this is light ch in ncert science book class 7

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Chapter-15
LIGHT

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What is light?
Light is a radiation or a form of energy
that our eyes can detect. Light
enables us to view our surroundings.
Light travels from one place to
another in a straight line For instance,
if you look at the flame of a candle
with a straight pipe we can easily view
the candle. However, if we bend the
pipe we cannot view the candle and
the light coming through it because it
is blocked.

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Reflection of Light
Whenever light hits an object it is either absorbed or reflected back.
Reflection of light can be defined as the phenomenon of an object throws back
the light that falls on it. Hence, the reflection of light changes its path.
A mirror is generally any shiny surface that can reflect back light.
A mirror that has a plane surface is called a Plane Mirror.
A mirror that is curved, it either bulges in or out, is called a Curved Mirror
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https://youtu.be/dwxaq4c9K6k
What is an image

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Image of An Object
Generally, when we look into a mirror, then we see our face. Actually, what we see in the mirror it
is exactly a reflection of our face, hence it is known as an image of our face. In this case, our face
is the object and what we see in the mirror is its image. The image of our face seen in the mirror is
formed where light rays, after reflection from the mirror, seems to originate from. The image of our
face appears to be situated behind the mirror.
There are two types of images:
Real image: It is an image which can be obtained on a screen, e.g. the image formed on a
cinema screen. When the light rays coming from an object actually meet at a point after reflection
from the mirror, then it results in the formation of a real image.
Virtual image: It is an image which cannot be obtained on a screen, e.g. image formed by a
plane mirror. When the light rays coming from an object appear to meet after reflection from the
mirror, then it results in the formation of virtual image. It is not possible to form a virtual image on
the screen because light rays actually do not pass the screen or cannot be received on a screen.
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What is an image
As a mirror reflects light, an image of the object that is in front of the mirror is formed on it.
The image of an object can be defined as the impression of the object created by the light on
the mirror.
The distance between the image and the mirror, and the object and the mirror always
remain the same.
If we increase or decrease the distance between the object and the mirror, the distance
between the image and the mirror also increases or decreases, respectively.
However, the size of the image formed on the mirror can vary with respect to the distance
between the object and the mirror.

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If the distance between the object and the mirror increases, the size of the image
decreases and vice-versa.
An image is said to be erect if the image is formed the same side up as that of the object
The image will be called Inverted if it is formed upside-down compared to the object
The image formed by the mirror is always left-right inverted. This means that the right side
of the object appears as the left side of the image, and the left side of the object appears at
the right side of the image.

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The Laws of Reflection of Light
Incident Ray - The light ray that falls on the reflecting surface is called an Incident Ray.
Reflected Ray –The light ray that gets reflected back from a reflecting surface is called a
Reflected Ray.
Normal - It is a line that is perpendicular to the reflected plane at the point of incidence of
Incident Ray

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Types of Reflection
Depending upon the surface of the reflecting object, the reflection of light can vary.
1.Diffused Reflection or Irregular Reflection: In this type of reflection, the light
rays that fall on the surface are reflected back in different directions irregularly.
This generally happens in the case of an irregular or roughly surfaced object.
2.Regular Reflection: In this type of reflection, the light rays that fall on the
surface of the reflecting object reflect back in a particular direction. The reflected
rays are always parallel to each other. This generally happens in case of a smooth
and shiny surface

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https://www.liveworksheets.com/rg19038fa ( work sheet)

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Spherical Mirrors
Spherical mirror, as the name suggests, has a sphere-like shape. It
appears as if it is a part of a sphere. There are two types of spherical
mirrors:
❖ Concave Mirror - It is a spherical mirror whose reflecting surface
is curved inwards.The concave mirror reflects the parallel rays of
light in such a way that after reflection, all the rays converge (or
meet) at one point called focus in front of the mirror. Since a
concave mirror converges a beam of parallel light rays. Therefore,
a concave mirror is also known as a converging mirror.

