light travels in a straight line...light travels in a straight line can be observed by keeping an...
TRANSCRIPT
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Light Travels In a Straight Line
Light travels in a straight line can be observed by keeping an object in
the path of light. In an atmosphere which is bit dusty, we can see light
traveling in a straight line. Light emerging from the torch, train and
lamps always travel in a straight line. Let us study in detail how does
light travel in a straight line.
Light Travels Along a Straight Line
Life without light would have been pretty dull. Light travels at a speed
of 186,000 miles per second. You must have observed that in your
house that whenever a beam of light enters a dark room through a tiny
hole in the window, the lightwave always travels in a straight line.
Let us carry out a small activity to show that lightwave travels along a
straight line. Take three CD’s and align them together. Align them in
such a way that all the CD’s line in a straight line. Now take a candle
and place it at the other end. Do make sure that the tip of the candle
and the holes of the CD’s all lie in the straight line. Ensure that the
height of the CD’s and the tip of the candle are same. Observe the
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flame of the candle. We are able to see the flame of the candle because
the light wave travels through the holes and reaches our eye.
Now if suppose we displace the center of the CD’s we observe that we
are not able to see the flame of the candle. Why does that happen?
This is because the light gets blocked. If the light could have the
ability to take a curve and travel, we could have seen the lightwave.
But since light travels in a straight line, we were unable to see the
flame of the candle when the CD is displaced. This proves that light
travels along a straight line.
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(Source: Wikipedia)
In the above picture, we can clearly see that light coming through the
holes in the window travel along a straight line.
Questions For You
Q1. The phenomenon in which the moon’s shadow falls on earth, or
the earth casts its shadow on the moon, is known as
A. Shadow
B. Lateral deviation
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C. Eclipse
D. Tides
Answer: C. The phenomenon in which the moon’s shadow falls on
earth or the earth casts its shadow on the moon is known as an eclipse.
During a solar eclipse, moon’s shadow falls on the earth. During a
lunar eclipse, earth’s shadow falls on the moon.
Q2. Two examples of non-luminous objects are
A. Stars and Moon
B. Burning candle, glowing bulb
C. The moon, a spoon
D. Stars, a spoon
Answer: C. Non-luminous objects are those that do not emit light. The
moon and the spoon do not emit light. So these two are good examples
of non-luminous objects.
Q3. We can see the objects only when
A. Reflected light from the object reaches our eye.
B. The objects absorb all the light.
C. When the objects allow all the light to pass through them.
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D. None of these.
Answer: A. Objects can only be seen when light falls on the object
and are reflected back to our eyes.
Mirrors
When you look in the mirror have you noticed something interesting
about you and the image of you in the mirror? Let us carry out a small
activity. Stand in front of the mirror and move your right hand. Now
lift your left hand. Did you notice that in the mirror the right appears
left and vice versa? To understand this let us talk about Mirrors.
Mirrors
A smooth and highly polished reflecting surface are is a mirror. Most
commonly used are plane mirrors.
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Lateral Inversions in Plane Mirrors
The word ‘AMBULANCE’ is opposite because when the driver of the
vehicle ahead of the ambulance looks in her/his mirror, the person can
read it as ‘AMBULANCE’ and give way to it.
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Spherical Mirrors
Suppose you are sitting at the dining table and you don’t like the food,
you start playing with the spoon. You look yourself in the spoon and
you notice that you look pretty funny. The moment you get the spoon
closer you get a magnified image and when taken far, you see an
inverted image. Do you know what’s really happening? To understand
what is happening lets us talk about the special class of mirrors known
as spherical mirrors.
Let us first understand the terms of spherical mirrors.
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● Radius of Curvature (c): It is the distance between Pole and the
Center of curvature.
● Center of Curvature (r): The Center of Curvature of a spherical
mirror is the point in the center of the mirror which passes
through the curve of the mirror and has the same tangent and
curvature at that point.
● Aperture: It is a point from which the reflection of light
actually happens.
● Pole (p): Pole is the midpoint of a mirror. It’s twice the focus.
● Focus: It is any point, where light rays parallel to the principal
axis, will converge after reflecting from the mirror.
● Principal axis: An imaginary line passing through the optical
center and the center of curvature of the spherical mirror.
● Focal Length: It is on the axis of a mirror where rays of light
are parallel to the axis converge after reflection or refraction.
Spherical mirrors are of two types: ● Convex Mirror
● Concave Mirror
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Concave Mirror
We are familiar that the spherical mirrors are not plane, they are
curved in one particular direction. They are curved inward. A concave
mirror is also known as the converging mirror as in these type of
mirrors light rays converge at a point after they strike and are getting
reflecting back from the reflecting surface of the mirror.
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A concave mirror produces real and inverted images except when the
object is placed very near to the mirror that pole (p) and the focus (f)
where the image produced is virtual and erect. The concave mirror is
used in shaving mirrors to see a large image of the face. It is also
useful in vehicle headlights and torches. Dentists also use a concave
mirror to get the bigger image of the teeth.
