Textbook Solutions: Lenses | Science and Technology Class 10 (Maharashtra SSC Board) PDF Download

 Page 1


Lenses 
Exercise 
Q. 1. Match the columns in the following table and explain them. 
 
 
Answer : 
 
(i) Farsightedness 
? It is also known as Hypermetropia. 
? In this defect of vision, the closer objects are blurred 
? It occurs due to the flat cornea or due to the incorrect curvature of the eye lens. 
(ii) Nearsightedness 
? It is also known as Myopia 
? In this defect vision of distant objects are blurred. 
? It occurs due to due to defect in eye lense as the image is formed in front of the retina 
and not on it. 
? It is most common types of an eye defect. 
Q. 2. Draw a figure explaining various terms related to a lens. 
Answer : i. Optical Centre: 
Page 2


Lenses 
Exercise 
Q. 1. Match the columns in the following table and explain them. 
 
 
Answer : 
 
(i) Farsightedness 
? It is also known as Hypermetropia. 
? In this defect of vision, the closer objects are blurred 
? It occurs due to the flat cornea or due to the incorrect curvature of the eye lens. 
(ii) Nearsightedness 
? It is also known as Myopia 
? In this defect vision of distant objects are blurred. 
? It occurs due to due to defect in eye lense as the image is formed in front of the retina 
and not on it. 
? It is most common types of an eye defect. 
Q. 2. Draw a figure explaining various terms related to a lens. 
Answer : i. Optical Centre: 
The center point of a lens that lies on the principal axis of the lens is called its optical 
center. The optical center is represented by letter C. 
ii. Principal Axis 
The principal axis is a straight line passing through the optical center and the center of 
curvature of two surfaces of a lens. It is represented by letter P 
iii. Principal Focus 
The principal focus is a point on its principal axis to which the light rays parallel to the 
principal axis converge (in case of convex lens) or appear to diverge (in case of the 
concave lens) after passing through it. It is represented by letter F 
iv. Focal Length 
The focal length of a lens is the distance between its optical center and principal focus. 
It is represented by letter f 
F = Principal focus 
2F = Pole of the lens 
C = Optical centre 
P = Principal axis 
 
Q. 3. At which position will you keep an object in front of a convex lens so as to 
get a real image of the same size as the object? Draw a figure. 
Answer : In case of convex lens when the object is at “2F1”(twice of focal length), a 
real, inverted image of the same size is formed at “2F2” 
Explanation: 
When the object is placed at the center of curvature of a lens then a ray of light AO 
which is parallel to the principal axis after refraction pass through the focus F along the 
Page 3


Lenses 
Exercise 
Q. 1. Match the columns in the following table and explain them. 
 
 
Answer : 
 
(i) Farsightedness 
? It is also known as Hypermetropia. 
? In this defect of vision, the closer objects are blurred 
? It occurs due to the flat cornea or due to the incorrect curvature of the eye lens. 
(ii) Nearsightedness 
? It is also known as Myopia 
? In this defect vision of distant objects are blurred. 
? It occurs due to due to defect in eye lense as the image is formed in front of the retina 
and not on it. 
? It is most common types of an eye defect. 
Q. 2. Draw a figure explaining various terms related to a lens. 
Answer : i. Optical Centre: 
The center point of a lens that lies on the principal axis of the lens is called its optical 
center. The optical center is represented by letter C. 
ii. Principal Axis 
The principal axis is a straight line passing through the optical center and the center of 
curvature of two surfaces of a lens. It is represented by letter P 
iii. Principal Focus 
The principal focus is a point on its principal axis to which the light rays parallel to the 
principal axis converge (in case of convex lens) or appear to diverge (in case of the 
concave lens) after passing through it. It is represented by letter F 
iv. Focal Length 
The focal length of a lens is the distance between its optical center and principal focus. 
It is represented by letter f 
F = Principal focus 
2F = Pole of the lens 
C = Optical centre 
P = Principal axis 
 