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Application of Concave Mirrors
● To see the large image of teeth of a
patient, concave mirrors are used by
the dentist.
● In torches, headlights of vehicles and
searchlights to get a strong, straight
beam of light, etc., concave mirrors
are used as reflectors.
● To see a large image of the face,
then concave mirrors are used as
shaving mirrors.

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❖ Convex Mirror - It is a spherical mirror whose reflecting surface is
curved outwards.After reflection from the convex mirror, the
parallel rays of light are spreading out. When the parallel rays of
light spread out, we can say that the rays of light are diverging.
Now, we can say that a beam of parallel light rays diverges
(spreads out) after reflection from a convex mirror.
Since a convex mirror diverges a beam of parallel light rays,
therefore, it is also known as a diverging mirror

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Uses of Convex Mirrors
● To see the traffic at the rear side or
backside on the road, convex mirrors are
used as rear view mirrors or side view
mirrors in vehicles such as cars,
scooters, buses, etc.
● Big convex mirrors are used as shop and
ATM security mirrors. By installing a
convex mirror in the shop, the shop
owner can keep an eye on the
customers.

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Image formed by Plane mirror
1. Always Virtual
2. Always erect
3. size is always same as object
4. Distance from mirror is always
same as object.
Images formed by Convex and Concave mirror

16. Nature of
image
Concave mirror Convex mirror
Real or virtual a.virtual image when the object is between focus and
pole
b. real for all other positions
always virtual
Erect or
inverted
a. erect image when the object is between focus and
pole
b. inverted for all other positions
always erect
size a. magnified when object is between Focus and pole
b. magnified when the object is between Focus and
center of curvature
c. diminished when the object is beyond curvature
always
diminished
distance from
the mirro
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Difference between image formed by concave and convex mirror

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Lenses
Since a lens is a piece of transparent glass bound by the two spherical
surfaces. Lenses are transparent so that light can pass through lenses.
Lenses are of two types:
❖ Convex Lens - A Convex Lens is curved outwards. It is thicker in the centre
and narrows down at the edges. It merges the light rays passing through it at
a certain point. Therefore, it is also called a Converging Lens.
❖ Concave Lens - A Concave Lens is curved inwards. It has wider edges and
a thinner centre. It reflects back the light that travels through it in different
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Uses of Convex Lenses
● Convex lenses are used as a magnifying glass.
● In the manufacturing of spectacles, camera, microscope,
telescope and binoculars, convex lenses are used
Uses of Concave Lenses
● In order to see the image of the person standing outside, concave
lenses are used in the peepholes in the door of hotel rooms.
● Concave lenses are used in making spectacles

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Images formed by Convex and Concave Lenses
A Convex lens forms an image that is:
real
inverted
the image is large and appears close to the lens
A Concave lens forms an image that is:
virtual
erect
small and appears far away

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Sunlight
What is a Rainbow?
A rainbow appears as an arc on the sky that contains a band of
seven colours – Red, orange, yellow, green, blue, indigo and violet.

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❖ A rainbow is a natural phenomenon in which the light rays of the sun are reflected and
refracted by the water droplets present in the atmosphere
❖ This also means that the white light of the sun contains seven coloured lights in it that
separate out due to refraction (called a Spectrum of Lights). This spectrum of white
light can be seen in the following:
➢ Rainbows
➢ Soap bubbles
➢ Surface of a CD
➢ Prisms

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Newton's Disc
The Newton’s disc can be obtained by dividing a disk into 7 partitions
and painting each of them with the seven colours of the rainbow.
When the disc is rotated at a fast pace in daylight all the colours tend
to mix together and the disc appears whitish in colour

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https://quizlet.com/au/459227640/law-of-reflection-flash-cards/
https://www.liveworksheets.com/rg19038fa ( work sheet)
https://youtu.be/dwxaq4c9K6k
https://youtu.be/PF4_liKm-WQ ( practical with reflection)
https://youtu.be/ETF2-Zz3J18 ( practical with light ray)