary empty space -
actually a dense sea of radiation. Vibrating
electrons in sun put out EM waves of frequencies
across the whole spectrum. Any body at any
temperature other than absolute zero, have
electrons that accelerate or vibrate and emit EM
radiation that permeates us, even if very low
frequency.
A thin beam of light is called a ray.
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We see because we have organs (our eyes) that
sense the intensity (brightness) and wavelength
(color) of light.
If the light is travelling through a uniform
medium, light travels in a straight line, and our
brain thinks the light is ALWAYS traveling in the
straight line. Our visual systems rely heavily
on this fact, 'back-projecting' rays that enter
our eyes, to the probable origin of the light
rays. So, if the light has traveled to your
eyes in a straight line the object is really
where it appears to be. However, if the light
entering your eyes has changed the path on the
way from origin, your brain will see the object
along the extended line entering your eyes.
The eyes do not know laws of physics.
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Reflection ? Mirrors
For now, we are only interested in the light that
is reflected at the surface.
The law of reflection

• The angle of reflection angle of incidence
• Incident ray,
• reflected ray
• and normal all lie
• in the same plane

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How can we get a bunch of parallel rays striking
a surface? A beam of light? LASER? YES! BUT
THERE IS ANOTHER WAY!!!! If the light source is
infinitely far away it is a perfect approximation
(example SUN).
A good approximation is when the source is far
enough compared to the dimension of the surface.
a small curved mirror
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- bunch of parallel rays encounters an obstacle
The type of reflection is dependent on the size
of the surface irregularities relative to the
incident wavelength (l).
DIFFUSE reflection Surface is rough relative to
the incident l. Light is reflected (scattered)
in all directions.
SPECULAR - 'mirror' reflection Surface particles
are small relative to the l. Light is reflected
in a single direction.
Fuzzy or no image
All reflections follow the law of reflection
Sharp image
smooth (flat) surface l gt irregularities in the
surface
Many natural surfaces act as a diffuse reflector
to some extent.
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IMAGE FORMED BY PLANE MIRROR An object is in
front of a plane mirror. The light is spreading
in all directions. Shown is the path of several
rays. This light reflects from the mirror.
The reflected light doesnt meet (intersect) in
the real space, but extended rays behind mirror
in the virtual space do. For the eyes it seems as
if these reflected light rays were coming from
another object back BEHIND the mirror at the
intersection of the extended rays !! We call this
virtual space because the light never really
exists back there....it just SEEMS to be coming
from there. We call this apparent source of the
light rays a VIRTUAL IMAGE. Different eyes at
different positions yet - the same image
location.
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Your eye cant tell the difference between an
object and its image. The light enters your eye
the same way it would without the mirror if there
really were an object there behind the mirror.
Mirror forms image of every point.
Mirrors appear to make rooms look larger.
The image is 1. virtual 2. the same height
(magnification of 1) 3. upright (in the same
direction) 4. equally distant from the mirror as
the object
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How Large Does A Mirror Need To Be To Show Your
Entire body?
A
B
If you measure the length AB youll find it will
be the half of your height the distance from
the mirror doesnt matter!!!
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You only need a mirror half as tall as you are to
see your whole self
Mr. Stanbrough's Classes
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The image of your right hand is your left hand
AMBULANCE is painted backward so that you see
it correctly in your real-view mirror
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LENSES
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Image formation with lenses

• converging lens (positive lens)
• diverging lens (negative lens)
• the human eye
• correcting for nearsightedness
• correcting for farsightedness
• optical instruments

• lenses are relatively simple optical devices
• the principle behind the operation of a lens is
  refraction? the bending of light as it passes
  from air into glass (or plastic)

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LENSES
REMEMBER LIGHT PASSES THROUGH A LENS
The lenses used in optical instruments
(eyeglasses, cameras, telescopes, ...) are made
from transparent materials that refract light.
Biconvex - Converging lenses
Imagine two prisms and bunch of parallel rays.
Crude lens two glass prism causing light rays
to converge after refraction but they do not
converge to one single point
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Improved lens parallel beam of rays will
converge to a single point on the axis after
emerging from the lens (after refraction) .
This point is called focal point F. And thats
exactly the definition of focal point (focus)
Optical (Principal) axis
F
Real focus parallel rays really meet at focus
after refraction through the lens. It is a real
image of an infinitely far object.
F
F
F
F
F
F

