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PIC SCIENCES
A VERSATILE IN TEACHING SCIENCES SINCE 2000
PROJECT WORKS
1. Collect the information about different type of eye defects from your nearest eye
specialist or optical hop and write a report on.
2. Collect the information about dispersion phenomenon occurs in the daily life.
3. Collect the information of historical development of the atomic theory.
4. Collect the information about the scientists who developed the atomic theories.
5. Make the s, p, and d Orbital models.
6. Collect the information about reactivity of II A group elements (Noble gases) from your
school library and prepare a report on their special characteristics when compared to
other elements in the periodic table.
7. Collect the information about regarding metallic character of elements of I A group and
pre pare report to support the idea of metallic character increases in a group as we move
from top to bottom.
8. Collect the information about the lenses available in an optical shop. Find out how the
focal lengths of a lens may be determined by the given power of the lens.
9. Take two watch glasses and affix them, pour two different liquids (ex. Water and
Navaratna oil) and now it will acts like a lens with two different materials. Put a light
source (object) in front of this lens and note the observations and write a report on it.
LAB ACTIVITIES
Conduct an experiment to produce rainbow in your class room and explain the procedure.
Conduct an experiment to find the refractive index of a prism.
Conduct an experiment to demonstrate the scattering of the light.
Aluminum does not react with water at room temperature but reacts with both dil. HCl
and NaOH solutions. Verify these statements experimentally. Write your observations
with chemical equations from the se observations. Can we conducts that Al is a
metalloid?
5. Conduct an experiment to focal length of the lens.
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PIC SCIENCES
A VERSATILE IN TEACHING SCIENCES SINCE 2000
1. Collect the information about dispersion phenomenon occurs in the daily life.
Rainbow Formation
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Dispersion of Light by Prisms
Rainbow Formation
Mirages
One of nature's most splendid masterpieces is the rainbow. A rainbow is an excellent
demonstration of the dispersion of light and one more piece of evidence that visible
light is composed of a spectrum of wavelengths, each associated with a distinct color.
To view a rainbow, your back must be to the sun as you look at an approximately 40
degree angle above the ground into a region of the atmosphere with suspended
droplets of water or even a light mist. Each individual droplet of water acts as a tiny
prism that both disperses the light and reflects it back to your eye. As you sight into the
sky, wavelengths of light associated with a specific color arrive at your eye from the
collection of droplets. The net effect of the vast array of droplets is that a circular arc
of ROYGBIV is seen across the sky. But just exactly how do the droplets of water
disperse and reflect the light? And why does the pattern always appear as ROYGBIV
from top to bottom? These are the questions that we will seek to understand on this
page of The Physics Classroom Tutorial. To understand these questions, we will need to
draw upon our understanding of refraction, internal reflection and dispersion.
The Path of Light through a Droplet
A collection of suspended water droplets in the atmosphere serves as a refractor of
light. The water represents a medium with a different optical density than the
surrounding air. Light waves refract when they cross over the boundary from one
medium to another. The decrease in speed upon entry of light into a water droplet
causes a bending of the path of light towards the normal. And upon exiting the droplet,
light speeds up and bends away from the normal. The droplet causes a deviation in the
path of light as it enters and exits the drop.
There are countless paths by which light rays from
the sun can pass through a drop. Each path is
characterized by this bending towards and away from
the normal. One path of great significance in the
discussion of rainbows is the path in which light
refracts into the droplet, internally reflects, and then
refracts out of the droplet. The diagram at the right
depicts such a path. A light ray from the sun enters
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the droplet with a slight downward trajectory. Upon refracting twice and reflecting
once, the light ray is dispersed and bent downward towards an observer on earth's
surface. Other entry locations into the droplet may result in similar paths or even in
light continuing through the droplet and out the opposite side without significant
internal reflection. But for the entry location shown in the diagram at the right, there is
an optimal concentration of light exiting the airborne droplet at an angle towards the
ground. As in the case of the refraction of light through prisms with nonparallel sides,
the refraction of light at two boundaries of the droplet results in the dispersion of light
into a spectrum of colors. The shorter wavelength blue and violet light refract a slightly
greater amount than the longer wavelength red light. Since the boundaries are not
parallel to each other, the double refraction results in a distinct separation of the
sunlight into its component colors.
The angle of deviation between the incoming light
rays from the sun and the refracted rays directed to
the observer's eyes is approximately 42 degrees for
the red light. Because of the tendency of shorter
wavelength blue light to refract more than red light,
its angle of deviation from the original sun rays is
approximately 40 degrees. As shown in the
diagram, the red light refracts out of the droplet at
a steeper angle toward an observer on the ground.
There are a multitude of paths by which the original ray can pass through a droplet and
subsequently angle towards the ground. Some of the paths are dependent upon which
part of the droplet the incident rays contact. Other paths are dependent upon the
location of the sun in the sky and the subsequent trajectory of the incoming rays
towards the droplet. Yet the greatest concentration of outgoing rays is found at these
40-42 degree angles of deviation. At these angles, the dispersed light is bright enough
to result in a rainbow display in the sky. Now that we understand the path of light
through an individual droplet, we can approach the topic of how the rainbow forms.
