Paheli noticed that if she held a paper under a specific lens in the bright sunlight, it caught fire.Give reason to explain this phenomenon and identify the nature of the lens.
Answers
You 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)
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 slits as shown in
Fig. 15.5. Spread a sheet of chart paper
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).
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
Explanation:
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