Q. 3. At which position will you keep an object in front of a convex lens so as to 
get a real image of the same size as the object? Draw a figure. 
Answer : In case of convex lens when the object is at “2F1”(twice of focal length), a 
real, inverted image of the same size is formed at “2F2” 
Explanation: 
When the object is placed at the center of curvature of a lens then a ray of light AO 
which is parallel to the principal axis after refraction pass through the focus F along the 
direction OF. While the other ray AC pass through the optical center C and goes straight 
without any deviation. These two refracted light rays intersect each other at point A’, on 
the other side of the lens at the center of curvature 2F. so, the image A’B’ formed in this 
case is at the center of curvature, of the same size as the object, real and inverted. 
 
Q. 4. Give scientific reasons: 
 
a. Simple microscope is used for watch repairs. 
b. One can sense colours only in bright light. 
c. We cannot clearly see an object kept at a distance less than 25 cm from the 
eye. 
Answer : (a) A simple microscope used in repairing watches has a convex lens of small 
focal length with magnification capacity up to 20 times. Therefore, it is used by watch 
repairers to check the smallest parts of the watch clearly and it does not cause any 
strain on eyes. 
(b) We can sense colours only in light because when it gets dark the cones ability to 
respond to light is lost and the rods continue to respond to available light. However 
cones cannot see color, everything looks like shades of black and white and gray. 
(c) Eyes can see its distant and close objects. It adjusts its focal length by contracting or 
relaxing ciliary muscles to see the objects. The ciliary muscles cannot be contracted 
below a certain minimum limit. Therefore we cannot see an object kept at a distance 
less than 25 cms from the eye. 
Q. 5. Explain the working of an astronomical telescope using refraction of light. 
Answer : In the astronomical telescopes the image is formed by bending of light also 
known as refraction so these telescopes are known as refracting telescopes. It is used 
to see the magnified images of heavenly bodies like stars, planets, etc. 
The construction of the telescope is given as: 
An astronomical telescope is made up of two convex lenses : 
i) an objective lens O and, 
ii) an eye piece E. 
Page 4


Lenses 
Exercise 
Q. 1. Match the columns in the following table and explain them. 
 
 
Answer : 
 
(i) Farsightedness 
? It is also known as Hypermetropia. 
? In this defect of vision, the closer objects are blurred 
? It occurs due to the flat cornea or due to the incorrect curvature of the eye lens. 
(ii) Nearsightedness 
? It is also known as Myopia 
? In this defect vision of distant objects are blurred. 
? It occurs due to due to defect in eye lense as the image is formed in front of the retina 
and not on it. 
? It is most common types of an eye defect. 
Q. 2. Draw a figure explaining various terms related to a lens. 
Answer : i. Optical Centre: 
The center point of a lens that lies on the principal axis of the lens is called its optical 
center. The optical center is represented by letter C. 
ii. Principal Axis 
The principal axis is a straight line passing through the optical center and the center of 
curvature of two surfaces of a lens. It is represented by letter P 
iii. Principal Focus 
The principal focus is a point on its principal axis to which the light rays parallel to the 
principal axis converge (in case of convex lens) or appear to diverge (in case of the 
concave lens) after passing through it. It is represented by letter F 
iv. Focal Length 
The focal length of a lens is the distance between its optical center and principal focus. 
It is represented by letter f 
F = Principal focus 
2F = Pole of the lens 
C = Optical centre 
P = Principal axis 
 
Q. 3. At which position will you keep an object in front of a convex lens so as to 
get a real image of the same size as the object? Draw a figure. 
Answer : In case of convex lens when the object is at “2F1”(twice of focal length), a 
real, inverted image of the same size is formed at “2F2” 
Explanation: 
When the object is placed at the center of curvature of a lens then a ray of light AO 
which is parallel to the principal axis after refraction pass through the focus F along the 
direction OF. While the other ray AC pass through the optical center C and goes straight 
without any deviation. These two refracted light rays intersect each other at point A’, on 
the other side of the lens at the center of curvature 2F. so, the image A’B’ formed in this 
case is at the center of curvature, of the same size as the object, real and inverted. 
 