Convex mirror
The convex mirror has a reflective surface that curves outward. These
mirrors are “always” form virtual, erect and diminished regardless of
the distance between the object and mirror.
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When parallel rays of light strike the mirror, they are reflected in a
way wherein they spread out or diverge. For this reason, a convex
mirror is also a diverging mirror too. If these reflected rays are
extended behind the mirror by dotted lines, they meet at a point. This
point is the focus of the convex mirror. The concave mirror is used in
the vehicle so that the driver is aware of the vehicle coming from
behind. They are also used in street light reflectors.
Question For You
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Q. The following is used in the rear-view mirrors of vehicles:
A. Plane Mirror
B. Convex Mirror
C. Convex Lens
D. Any of these
Answer: B. The Convex Mirror is used in the rear-view mirrors of
vehicles as it forms upright/erect images and has a wider field of view.
Reflection of Light
We ‘see’ a lot of things around us on a daily basis. But how are we
able to do so? You might answer saying it is because of our eyes. Isn’t
it? And you aren’t wrong. But is it just the eyes? What about the light
bounced back from the things around us and enabling us to see things
better? What is this phenomenon? It’s because of ‘Reflection of
Light’. Let us study more about it below.
What is Reflection of Light?
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When a ray of light falls on any object (polished, smooth, shiny
object), light from that object bounces back those rays of light to our
eyes and this is known as “Reflection” or “Reflection of Light”.
This phenomenon is what enables us to look at the world around us
based. Before, after and during reflection light travels in a straight
line. For example, twinkling of stars or light reflected by a mirror.
Laws of Reflection
In the diagram given above, the ray of light that approaches the mirror
is known as “Incident Ray”. The ray that leaves the mirror is known as
“Reflected Ray”.
At the point of incidence where the incident ray strikes the mirror, a
perpendicular line is drawn known as the “Normal”. This normal is
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what divides the incident ray and the reflected ray equally and gives
us the “Angle of Incidence” (Qi) and “Angle of Reflection” (Qr).
Hence the above information gives us the “Laws of Reflection of
Light” which state that :
a. The angle of incidence is equal to the angle of reflection.
b. The incident ray, the normal and the reflected ray, all lie in the same
plane.
Types of Reflection:
There are two types of reflection :
a. Specular/ Regular reflection
b. Diffused/ Irregular reflection
a. Specular/ Regular reflection:
Specular/Regular reflection is the mirror-like reflection of rays of
light. In such a type of reflection, the rays of light that are reflected
from a smooth and shiny object such as a mirror, are reflected at a
definitive angle and each incident ray which is reflected along with the
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reflected ray has the same angle to the normal as the incident ray.
Thus, this type of phenomena causes the formation of an image.
b. Diffused/ Irregular Reflection:
Diffused/Irregular reflection is a non-mirror-like reflection of light. In
such a type of reflection rays of light that hit an irregular object with a
rough surface, are reflected back and scatter in all directions. Here, the
incident ray which is reflected along with reflected ray doesn’t have
the same angle to the normal as the incident ray.
Thus, this type of reflection doesn’t form an image.
Image Formation by Plane Parallel Mirrors
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As shown in the figure, infinite images are formed when an object
placed is placed between two parallel plane mirrors.
Consider an object placed in between the mirrors and we can actually
see the image, in this case, the number of images would range from 1
to X (X being a very large number depending on your position relative
to the mirror and object, and X not infinity).
Angle between mirrors = θ=O
Hence,
Number of images formed = 360/θ
So, in case of parallel mirrors, we get infinite images.
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Solved Example For You
Q. Which of the following phenomenon has two angles that are equal
to each other?
a. Reflection
b. Refraction
c. Diffraction
d. None of the above
Sol: a. Reflection
According to Snell’s law of reflection, the angle of reflection with
normal is equal to the angle of incidence with that normal and all these
lies in the same plane.
Sunlight- White or Coloured
As we all know that Sun is the major source of light. But what is the
color of the sunlight? We see the rainbow in the sky consisting of
different colors that usually appears after the rains when the sunlight is
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low. Also, the sky appears orange or red during sunset. So is sunlight
colorful? Let us learn this in detail and see what is the actual color of
sunlight.
Sunlight- White or Coloured
White light is called as white because it consists of seven colors. The
sunlight splits into seven colors namely violet, indigo, blue, green,
orange, and red. We usually call it as VIBGYOR. When we mix all
these colors we just get one light which is the WHITE light. Let us
carry out a small activity to show that sunlight has seven different
colors.
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With the help of a narrow beam of light, a glass prism, and a white
wall it is possible to produce the band of seven colors using white
light. Keep this arrangement near the window. Place the glass prism in
such manner that the sunlight through the window falls on one side of
the prism and then on the white wall. You can see that the light
reflected on the wall has several colors. The prism splits the white
light into seven different colors. This splitting of white light into many
colors is called as a dispersion of light.
