NCERT Textbook - Light Class 7 Notes | EduRev

Science Class 7

Class 7 : NCERT Textbook - Light Class 7 Notes | EduRev

 Page 1


SCIENCE 174
Light
15
Y
ou might have seen a beam of
sunlight when it enters a room
through a narrow opening or a hole.
You may have also seen beams of light from
the headlamps of scooters, cars and engines
of trains [Fig. 15.1 (a)]. Similarly, a beam
of light can be seen from a torch. Some of
(a) Rail engine
Fig. 15.1  Beams of light
(b) Light house
Fig. 15.2  Looking at a candle through a straight and a bent pipe
you may have seen a beam of searchlight
from a light house or from an airport
tower [Fig. 15.1 (b)].
What do these experiences suggest?
15.1 LIGHT TRAVELS ALONG A
STRAIGHT LINE
Boojho recalls an activity he performed
in Class VI. In that activity he looked
(a) (b)
2020-21
Page 2


SCIENCE 174
Light
15
Y
ou might have seen a beam of
sunlight when it enters a room
through a narrow opening or a hole.
You may have also seen beams of light from
the headlamps of scooters, cars and engines
of trains [Fig. 15.1 (a)]. Similarly, a beam
of light can be seen from a torch. Some of
(a) Rail engine
Fig. 15.1  Beams of light
(b) Light house
Fig. 15.2  Looking at a candle through a straight and a bent pipe
you may have seen a beam of searchlight
from a light house or from an airport
tower [Fig. 15.1 (b)].
What do these experiences suggest?
15.1 LIGHT TRAVELS ALONG A
STRAIGHT LINE
Boojho recalls an activity he performed
in Class VI. In that activity he looked
(a) (b)
2020-21
LIGHT 175
Fig. 15.3  Reflection of objects in water
at a lighted candle first through
a straight pipe and then through
a bent pipe (Fig. 15.2). Why was
Boojho not able to see the candle
flame through a bent pipe?
This activity showed that
light travels along straight lines.
How can we change the path
of light? Do you know, what
happens when light falls on a
polished or a shiny surface?
15.2  REFLECTION OF LIGHT
One way to change the direction
of light is to let it fall on a shiny
surface. For example, a shining
stainless steel plate or a shining
steel spoon can change the
direction of light. The surface of
water can also act like a mirror
and change the path of light.
Have you ever seen the reflection
of trees or buildings in water
(Fig. 15.3)?
Any polished or a shiny surface can
act as a mirror. What happens when
light falls on a mirror?
You have learnt in Class VI that a
mirror changes the direction of light that
falls on it. This change of direction by a
mirror is called reflection of light. Can
you recall the activity in which you got
the light of a torch reflected from a
mirror? Let us perform a similar activity.
Activity 15.1
Take a torch. Cover its glass with a chart
paper which has three narrow slits as
shown in Fig. 15.5. Spread a sheet of
Paheli remembers the story of the lion and the
rabbit from the Panchtantra, in which the
rabbit fooled the lion by showing him his
reflection in water (Fig. 15.4).
Fig. 15.4  Reflection of the lion in water
2020-21
Page 3