or

thin converging lenses ignore double refraction
in drawings light ray goes to the middle of the
lens and refracts there
For simplicity well consider only biconvex
symmetrical lens- equal focal lengths ( f )
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REMEMBER LIGHT PASSES THROUGH A LENS
converging lens
focal point F
?a converging lens focuses parallel rays to a
point called the focal point. ? a thicker lens
has a shorter focal length
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A converging lens is used tofocus rays from the
sun to a point
since the sun is very far from the lens, the rays
are nearly parallel
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Standard rays to help us draw an image formed by
a lens
Converging lenses
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Image formation by a converging lens
image
F
2F
object
?If the object is located at a distance beyond 2F
from the lens, the image is inverted and smaller
than the object. ?The image is called a REAL
image since light rays actually converge at the
image location (to remind you there are plenty
of rays converging there we drew only two of
them
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converging lens is used in a camera to focus
light onto the film
Object between infinity and 2F p
gt 2f Image real, inverted, smaller. That
arrangement is used in camera.
when you focus a camera, you adjust the distance
between the lens and the film depending on the
object location.
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Object between 2F F f lt p lt
2f Image real, inverted, enlarged.
image
F
2F
object
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That arrangement is used in a slide or film
projector.
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Object between F lens p lt f
Image virtual, upright, enlarged.
object
image
By placing the lens close to the object we get a
magnified virtual image.
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The Thin-Lens Equation and the Magnification
Equation
f is for a converging lens
object is real u is object is virtual
u is image is real v is image is
virtual v is
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Aberrations
In an ideal lens, all light rays from one point
of the object would meet at the same point of the
image, forming a clear image. The influences
which cause different rays to converge to
different points are called aberrations.
object
blurred image
Lenses do not form perfect images, and there is
always some degree of distortion or aberration
introduced by the lens which causes the image to
be an imperfect replica of the object. Careful
design of the lens system for a particular
application ensures that the aberration is
minimized. There are several different types of
aberration which can affect image quality.
(Wikipedia)
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Spherical Aberration occurs because spherical
surfaces are not the ideal shape with which to
make a lens, but they are by far the simplest
shape to which glass can be ground and polished
(the least expensive) and so are often used.
paralel light rays striking the outer edges of a
lens are focused in a slightly different place
than beams close to the axis.
perfect lens
spherical lens
This problem is not limited to parallel light.
Any incident ray which strikes the outer edges of
the lens is subject to this departure from the
expected or proper course for the ideal lens.
This manifests itself as a blurring of the image.
Lenses in which closer-to-ideal, non-spherical
surfaces are used are called aspheric lenses.
object
object
Correction for spherical aberration this or money
image
blurred image
cover
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Chromatic Aberration
A lens will not focus different colors in exactly
the same place because the focal length depends
on refraction and the index of refraction for
blue light (short wavelengths) is larger than
that of red light (long wavelengths). The amount
of chromatic aberration depends on the dispersion
of the glass.
One way to minimize this aberration is to use
glasses of different dispersion in a doublet or
other combination
This effect can be reduced by having a
combination of a convex and a concave lens made
of glasses having different refractive indices.
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Chromatic aberration can be minimized using
additional lenses
In an Achromat, the second lens cancels the
dispersion of the first.
Achromats use two different materials, and one
has a negative focal length.
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Sight the human eye

• Physics of the human eye
• Corrections for abnormal vision
• Nearsightedness
• Farsightedness

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The Eye

• light enters through the cornea
• the iris controls the amount of light that gets
  in, a muscle can close it or open it, the iris is
  the colored part
• the lens is filled with a jelly-like substance
  the ciliary muscle can change the shape of the
  lens and thus change its focal length

? by changing the focal length,
(accommodation) the lens is able to focus light
onto the retina for objects located at various
distances
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The human eye resembles a camera in its basic
structure. Light passé through a lens. A
diaphragm, called iris (the colored part of your
eye), adjusts automatically to control the amount
of light entering the eye. The hole through which
light passes (the pupil) is black because no
light is reflected from it (its a hole), and
very little light is reflected back out from the
interior of eye. The retina, which plays the role
of the film in a camera is on the curved rear
surface. It consists of array of nerves and
receptors known as rods and cones which act to
change light energy into electrical signals that
travel along the nerves.
The reconstruction of the image from all these
tiny receptors is done mainly in the brain. The
sharpest image and the best color discrimination
are made at the center of retina, where the cones
are very closed packed. There is no shutter in
the eye. The equivalent operation is carried out
by the nervous system, which analyzes the signals
to form images at the rate of about 30 per
second. Movies (US television) operate by taking
a series of still pictures at a rate of 24 (30)
per second. The rapid projection of these on the
screen gives the appearance of motion.
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The relaxed eye can easily focus on distant
objects. To focus on close objects the lens is
squeezed to shorten its focal length, making it
possible to converge the rays onto the retina.
The near point is the distance at which the
closest object can be seen clearly. It recedes
with age. The far point is the farthest
distance at which an object can be seen clearly
Normal eye (a sort of average) is defined as one
having a near point of 25 cm and far point at
infinity.
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Near-sightedness (myopia)
eye tends to refract light more than usual
In nearsightedness, a person can see nearby
objects well, but has difficulty seeing distant
objects. Objects focus before the retina. This
is usually caused by an eye that is too long or a
lens system that has too much power to focus.
Myopia is corrected with a negative-focal-length
lens (diverging lens). This lens causes the
light to diverge slightly before it enters the
eye which then converge light at the retina.
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Far-sightedness (hyperopia)

• Far-sightedness (hyperopia) occurs when the focal
  point is beyond the retina. Such a person can
  see distant objects well, but has difficulty
  seeing nearby objects. This is caused by an eye
  that is too short, or a lens system that has too
  little focusing power. When a farsighted person
  tries to focus on a close object the lens cannot
  be squeezed enough to focus on the retina. Images
  of closed objects are focused behind the retina,
  and can not be seen clearly.
• Hyperopia is corrected with a positive-focal-leng
  th lens (converging lense). The lens slightly
  converges the light before it enters the eye.

As we age, our lens hardens, so were less able
to adjust and more likely to experience
far-sightedness. Hence bifocals.
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The optometrists do not specify the focal length
of the correctional lenses directly. Instead they
use concept of refractive power to describe how
much a lens refract the incident light.
Refractive power P of a lens
unit diopter (D m-1)
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The Apparent Size of an Object
The size of the image on the retina of the
observer depends on i) the real size of the
object, h ii) the distance of the object from the
observer, u.
These two factors (h and u), determine the size
of the angle subtended at the eye by the object.
The apparent size of an object is proportional
to this angle. You can see how it changes. The
image formed on retina is bigger if the angle is
bigger.
Optical instruments provide magnification by
increasing the size of the angle subtended at the
eye.