Q. 4. Give scientific reasons: 
 
a. Simple microscope is used for watch repairs. 
b. One can sense colours only in bright light. 
c. We cannot clearly see an object kept at a distance less than 25 cm from the 
eye. 
Answer : (a) A simple microscope used in repairing watches has a convex lens of small 
focal length with magnification capacity up to 20 times. Therefore, it is used by watch 
repairers to check the smallest parts of the watch clearly and it does not cause any 
strain on eyes. 
(b) We can sense colours only in light because when it gets dark the cones ability to 
respond to light is lost and the rods continue to respond to available light. However 
cones cannot see color, everything looks like shades of black and white and gray. 
(c) Eyes can see its distant and close objects. It adjusts its focal length by contracting or 
relaxing ciliary muscles to see the objects. The ciliary muscles cannot be contracted 
below a certain minimum limit. Therefore we cannot see an object kept at a distance 
less than 25 cms from the eye. 
Q. 5. Explain the working of an astronomical telescope using refraction of light. 
Answer : In the astronomical telescopes the image is formed by bending of light also 
known as refraction so these telescopes are known as refracting telescopes. It is used 
to see the magnified images of heavenly bodies like stars, planets, etc. 
The construction of the telescope is given as: 
An astronomical telescope is made up of two convex lenses : 
i) an objective lens O and, 
ii) an eye piece E. 
The focal length fo of the objective lens of astronomical telescope is large as compared 
to the focal length fe of the eye piece. 
Working of the telescope: 
The ray diagram to show the working of the astronomical telescope is shown in figure 
below: 
 
i. A parallel beam of light from a star or a satellite falls on the objective lens of the 
telescope. 
ii. The objective lens forms a real, inverted and diminished image A’B’ of the heavenly 
body. 
iii. This image (A’B’) now acts as an object for the eye piece E, whose position is 
adjusted so that the image lies between the focus fe’ and the optical centre C2of the eye 
piece. 
iv. Now the eye piece forms a virtual, inverted and highly magnified image of object at 
infinity. When the final image of an object is formed at infinity, the telescope is said to be 
in ‘normal adjustment’. 
It should be noted that, the final image of object (such as stars, planets or satellites) 
formed by an astronomical telescope is always inverted with respect to the object. But it 
does not matter whether the image formed by an astronomical telescope is inverted or 
not, as all the heavenly bodies are usually spherical is shape. Astronomical telescopes 
bend light called as refraction. Due to this refraction the light rays which are parallel, 
converge at a focal point and those lines that are not parallel converge upon focal 
plane. The telescope converts a bundle of parallel rays and makes angle a and 
angle ß with a second parallel bundle. The ratio of  gives angular magnification to the 
telescope. 
Page 5


Lenses 
Exercise 
Q. 1. Match the columns in the following table and explain them. 
 
 
Answer : 
 
(i) Farsightedness 
? It is also known as Hypermetropia. 
? In this defect of vision, the closer objects are blurred 
? It occurs due to the flat cornea or due to the incorrect curvature of the eye lens. 
(ii) Nearsightedness 
? It is also known as Myopia 
? In this defect vision of distant objects are blurred. 
? It occurs due to due to defect in eye lense as the image is formed in front of the retina 
and not on it. 
? It is most common types of an eye defect. 
Q. 2. Draw a figure explaining various terms related to a lens. 
Answer : i. Optical Centre: 
The center point of a lens that lies on the principal axis of the lens is called its optical 
center. The optical center is represented by letter C. 
ii. Principal Axis 
The principal axis is a straight line passing through the optical center and the center of 
curvature of two surfaces of a lens. It is represented by letter P 
iii. Principal Focus 
The principal focus is a point on its principal axis to which the light rays parallel to the 
principal axis converge (in case of convex lens) or appear to diverge (in case of the 
concave lens) after passing through it. It is represented by letter F 
iv. Focal Length 
The focal length of a lens is the distance between its optical center and principal focus. 
It is represented by letter f 
F = Principal focus 
2F = Pole of the lens 
C = Optical centre 
P = Principal axis 
 