SCIENCE 174
Light
15
Y
ou might have seen a beam of
sunlight when it enters a room
through a narrow opening or a hole.
You may have also seen beams of light from
the headlamps of scooters, cars and engines
of trains [Fig. 15.1 (a)]. Similarly, a beam
of light can be seen from a torch. Some of
(a) Rail engine
Fig. 15.1  Beams of light
(b) Light house
Fig. 15.2  Looking at a candle through a straight and a bent pipe
you may have seen a beam of searchlight
from a light house or from an airport
tower [Fig. 15.1 (b)].
What do these experiences suggest?
15.1 LIGHT TRAVELS ALONG A
STRAIGHT LINE
Boojho recalls an activity he performed
in Class VI. In that activity he looked
(a) (b)
2020-21
LIGHT 175
Fig. 15.3  Reflection of objects in water
at a lighted candle first through
a straight pipe and then through
a bent pipe (Fig. 15.2). Why was
Boojho not able to see the candle
flame through a bent pipe?
This activity showed that
light travels along straight lines.
How can we change the path
of light? Do you know, what
happens when light falls on a
polished or a shiny surface?
15.2  REFLECTION OF LIGHT
One way to change the direction
of light is to let it fall on a shiny
surface. For example, a shining
stainless steel plate or a shining
steel spoon can change the
direction of light. The surface of
water can also act like a mirror
and change the path of light.
Have you ever seen the reflection
of trees or buildings in water
(Fig. 15.3)?
Any polished or a shiny surface can
act as a mirror. What happens when
light falls on a mirror?
You have learnt in Class VI that a
mirror changes the direction of light that
falls on it. This change of direction by a
mirror is called reflection of light. Can
you recall the activity in which you got
the light of a torch reflected from a
mirror? Let us perform a similar activity.
Activity 15.1
Take a torch. Cover its glass with a chart
paper which has three narrow slits as
shown in Fig. 15.5. Spread a sheet of
Paheli remembers the story of the lion and the
rabbit from the Panchtantra, in which the
rabbit fooled the lion by showing him his
reflection in water (Fig. 15.4).
Fig. 15.4  Reflection of the lion in water
2020-21
SCIENCE 176
chart paper on a smooth wooden board.
Fix a plane mirror strip vertically on the
chart paper (Fig. 15.5). Now direct the
beam of light on the mirror from the
torch with slits. Place the torch in such
a way that its light is seen along the
chart paper on the board. Now adjust
its position so that the light from the
torch strikes the plane mirror at an
angle (Fig. 15.5).
Does the mirror change the direction
of light that falls on it? Now move the
torch slightly to either side. Do you find
any change in the direction of reflected
light?
Look into the mirror along the
direction of the reflected light. Do you
see the slits in the mirror? This is the
image of the slits.
This activity shows how light gets
reflected from a plane mirror.
Let us play around with the images
formed in mirrors and know a little more
about them.
Fig. 15.6  Image of a candle in a plane mirror
Fig. 15.5  Reflection of light from a mirror
Paheli wants to know, what
makes things visible to us?
Boojho thinks that objects are
visible only when light reflected
from them reaches our eyes. Do
you agree with him?
CAUTION
Handle the lighted candle with care.
It is better if this activity is performed
in the presence of a teacher or an elder
person.
Place a lighted candle in front of a plane
mirror. Try to see the flame of the candle
in the mirror. It appears as if a similar
candle is placed behind the mirror. The
candle, which appears behind the
mirror, is the image of the candle formed
by the mirror (Fig. 15.6). The candle
itself is the object.
Now move the candle to different
positions in front of the mirror. Observe
the image in each case.
Activity 15.2
2020-21
Page 4


