Download f - Uplift Education

Which of these lamps is emitting EM radiation?
1. Lamp A
2. Lamp B
3. Both
4. Neither
A
B
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 visible—it emits infrared
waves, which aren’t 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!
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.
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.
Reflection  Mirrors
For now, we are only interested in the light that is reflected at
the surface.
The law of reflection
r
=
i
• The angle of reflection = angle of incidence
• Incident ray,
normal
reflected ray
r
i
reflected
and normal all lie
Incident
ray
in the same plane
ray
mirror
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
- 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).
SPECULAR - 'mirror' reflection
Surface particles are small
relative to the l.
Light is reflected in a
single direction.
Sharp
image
DIFFUSE reflection
Surface is rough relative to the
incident l.
Light is reflected (scattered)
in all directions.
All reflections follow
the law of reflection
smooth (flat) surface
l > irregularities in the surface
Fuzzy or
no image
rough surface –
_ irregularities
l<
Many natural surfaces act as
a diffuse reflector to some extent.
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 doesn’t 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.
Mirror forms image
of every point.
Your eye can’t 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.
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
How Large Does A Mirror Need To Be
To Show Your Entire body?
A
B
If you measure the length AB you’ll find it will be
the half of your height
– the distance from the mirror doesn’t matter!!!
You only need a mirror half as tall as you are
to see your whole self
Mr. Stanbrough's Classes
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
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)
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
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 that’s exactly the definition of focal point (focus)
Optical (Principal) axis
F
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
thin converging lenses: ignore
double refraction in drawings: light
ray goes to the middle of the lens
and refracts there
=
F
F
or
F
F
For simplicity: we’ll consider
only biconvex symmetrical
lens- equal focal lengths ( f )
converging lens
– REMEMBER :
LIGHT PASSES THROUGH A 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
A converging lens is used to
focus rays from the sun to a point
since the sun is very
far from the lens, the
rays are nearly
parallel
Standard rays to help us draw an image
formed by a lens
Converging lenses
F
F
F
F
F
F
Image formation by a
converging lens
image
object
2F
F
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
converging lens is used in a camera to
focus light onto the film
Object: between infinity and 2F
p > 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.
Object: between 2F & F
f < p < 2f
Image: real, inverted, enlarged.
image
2F
F
object
That arrangement is used in
a slide or film projector.
Bulb
Screen
Object
upside-down
Real
image
Projector
Object: between F & lens
p<f
Image: virtual, upright, enlarged.
image
2F
F object
F
By placing the lens close to the object
we get a magnified virtual image.
The Thin-Lens Equation and the
Magnification Equation
11 1
u v f
hi v
m 
h u
f
is + for a converging lens
object is real: u is +
image is real: v is +
object is virtual: u is –
image is virtual: v is –
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)
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.
perfect lens
spherical lens
paralel light rays striking the
outer edges of a lens are
focused in a slightly different
place than beams close to
the axis.
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
blurred
image
image
cover
Correction for
spherical
aberration
this or money
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.
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.
Sight – the human eye
• Physics of the human eye
• Corrections for abnormal vision
• Nearsightedness
• Farsightedness
• light enters through
the cornea
The Eye
• 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
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 (it’s 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.
The relaxed eye can easily focus on distant objects. To focus
on close objects the lens is squeezed to shorten it’s 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.
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.
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 positivefocal-length lens (converging lense).
The lens slightly converges the light
before it enters the eye.
As we age, our lens hardens, so we’re less able to adjust and more
likely to experience far-sightedness. Hence “bifocals.”
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:
1
P
f
unit: diopter (D = m-1)
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.