Optics Kit User Manual
Optics Kit User Manual
Optics Kit User Manual
USER MANUAL
Version 1.0
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Dear Teacher,
Welcome to the world of Optics! As you read on, in the next few
minutes, we will introduce you to a variety of experiments you
can show to the students of class VIII-X that will further the
understanding of the properties of light. You will notice that as
you go along, the kit will help you reach out to the students more
effectively and they will enthusiastically participate in class room
discussions.
To get the best out of the kit, form groups of five students and let
them do the experiments collaborating among themselves; they
will understand the subject better. Interesting quizzes can be
conducted for the groups to test their understanding.
The kit is ruggedized, assuming that the students will be working
on the kit, hands on.
A. INTRODUCTION
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Insert the triple slit in the ray box. Adjust the movable Planoconvex platform to obtain triple rays through the aperture
and parallel to the display panel as you swing the panel
away from the aperture. Observe the triple rays, parallel to
each other, beaming on the display panel. The central beam
of light is the principle axis. Each time the triple slit is used
to conduct experiments, ensure proper propagation of the
rays as detailed as above.
C.
ABOUT LENSES
A lens is
merely
a
carefully
ground
or
moulded
piece
of
transparent material that
refracts light rays in such
as way as to form an
image. Lenses can be thought of as a series of tiny refracting
prisms, each of which refracts light to produce their own
image. When these prisms act together, they produce a bright
image focused at a point.
Types of Lenses
There are a variety of types of lenses. Our focus will be upon
lenses that are symmetrical across their horizontal axis known as the principal axis. Lenses can be categorised as
converging lenses and diverging lenses. A converging lens is
a lens that converges rays of light that are travelling parallel to
its principal axis. A diverging lens is a lens that diverges rays
of light that are travelling parallel to its principal axis.
A double convex lens is symmetrical across both its
horizontal and vertical axis. Each of the lens' two faces can be
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Language of Lenses
As we begin to discuss the refraction of light rays and the
formation of images by these two types of lenses, we will need
to use a variety of terms. If a symmetrical lens were thought of
as being a slice of a sphere, then there would be a line passing
through the center of the sphere and attaching to the exact
center of the lens. This imaginary line is known as
the principal axis. A lens also has an imaginary vertical
axis that
bisects
the
symmetrical
lens
into
halves. If the light rays
converge
(as
in
a
converging lens), then they
will converge to a point on the principal axis. This point is
known as the focal point of the converging lens. If the light
rays diverge (as in a diverging lens), then the diverging rays
can be traced backwards until they intersect at a point on the
principal axis. This intersection point is known as the focal
point of a diverging lens. The focal point is denoted by the
letter f on the diagrams below. Note that each lens has two
focal points - one on each side of the lens. Unlike mirrors,
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8. Laws of reflection:
The
reflection of
light from a
plane mirror
can
be
summarized
by
the
following
laws:
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Fix a strip of paper on the display panel. Insert the single slit
in the ray box and adjust the Plano convex lens so that a
single ray appears on the display panel. Position the
protractor on the display panel so that the mirror strip is
facing the ray and the center of the protractor and the zero
reading on the protractor is aligned and in line with the ray.
Slowly rotate the protractor anticlockwise on its axis and
observe the reflected ray. Note that the reading of the
incident ray is always equal to the reading of the reflected
ray.
At some point, mark a b and c d on the incident and the
reflected rays. Mark e f along the mirror strip on the
protractor. Mark the point h, coinciding with the zero
reading on the protractor. Remove the paper strip from the
display panel and join a b, c d and e f. Let a b and c d meet e
f at g. Bisect the angle formed by a b and c d and observe
that it passes through the point h. Note that:
Angle of incidence = angle of reflection.
Try with various angles of incidence and note that the law
holds true for all angles.
9. Multiple reflections - mirrors at an angle:
Ensure single ray on the display panel. Place the two plane
mirror bars facing each other as shown in the ray diagram.
The mirrors are at an angle. This phenomenon is called
multiple reflections. Observe the nature of reflected rays.
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10.
Move the mirror bars so that they are parallel to each other.
Observe that the rays moving back and forth are alternately
parallel to each other.
11.
Principle of periscope:
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12.
of it
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13.
14.
Use the triple slit. Position the semi-circular slab such that
the curved surface is facing the incident rays. Observe that
the rays converge to a point on the normal ray.
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15.
Critical angle:
Use the single slit. Position the semi-circular slab such that
the curved surface faces the incident ray. Being normal to
the curved surface, the incident ray does not bend as it
passes through the slab. It refracts away from the normal
when it comes out of the slab since it is travelling from a
denser medium to a rarer medium.
Slowly increase the angle of incidence by rotating the slab
on its axis, anti-clockwise. Note that the angle of refraction
also increases. At a particular angle of incidence, the
refracted ray emerging out of the slab grazes the plane
surface of the slab. Notice that the angle of refraction is 90 0.
The angle of incidence for which the angle of refraction
becomes 900 is called critical angle.