The Formation of the Rainbow
A rainbow is most often viewed as a circular arc in the sky. An observer on the ground
observes a half-circle of color with red being the color perceived on the outside or top
of the bow. Those who are fortunate enough to have seen a rainbow from an airplane
in the sky may know that a rainbow can actually be a complete circle. Observers on the
ground only view the top half of the circle since the bottom half of the circular arc is
prevented by the presence of the ground (and the rather obvious fact that suspended
water droplets aren't present below ground). Yet observers in an airborne plane can
often look both upward and downward to view the complete circular bow.
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The circle (or half-circle) results because there are a collection of suspended droplets in
the atmosphere that are capable concentrating the dispersed light at angles of deviation
of 40-42 degrees relative to the original path of light from the sun. These droplets
actually form a circular arc, with each droplet within the arc dispersing light and
reflecting it back towards the observer. Every droplet within the arc is refracting and
dispersing the entire visible light spectrum (ROYGBIV). As described above, the red
light is refracted out of a droplet at steeper angles towards the ground than the blue
light. Thus, when an observer sights at a steeper angle with respect to the ground,
droplets of water within this line of sight are refracting the red light to the observer's
eye. The blue light from these same droplets is directed at a less steep angle and is
directed along a trajectory that passes over the observer's head. Thus, it is the red light
that is seen when looking at the steeper angles relative to the ground. Similarly, when
sighting at less steep angles, droplets of water within this line of sight are directing blue
light to the observer's eye while the red light is directed downwards at a more steep
angle towards the observer's feet. This discussion explains why it is the red light that is
observed at the top and on the outer perimeter of a rainbow and the blue light that is
observed on the bottom and the inner perimeter of the rainbow.
Rainbows are not limited to the dispersion of light by raindrops. The splashing of water
at the base of a waterfall caused a mist of water in the air that often results in the
formation of rainbows. A backyard water sprinkler is another common source of a
rainbow. Bright sunlight, suspended droplets of water and the proper angle of sighting
are the three necessary components for viewing one of nature's most splendid
masterpieces.
Dispersion is defined as the spreading of white light into
its full spectrum of wavelengths.
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LEARNING OBJECTIVE[ EDIT ]
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Describe production of rainbows by a combination of refraction and reflection
processes
KEY POINTS[ EDIT ]
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Dispersion occurs whenever there is a process that changes the direction of light in a manner
that depends on wavelength. Dispersion can occur for any type of wave and always involves
wavelength-dependent processes.
For a given medium, n increases as wavelength decreases and is greatest for violet light. Thus
violet light is bent more than red light, as can be seen with a prism.
In a rainbow, light enters a drop of water and is reflected from the back of the drop. The light
is refracted both as it enters and as it leaves the drop.
refraction
Changing of a light ray's direction when it passes through variations in matter.
We see about six colors in a rainbow—red, orange, yellow, green, blue, and violet; sometimes
indigo is listed, too. These colors are associated with different wavelengths of light. White light,
in particular, is a fairly uniform mixture of all visible wavelengths. Sunlight, considered to be
white, actually appears to be a bit yellow because of its mixture of wavelengths, but it does
contain all visible wavelengths. The sequence of colors in rainbows is the same sequence as the
colors plotted versus wavelength . What this implies is that white light is spread out according to
wavelength in a rainbow. Dispersion is defined as the spreading of white light into its
full spectrum of wavelengths. More technically, dispersion occurs whenever there is a process
that changes the direction of light in a manner that depends on wavelength. Dispersion, as a
general phenomenon, can occur for any type of wave and always involves wavelength-dependent
processes.
Colors of a Rainbow
Even though rainbows are associated with seven colors, the rainbow is a continuous distribution of
colors according to wavelengths.
Refraction is responsible for dispersion in rainbows and many other situations. The angle of
refraction depends on the index of refraction, as we saw in the Law of Refraction. We know that
the index of refraction n depends on the medium. But for a given medium, n also depends on
wavelength. Note that, for a given medium, n increases as wavelength decreases and is greatest
for violet light. Thus violet light is bent more than red light and the light is dispersed into the
same sequence of wavelengths.
Pure Light and Light Dispersion
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(a) A pure wavelength of light falls onto a prism and is refracted at both surfaces. (b) White light is
dispersed by the prism (shown exaggerated). Since the index of refraction varies with wavelength,
the angles of refraction vary with wavelength. A sequence of red to violet is produced, because the
index of refraction increases steadily with decreasing wavelength.
Rainbows are produced by a combination of refraction and reflection. You may have noticed that
you see a rainbow only when you look away from the sun. Light enters a drop of water and is
reflected from the back of the drop . The light is refracted both as it enters and as it leaves the
drop. Since the index of refraction of water varies with wavelength, the light is dispersed, and a
rainbow is observed. (There is no dispersion caused by reflection at the back surface, since the
law of reflection does not depend on wavelength. ) The actual rainbow of colors seen by an
observer depends on the myriad of rays being refracted and reflected toward the observer's eyes
from numerous drops of water. The arc of a rainbow comes from the need to be looking at a
specific angle relative to the direction of the sun.
Light Reflecting on Water Droplet
Part of the light falling on this water drop enters and is reflected from the back of the drop. This
light is refracted and dispersed both as it enters and as it leaves the drop.
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