Q. 3. At which position will you keep an object in front of a convex lens so as to 
get a real image of the same size as the object? Draw a figure. 
Answer : In case of convex lens when the object is at “2F1”(twice of focal length), a 
real, inverted image of the same size is formed at “2F2” 
Explanation: 
When the object is placed at the center of curvature of a lens then a ray of light AO 
which is parallel to the principal axis after refraction pass through the focus F along the 
direction OF. While the other ray AC pass through the optical center C and goes straight 
without any deviation. These two refracted light rays intersect each other at point A’, on 
the other side of the lens at the center of curvature 2F. so, the image A’B’ formed in this 
case is at the center of curvature, of the same size as the object, real and inverted. 
 
Q. 4. Give scientific reasons: 
 
a. Simple microscope is used for watch repairs. 
b. One can sense colours only in bright light. 
c. We cannot clearly see an object kept at a distance less than 25 cm from the 
eye. 
Answer : (a) A simple microscope used in repairing watches has a convex lens of small 
focal length with magnification capacity up to 20 times. Therefore, it is used by watch 
repairers to check the smallest parts of the watch clearly and it does not cause any 
strain on eyes. 
(b) We can sense colours only in light because when it gets dark the cones ability to 
respond to light is lost and the rods continue to respond to available light. However 
cones cannot see color, everything looks like shades of black and white and gray. 
(c) Eyes can see its distant and close objects. It adjusts its focal length by contracting or 
relaxing ciliary muscles to see the objects. The ciliary muscles cannot be contracted 
below a certain minimum limit. Therefore we cannot see an object kept at a distance 
less than 25 cms from the eye. 
Q. 5. Explain the working of an astronomical telescope using refraction of light. 
Answer : In the astronomical telescopes the image is formed by bending of light also 
known as refraction so these telescopes are known as refracting telescopes. It is used 
to see the magnified images of heavenly bodies like stars, planets, etc. 
The construction of the telescope is given as: 
An astronomical telescope is made up of two convex lenses : 
i) an objective lens O and, 
ii) an eye piece E. 
The focal length fo of the objective lens of astronomical telescope is large as compared 
to the focal length fe of the eye piece. 
Working of the telescope: 
The ray diagram to show the working of the astronomical telescope is shown in figure 
below: 
 
i. A parallel beam of light from a star or a satellite falls on the objective lens of the 
telescope. 
ii. The objective lens forms a real, inverted and diminished image A’B’ of the heavenly 
body. 
iii. This image (A’B’) now acts as an object for the eye piece E, whose position is 
adjusted so that the image lies between the focus fe’ and the optical centre C2of the eye 
piece. 
iv. Now the eye piece forms a virtual, inverted and highly magnified image of object at 
infinity. When the final image of an object is formed at infinity, the telescope is said to be 
in ‘normal adjustment’. 
It should be noted that, the final image of object (such as stars, planets or satellites) 
formed by an astronomical telescope is always inverted with respect to the object. But it 
does not matter whether the image formed by an astronomical telescope is inverted or 
not, as all the heavenly bodies are usually spherical is shape. Astronomical telescopes 
bend light called as refraction. Due to this refraction the light rays which are parallel, 
converge at a focal point and those lines that are not parallel converge upon focal 
plane. The telescope converts a bundle of parallel rays and makes angle a and 
angle ß with a second parallel bundle. The ratio of  gives angular magnification to the 
telescope. 
Q. 6. A. Distinguish between: 
 
Farsightedness and Nearsightedness 
Answer :  
 
 
 
The diagram is given below: 
 
Q. 6. B. Distinguish between: 
 
Concave lens and Convex Lens 
Answer :