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16.
Continue with the previous experiment. Turn the semicircular slab such that the angle of incidence increases. The
incidence ray is totally reflected inside the slab. This
phenomenon is called total internal reflection.
17.
18.
notice that the top and bottom rays diverge away from the
normal ray. Mark the position of the lens and two points on
each of the divergent rays. Take out the paper strip and
extend the diverging rays backwards until they meet at a
point on the normal ray. This is the focal point of the lens and
the distance between the focal point and the lens is the focal
length of the lens. Try with the other concave lens and
determine its focal length.
19.
21.
Dispersion of light:
Use the single slit and position the prism such that the
incident ray meets the prism as shown in the ray diagram.
Observe that the white incident ray as it emerges from the
prism splits into its constituent colours (VIBGYOR).This
phenomenon is called dispersion of light. Hold a sheet of
paper below the display window and observe the vibrant
colors are displayed on the paper.
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22.
Total internal reflection using the right-angled
prism:
Use the single slit slab. Position the right angled prism as
shown in the ray diagram. Observe that the incident ray
totally reflects perpendicular to the incident ray.
23.
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Use the triple slit slab. An assembly of two prisms and a slab is
provided in the kit, with the apex of two prisms on either side
of the slab facing inwards. Position the assembly such that the
incident ray passes through the slab. Observe that the parallel
rays on either side of the normal ray diverge away
from the normal ray. You may position a strip of paper on the
display panel and mark points on the diverging rays, normal
ray and the position of the
assembly. Extend the diverging rays backwards so that they
converge on the normal ray. The point where the diverging rays
meet the normal ray is the focal point of the concave assembly
and the distance between the focal point and the assembly, its
focal length. The assembly of prisms hence acts as a concave
lens.
25.
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Insert
the
double
convex mirror
(focal length
10cms) in the
second
slot
on the display panel. Use the triple slit and observe that the
rays converge on to the principle axis at the focal point.
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Farsightedness
or hyperopia is
the inability of the
eye to focus on
nearby
objects.
The
farsighted
eye has no difficulty viewing distant objects. But the ability
to view nearby objects requires a different lens shape - a
shape that the farsighted eye is unable to assume.
Subsequently, the farsighted eye is unable to focus on
nearby objects that result in the lens of the eye no longer
able to assume the high curvature that is required to view
nearby objects. The lens' power to refract light has
diminished and the images of nearby objects are focused at
a location behind the retina. On the retinal surface, where
the light-detecting nerve cells are located, the image is not
focused. These nerve cells thus detect a blurry image of
nearby objects.
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The cure for the farsighted eye centers on assisting the lens
in refracting the light. Since the lens can no longer assume
the convex and highly curved shape that is required to view
nearby objects, it needs some help. Thus, the farsighted eye
is assisted by the use of a converging lens. This converging
lens will refract light before it enters the eye and
subsequently decreases the image distance. By beginning
the refraction process prior to light reaching the eye, the
image of nearby objects is once again focused upon the
retinal surface.
Insert the double convex lens (focal length 10cms) in the second
slot on the display panel. Use the triple slit and observe that the
rays converge on to the principle axis at the focal point. Assume
the convex lens to be the eye lens. If the retina is at the focal
point, normal vision occurs. If the retina is closer to the lens
compared to the focal point, closer object appears blurred. Now
insert a converging lens in the first slot and note that the focal
length decreases. Hence,
by
lens
using a converging
of the right focal
length, the image of
the
distant object can
be
made to occur on
the Retina, there by correcting the eye defect.
Notice the double convex lens in the second slot. This is deemed
as the eye lens. The focal point is behind the retina, indicating
that a correction is needed to push the focal point to the surface
of the retina.
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27.
Type: Real
Case 3: The object is located between C
and F
Image Location: Beyond C
Orientation: Inverted
Size: Larger than the object
Type: Real
between the mirror and the object and obtain the image on
the paper. Observe the LOST properties of the image.
28.
at the 2F
between the 2F point and the
at the focal point (F)
in front of the focal point (F)
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Orientation: Inverted
Size: Same as Object
Type: Real
Case 3: The object is located between 2F and F
Image location: Beyond 2F
Orientation: Inverted
Size: Larger than the Object
Type: Real
Light rays diverge upon refraction; for this reason, the image
location can only be found by extending the refracted rays
backwards on the object's side the lens. The
point of their intersection is the virtual image
location. It would appear to any observer as
though light from the object were diverging from
this location. Any attempt to project such an
image upon a sheet of paper would fail since light does not
actually pass through the image location.
You will need:
a. A two-cell LED torch.
b. An object, as shown, cut from paper and pasted on the
torch.
c. A 10cm focal length double convex lens (supplied)
d. A mirror/lens holding stand
Place the torch in front of the Lens. Place a sheet of paper
behind the lens; capture the image on the paper by moving
the paper towards or away from the lens. Observe the image
characteristics as the object is moved to different locations.
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