1. What is a lens?
2. What is the other name of convex lens
3. What do you understand by the term refraction of light?
4. If speed of light in vacuum = 3.0×10ms , and refractive index of water = 4/3, what is the 8-1 speed of light in water?
5. What is Snell’s law of refraction?
6. Can one burn a piece of paper in daylight by just using a convex lens instead of a match or any direct flame? Support your answer with the help of an appropriate ray diagram.
7.Discuss ray diagrams for various positions of the object in front of concave lens.
8.If an object of 7 cm height is placed at a distance of 12 cm from a convex lens of focal length 8cm, find the position, nature and height of the image formed.
9. Explain the formation of mirage in deserts.
SOLUTIONS
1. A lens is an optical system with two refracting surfaces.
2. Converging lens or positive lens.
3. The bending of light when it travels from one medium into another.
4. We have
Refractive index of water = speed of light in vacuum/speed of light in water
4
3
.
0
×
10
ms
8 -
1
=
3
speed
of
light
in
water
×
×
Speed of light in water = (3.0
10
ms
)
3/4
8
–1
×
= (9.0/4)
10
ms
8
- 1
×
= 2.25
10
ms
8
- 1
5. According to Snell’s law of refraction:
When a ray of light undergoes refraction, (while passing from a medium 1 to another medium
2), the ratio of the sine of the angle of incidence to the sine of the angle of refraction, is a
constant. We call this (constant) ratio as the refractive index of medium 2 with respect to
medium 1 and denote it by
. Thus,
= sin i/sin r.
µ
µ
2
2
1
1
6. Yes, it is possible to burn a piece of paper, in daylight, by using a convex lens instead of a
match or any other direct flame.
We know that a convex lens brings a beam of parallel incident rays to a focus at its focal point.
The rays of the sun (practically an infinite distance source) form a parallel incident
beam. When they are allowed to fall on a convex lens, they get focused at its focal point. If a
piece of paper is kept at this focal point, all the (large) heat energy of this parallel beam gets
concentrated at this point. This (concentrated heat energy) is often sufficient to burn the piece
of paper. The required ray diagram is as shown in Figure.
Principal
•
•
F
•
Axis
F
C
7. Image formation by a concave lens.
S.N. Position of
Position of
Nature of
Uses
object
image
image
1. At infinity Appears at the
Virtual erect and
Spectacles for
principal focus on
diminished
short sightedness
the same side as
that of the object
2. Between infinity
Appears between
Virtual, erect and
Spectacles for
and the lens
the principal
diminished
short sightedness
focus and the lens
8. For convex lens: u = –12 cm; f = + 8 cm; v =?
1 =
1
1
1 +
1
1
Now
or
-
=
v
u
f
v
f
u
1
1 -
1
1
1
=
+
=
v
8
cm
-
12
cm
8
cm
12
cm
( -
12
8
)
cm
4
1
=
=
=
96
cm
96
cm
24
cm
2
Distance of image, v = + 24 cm on the right side of lens.
24
cm
v
Now magnification, m =
=
= –1
-
u
12
cm
Image is magnified (i.e., larger than the object), but inverted (due to negative sign of m).
h
h
Also m =
= – 2 =
im ag e
i ma g e
h
+
7
cm
o bj e ct
or height of image, h
= – 2
7 cm = – 14 cm or 14 cm inverted.
i ma ge
9. An
optical mirage is an image of an
object that appears to be in a location other
than the correct one, as the result of
abnormal atmospheric conditions
. Heat
radiating from a hot earth surface, such as a
desert, causes a reduction in air density just
above the surface. Hence the density of the air
increases from bottom to top continuously. The
boundaries
between the hotter and colder layers
bend light rays from a distant object
continuously. After some time the rays undergo total internal reflection and when the rays
finally enter the observer’s eyes as shown in figure the image produced by the rays appears inve rted
and below the real object. Just as an image reflected in water appears when observed from a
distance.