SCIENCE 174
Light
15
Y
ou might have seen a beam of
sunlight when it enters a room
through a narrow opening or a hole.
You may have also seen beams of light from
the headlamps of scooters, cars and engines
of trains [Fig. 15.1 (a)]. Similarly, a beam
of light can be seen from a torch. Some of
(a) Rail engine
Fig. 15.1  Beams of light
(b) Light house
Fig. 15.2  Looking at a candle through a straight and a bent pipe
you may have seen a beam of searchlight
from a light house or from an airport
tower [Fig. 15.1 (b)].
What do these experiences suggest?
15.1 LIGHT TRAVELS ALONG A
STRAIGHT LINE
Boojho recalls an activity he performed
in Class VI. In that activity he looked
(a) (b)
2020-21
LIGHT 175
Fig. 15.3  Reflection of objects in water
at a lighted candle first through
a straight pipe and then through
a bent pipe (Fig. 15.2). Why was
Boojho not able to see the candle
flame through a bent pipe?
This activity showed that
light travels along straight lines.
How can we change the path
of light? Do you know, what
happens when light falls on a
polished or a shiny surface?
15.2  REFLECTION OF LIGHT
One way to change the direction
of light is to let it fall on a shiny
surface. For example, a shining
stainless steel plate or a shining
steel spoon can change the
direction of light. The surface of
water can also act like a mirror
and change the path of light.
Have you ever seen the reflection
of trees or buildings in water
(Fig. 15.3)?
Any polished or a shiny surface can
act as a mirror. What happens when
light falls on a mirror?
You have learnt in Class VI that a
mirror changes the direction of light that
falls on it. This change of direction by a
mirror is called reflection of light. Can
you recall the activity in which you got
the light of a torch reflected from a
mirror? Let us perform a similar activity.
Activity 15.1
Take a torch. Cover its glass with a chart
paper which has three narrow slits as
shown in Fig. 15.5. Spread a sheet of
Paheli remembers the story of the lion and the
rabbit from the Panchtantra, in which the
rabbit fooled the lion by showing him his
reflection in water (Fig. 15.4).
Fig. 15.4  Reflection of the lion in water
2020-21
SCIENCE 176
chart paper on a smooth wooden board.
Fix a plane mirror strip vertically on the
chart paper (Fig. 15.5). Now direct the
beam of light on the mirror from the
torch with slits. Place the torch in such
a way that its light is seen along the
chart paper on the board. Now adjust
its position so that the light from the
torch strikes the plane mirror at an
angle (Fig. 15.5).
Does the mirror change the direction
of light that falls on it? Now move the
torch slightly to either side. Do you find
any change in the direction of reflected
light?
Look into the mirror along the
direction of the reflected light. Do you
see the slits in the mirror? This is the
image of the slits.
This activity shows how light gets
reflected from a plane mirror.
Let us play around with the images
formed in mirrors and know a little more
about them.
Fig. 15.6  Image of a candle in a plane mirror
Fig. 15.5  Reflection of light from a mirror
Paheli wants to know, what
makes things visible to us?
Boojho thinks that objects are
visible only when light reflected
from them reaches our eyes. Do
you agree with him?
CAUTION
Handle the lighted candle with care.
It is better if this activity is performed
in the presence of a teacher or an elder
person.
Place a lighted candle in front of a plane
mirror. Try to see the flame of the candle
in the mirror. It appears as if a similar
candle is placed behind the mirror. The
candle, which appears behind the
mirror, is the image of the candle formed
by the mirror (Fig. 15.6). The candle
itself is the object.
Now move the candle to different
positions in front of the mirror. Observe
the image in each case.
Activity 15.2
2020-21
LIGHT 177
cannot be obtained on the screen in
either case.
What about the distance of the image
from mirror? Let us perform another
activity.
Activity 15.3
Take a chess board. If a chess board is
not available, draw on a chart paper 64
(8×8) squares of equal size. Draw a thick
line in the middle of the paper. Fix a
plane mirror vertically on this line. Place
any small object, such as a pencil
sharpner, at the boundary of the third
square counting from the mirror
(Fig. 15.7). Note the position of the
image. Now shift the object to the
boundary of the fourth square. Again
note the position of the image. Did you
find any relation between the distance
of the image from the mirror and that of
the object in front of it?
Was the image upright in each case?
Did the flame appear on top of the candle
as in the object? Such an image is called
erect. An image formed by a plane
mirror is erect and of the same size as
the object.
Now place a vertical screen behind
the mirror. Try to obtain the image of
the candle on this screen. Can you get
the image on the screen? Now place the
screen in front of the mirror. Can you
get the image on the screen now? You
will find that the image of the candle
Boojho noted in his notebook: Is
it not surprising that my image is
of the same size as me whether
the mirror is small or large?
Fig. 15.7  Locating image in a plane mirror
Paheli made a note in her notebook:
In a plane mirror the image is
formed behind the mirror. It is erect,
of the same size and is at the same
distance from the mirror as the
object is in front of it.
You will find that the image
is at the same distance behind
the mirror as the object is in front
of it. Now verify this by placing
the object anywhere on the chart
paper.
2020-21
Page 5