This shows that the sunlight consists of several colors. Sometimes in
the rainbow, you may not see all the seven colors. This is because of
the colors overlap each other. The degree of bending of the light’s
path depends on the angle that the incident beam of light makes with
the surface, and on the ratio between the refractive indices of the two
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media (Snell’s law) and results in splitting all the seven colors present
in white light.
Spectrum
It is always not necessary that the source of light should always be
white to get a spectrum. The composite light which contains a range of
three to four colors also produces a band of three to four colors. A
glowing 40W tungsten filament bulb does not produce pure white light.
It is a source of composite light and will also produce a spectrum but
the spectrum may not be the same as the spectrum of white light. The
type of spectrum depends upon the nature of the source of light.
Dispersion of Light In Daily Life
Examples of dispersion in our daily life:
● After the rains, we see the rainbow in the sky which is due to
the dispersion of the sunlight.
● During rainy days when the roads are wet and you drive the car
or ride a bike sometimes the water spills on the road. When the
petrol mixes with the water we can see different colors.
● When sunlight passes through the prism the light splits into
different colors.
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● Dispersion of colors in soap bubbles.
● Dispersion of colors on CDs.
Question For You
Q. Blue color of sea water is due to:
A. Interference of sunlight reflected from the water surface.
B. Scattering of sunlight by water molecules
C. Image of sky in water
D. Refraction of sunlight
Ans: A. The ocean looks blue because red, orange and yellow are absorbed
more strongly by the water than the blue.
Images Formed By Lenses
Have you ever noticed the watch repairer? How does he manage to
locate and fix such tiny particles of the watch? What do you think
does he uses to view such small parts? Yes, he uses an eyepiece that
has a lens that magnifies the parts of the watch. So let us now study
the types of lenses and also the images formed by lenses.
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Images Formed by Lenses
A lens is a part of a transparent thick glass which is bounded by two
spherical surfaces. It is an optical device through which the rays of
light converge or diverge before transmitting.
Types of Images Formed by Lenses
Lenses are of two kinds: Convex Lenses and Concave Lenses.
Convex Lenses
A convex lens is thicker in the middle and thinner at the edges. A
convex lens is also known as a “biconvex lens” because of two
spherical surfaces bulging outwards.
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Convex lenses include lenses that are plano-convex (i.e. these lenses
are flat on one side and bulged outward on the other), and convex
meniscus (i.e. these lenses are curved inward on one side and on the
outer side it’s curved more strongly).
A concave lens is thicker at the edges and thinner in the middle. A
concave lens is also known as a “biconcave lens” because of two
spherical surfaces bulging inwards.
Concave lenses include lenses like plano-concave (i.e. these lenses are
flat on one side and curved inward on the other), and concave
meniscus (i.e. these lenses are curved inward on one side and on the
outer side it’s curved less strongly).
Image Formation by Concave Lens
In case of the concave lens, we always get erect images, diminished
images and virtual images.
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Object location Image location Image nature Image size
Infinity At F2 Virtual and Erect Highly Diminished
Beyond infinity
and 0 Between F1 and
Optical centre Virtual and Erect Diminished
Here if the object is very far away, the images formed by lenses will
be all the more diminished.
Image Formation by Convex Lens
In case of a convex lens, if we bring the object close to the lens, the
size of the image keeps on increasing. As you bring the object more
close to the lens, we get the image all the more enlarged. So here we
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can say that the images formed can be of a variety of types. We can
have diminished inverted image, small sizes inverted image, enlarged
inverted image, enlarged erect image. So in a concave lens, there is a
possibility of getting a real as well as an inverted image.
Object location Image location Image nature Image size
Infinity At F2 Real and Inverted Diminished
Beyond 2F1 Between 2F2 and
F2 Real and Inverted Diminished
Between 2F1 and
F1 Beyond 2F2 Real and Inverted Enlarged
At F1 At infinity Real and Inverted Enlarged
At 2 F1 At 2F2 Real and Inverted Same size
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Between F1 and 0 On the same side
as object Virtual and Erect Enlarged
Images Formed in Different Lenses
Which lens would you use as a magnifying glass?
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Looking at the image what do you think? What kind of image is
formed? Yes, an enlarged image is formed. We cannot get such image
using a concave lens because the concave lens always produces the
diminished image. So this is a convex lens.
Images through lenses as real, virtual, erect or magnified
The figure shows the virtual image formed by the convex lens and real
image formed by a concave lens.
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Learn more about the Different type of Mirrors here.
Solved Examples for You
Question. How should people wearing spectacles work with a
microscope?
A. Should keep wearing their spectacles
B. They should never use the microscope
C. Should take off their spectacles
D. They should either keep wearing their spectacles or take off
their spectacles.
Solution: Option C. If operator using microscope usually wears
spectacles for working at their PC’ s, they often need to remove them
while looking through the microscope so they can align their eyes
correctly.
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