2. What is the other name of convex lens
3. What do you understand by the term refraction of light?
4. If speed of light in vacuum = 3.0×10ms , and refractive index of water = 4/3, what is the 8-1 speed of light in water?
5. What is Snell’s law of refraction?
6. Can one burn a piece of paper in daylight by just using a convex lens instead of a match or any direct flame? Support your answer with the help of an appropriate ray diagram.
7.Discuss ray diagrams for various positions of the object in front of concave lens.
8.If an object of 7 cm height is placed at a distance of 12 cm from a convex lens of focal length 8cm, find the position, nature and height of the image formed.
9. Explain the formation of mirage in deserts.
SOLUTIONS
1. A lens is an optical system with two refracting surfaces.
2. Converging lens or positive lens.
3. The bending of light when it travels from one medium into another.
4. We have
Refractive index of water = speed of light in vacuum/speed of light in water
4
3
.
0
×
10
ms
8 -
1
=
3
speed
of
light
in
water
×
×
Speed of light in water = (3.0
10
ms
)
3/4
8
–1
×
= (9.0/4)
10
ms
8
- 1
×
= 2.25
10
ms
8
- 1
5. According to Snell’s law of refraction:
When a ray of light undergoes refraction, (while passing from a medium 1 to another medium
2), the ratio of the sine of the angle of incidence to the sine of the angle of refraction, is a
constant. We call this (constant) ratio as the refractive index of medium 2 with respect to
medium 1 and denote it by
. Thus,
= sin i/sin r.
µ
µ
2
2
1
1
6. Yes, it is possible to burn a piece of paper, in daylight, by using a convex lens instead of a
match or any other direct flame.
We know that a convex lens brings a beam of parallel incident rays to a focus at its focal point.
The rays of the sun (practically an infinite distance source) form a parallel incident
beam. When they are allowed to fall on a convex lens, they get focused at its focal point. If a
piece of paper is kept at this focal point, all the (large) heat energy of this parallel beam gets
concentrated at this point. This (concentrated heat energy) is often sufficient to burn the piece
of paper. The required ray diagram is as shown in Figure.
Principal
•
•
F
•
Axis
F
C
7. Image formation by a concave lens.
S.N. Position of
Position of
Nature of
Uses
object
image
image
1. At infinity Appears at the
Virtual erect and
Spectacles for
principal focus on
diminished
short sightedness
the same side as
that of the object
2. Between infinity
Appears between
Virtual, erect and
Spectacles for
and the lens
the principal
diminished
short sightedness
focus and the lens
8. For convex lens: u = –12 cm; f = + 8 cm; v =?
1 =
1
1
1 +
1
1
Now
or
-
=
v
u
f
v
f
u
1
1 -
1
1
1
=
+
=
v
8
cm
-
12
cm
8
cm
12
cm
( -
12
8
)
cm
4
1
=
=
=
96
cm
96
cm
24
cm
2
Distance of image, v = + 24 cm on the right side of lens.
24
cm
v
Now magnification, m =
=
= –1
-
u
12
cm
Image is magnified (i.e., larger than the object), but inverted (due to negative sign of m).
h
h
Also m =
= – 2 =
im ag e
i ma g e
h
+
7
cm
o bj e ct
or height of image, h
= – 2
7 cm = – 14 cm or 14 cm inverted.
i ma ge
9. An
optical mirage is an image of an
object that appears to be in a location other
than the correct one, as the result of
abnormal atmospheric conditions
. Heat
radiating from a hot earth surface, such as a
desert, causes a reduction in air density just
above the surface. Hence the density of the air
increases from bottom to top continuously. The
boundaries
between the hotter and colder layers
bend light rays from a distant object
continuously. After some time the rays undergo total internal reflection and when the rays
finally enter the observer’s eyes as shown in figure the image produced by the rays appears inve rted
and below the real object. Just as an image reflected in water appears when observed from a
distance.
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