SCIENCE 174
Light
15
Y
ou might have seen a beam of
sunlight when it enters a room
through a narrow opening or a hole.
You may have also seen beams of light from
the headlamps of scooters, cars and engines
of trains [Fig. 15.1 (a)]. Similarly, a beam
of light can be seen from a torch. Some of
(a) Rail engine
Fig. 15.1  Beams of light
(b) Light house
Fig. 15.2  Looking at a candle through a straight and a bent pipe
you may have seen a beam of searchlight
from a light house or from an airport
tower [Fig. 15.1 (b)].
What do these experiences suggest?
15.1 LIGHT TRAVELS ALONG A
STRAIGHT LINE
Boojho recalls an activity he performed
in Class VI. In that activity he looked
(a) (b)
2020-21
LIGHT 175
Fig. 15.3  Reflection of objects in water
at a lighted candle first through
a straight pipe and then through
a bent pipe (Fig. 15.2). Why was
Boojho not able to see the candle
flame through a bent pipe?
This activity showed that
light travels along straight lines.
How can we change the path
of light? Do you know, what
happens when light falls on a
polished or a shiny surface?
15.2  REFLECTION OF LIGHT
One way to change the direction
of light is to let it fall on a shiny
surface. For example, a shining
stainless steel plate or a shining
steel spoon can change the
direction of light. The surface of
water can also act like a mirror
and change the path of light.
Have you ever seen the reflection
of trees or buildings in water
(Fig. 15.3)?
Any polished or a shiny surface can
act as a mirror. What happens when
light falls on a mirror?
You have learnt in Class VI that a
mirror changes the direction of light that
falls on it. This change of direction by a
mirror is called reflection of light. Can
you recall the activity in which you got
the light of a torch reflected from a
mirror? Let us perform a similar activity.
Activity 15.1
Take a torch. Cover its glass with a chart
paper which has three narrow slits as
shown in Fig. 15.5. Spread a sheet of
Paheli remembers the story of the lion and the
rabbit from the Panchtantra, in which the
rabbit fooled the lion by showing him his
reflection in water (Fig. 15.4).
Fig. 15.4  Reflection of the lion in water
2020-21
SCIENCE 176
chart paper on a smooth wooden board.
Fix a plane mirror strip vertically on the
chart paper (Fig. 15.5). Now direct the
beam of light on the mirror from the
torch with slits. Place the torch in such
a way that its light is seen along the
chart paper on the board. Now adjust
its position so that the light from the
torch strikes the plane mirror at an
angle (Fig. 15.5).
Does the mirror change the direction
of light that falls on it? Now move the
torch slightly to either side. Do you find
any change in the direction of reflected
light?
Look into the mirror along the
direction of the reflected light. Do you
see the slits in the mirror? This is the
image of the slits.
This activity shows how light gets
reflected from a plane mirror.
Let us play around with the images
formed in mirrors and know a little more
about them.
Fig. 15.6  Image of a candle in a plane mirror
Fig. 15.5  Reflection of light from a mirror
Paheli wants to know, what
makes things visible to us?
Boojho thinks that objects are
visible only when light reflected
from them reaches our eyes. Do
you agree with him?
CAUTION
Handle the lighted candle with care.
It is better if this activity is performed
in the presence of a teacher or an elder
person.
Place a lighted candle in front of a plane
mirror. Try to see the flame of the candle
in the mirror. It appears as if a similar
candle is placed behind the mirror. The
candle, which appears behind the
mirror, is the image of the candle formed
by the mirror (Fig. 15.6). The candle
itself is the object.
Now move the candle to different
positions in front of the mirror. Observe
the image in each case.
Activity 15.2
2020-21
LIGHT 177
cannot be obtained on the screen in
either case.
What about the distance of the image
from mirror? Let us perform another
activity.
Activity 15.3
Take a chess board. If a chess board is
not available, draw on a chart paper 64
(8×8) squares of equal size. Draw a thick
line in the middle of the paper. Fix a
plane mirror vertically on this line. Place
any small object, such as a pencil
sharpner, at the boundary of the third
square counting from the mirror
(Fig. 15.7). Note the position of the
image. Now shift the object to the
boundary of the fourth square. Again
note the position of the image. Did you
find any relation between the distance
of the image from the mirror and that of
the object in front of it?
Was the image upright in each case?
Did the flame appear on top of the candle
as in the object? Such an image is called
erect. An image formed by a plane
mirror is erect and of the same size as
the object.
Now place a vertical screen behind
the mirror. Try to obtain the image of
the candle on this screen. Can you get
the image on the screen? Now place the
screen in front of the mirror. Can you
get the image on the screen now? You
will find that the image of the candle
Boojho noted in his notebook: Is
it not surprising that my image is
of the same size as me whether
the mirror is small or large?
Fig. 15.7  Locating image in a plane mirror
Paheli made a note in her notebook:
In a plane mirror the image is
formed behind the mirror. It is erect,
of the same size and is at the same
distance from the mirror as the
object is in front of it.
You will find that the image
is at the same distance behind
the mirror as the object is in front
of it. Now verify this by placing
the object anywhere on the chart
paper.
2020-21
SCIENCE 178
Fig. 15.8  Left hand appears on the right side in
the image
Fig. 15.9  An ambulance
mirror. How does it appear in the
mirror?
15.3  RIGHT OR LEFT!
When you see your image in a plane
mirror, is it exactly like you? Have you
ever noticed that there is one interesting
difference between you and your image
in a mirror? Let us find out.
Activity 15.4
Stand in front of a plane mirror and look
at your image. Raise your left hand.
Which hand does your image raise
(Fig. 15.8)? Now touch your right ear.
Which ear does your hand touch in your
image? Observe carefully. You will find
that in the mirror the ‘right’ appears ‘left’
and the ‘left’ appears ‘right’. Note that
only sides are interchanged; the image
does not appear upside down.
Now write down your name on a piece
of paper and hold it in front of a plane
Boojho saw an ambulance on
the road. He was surprised to
see that the word ‘AMBULANCE’
in front was written in a strange
manner.
Can you now understand
why the word ‘AMBULANCE’ is written
as in Fig. 15.9? When the driver of a
vehicle ahead of an ambulance looks in
her/his rear view mirror, she/he can
read ‘AMBULANCE’ written on it and
give way to it. It is the duty of every one
of us to allow an ambulance to pass
without blocking its way.
You might have observed that in the
side mirror of a scooter or a car the
images of all the objects appear smaller
than the objects themselves. Have you
ever wondered why is it so?
2020-21
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