Selina Textbook Solutions: Refraction Through a Lens | Physics Class 10 ICSE PDF Download

 Page 1


Class X Chapter – 05 Refraction Through A lens  Physics 
_____________________________________________________________________________ 
                     
 
EXERCISE - 5 (A) 
 
Question 1: 
What is a lens?  
Solution 1: 
A lens is a transparent refracting medium bounded by two curved surfaces which are generally 
spherical. 
 
 
Question 2: 
Name the different kinds of lenses. Draw diagrams to illustrate them. 
Solution 2: 
Lenses are of two types : 
(i) Convex or converging lens, and 
(ii) Concave or diverging lens. 
 
  
 
 
Question 3: 
State two differences between a convex and a concave lens. 
Solution 3: 
Convex lens: 
(i) It converge the incident rays towards the principal axis. 
(ii) It has a real focus. 
Concave lens: 
(i) It diverges the incident rays away from the principal axis. 
It has a virtual focus.  
 
 
Question 4: 
Which lens is converging: An equiconcave lens or an equiconvex lens. 
Solution 4: 
Equiconvex lens is converging.  
 
 
 
Page 2


Class X Chapter – 05 Refraction Through A lens  Physics 
_____________________________________________________________________________ 
                     
 
EXERCISE - 5 (A) 
 
Question 1: 
What is a lens?  
Solution 1: 
A lens is a transparent refracting medium bounded by two curved surfaces which are generally 
spherical. 
 
 
Question 2: 
Name the different kinds of lenses. Draw diagrams to illustrate them. 
Solution 2: 
Lenses are of two types : 
(i) Convex or converging lens, and 
(ii) Concave or diverging lens. 
 
  
 
 
Question 3: 
State two differences between a convex and a concave lens. 
Solution 3: 
Convex lens: 
(i) It converge the incident rays towards the principal axis. 
(ii) It has a real focus. 
Concave lens: 
(i) It diverges the incident rays away from the principal axis. 
It has a virtual focus.  
 
 
Question 4: 
Which lens is converging: An equiconcave lens or an equiconvex lens. 
Solution 4: 
Equiconvex lens is converging.  
 
 
 
Class X Chapter – 05 Refraction Through A lens  Physics 
_____________________________________________________________________________ 
 
Question 5: 
Out of the two lenses, one concave and the other convex, state which one will show the divergent 
action on a light beam. 
Solution 5: 
Concave lens will show the divergent action on a light beam.  
 
 
Question 6: 
Show by a diagram the refraction of two light rays incident parallel to the principal axis on a 
convex lens by treating it as a combination of a glass block and two triangular glass prisms. 
Solution 6: 
 
 
As shown in the figure the convex lens has two glass prisms and one glass block. One of the 
glass prisms is situated above the glass block and one below the block.  
 
 
Question 7: 
Show by a diagram, the refraction of two light rays incident parallel to the principal axis on a 
concave lens by treating it as a combination of a glass block and two triangular glass prisms. 
Solution 7: 
 
As shown in the figure the concave lens has two glass prisms and one glass block. One of the 
glass prisms is situated above the glass block and one below the block.  
 
 
Question 8: 
How does the action of a convex lens differ from that of a concave lens on a parallel beam of 
light incident on them? Draw diagram to illustrate your answer. 
Page 3


Class X Chapter – 05 Refraction Through A lens  Physics 
_____________________________________________________________________________ 
                     
 
EXERCISE - 5 (A) 
 
Question 1: 
What is a lens?  
Solution 1: 
A lens is a transparent refracting medium bounded by two curved surfaces which are generally 
spherical. 
 
 
Question 2: 
Name the different kinds of lenses. Draw diagrams to illustrate them. 
Solution 2: 
Lenses are of two types : 
(i) Convex or converging lens, and 
(ii) Concave or diverging lens. 
 
  
 
 
Question 3: 
State two differences between a convex and a concave lens. 
Solution 3: 
Convex lens: 
(i) It converge the incident rays towards the principal axis. 
(ii) It has a real focus. 
Concave lens: 
(i) It diverges the incident rays away from the principal axis. 
It has a virtual focus.  
 
 
Question 4: 
Which lens is converging: An equiconcave lens or an equiconvex lens. 
Solution 4: 
Equiconvex lens is converging.  
 
 
 
Class X Chapter – 05 Refraction Through A lens  Physics 
_____________________________________________________________________________ 
 
Question 5: 
Out of the two lenses, one concave and the other convex, state which one will show the divergent 
action on a light beam. 
Solution 5: 
Concave lens will show the divergent action on a light beam.  
 
 
Question 6: 
Show by a diagram the refraction of two light rays incident parallel to the principal axis on a 
convex lens by treating it as a combination of a glass block and two triangular glass prisms. 
Solution 6: 
 
 
As shown in the figure the convex lens has two glass prisms and one glass block. One of the 
glass prisms is situated above the glass block and one below the block.  
 
 
Question 7: 
Show by a diagram, the refraction of two light rays incident parallel to the principal axis on a 
concave lens by treating it as a combination of a glass block and two triangular glass prisms. 
Solution 7: 
 
As shown in the figure the concave lens has two glass prisms and one glass block. One of the 
glass prisms is situated above the glass block and one below the block.  
 
 
Question 8: 
How does the action of a convex lens differ from that of a concave lens on a parallel beam of 
light incident on them? Draw diagram to illustrate your answer. 
Class X Chapter – 05 Refraction Through A lens  Physics 
_____________________________________________________________________________ 
 
Solution 8: 
If a parallel beam of light is incident on a convex lens then the upper part of the lens bends the 
incident ray downwards. The lower part bens the ray upwards while the central part passes the 
ray undeviated. 
 
 
 
But in case of a concave lens the upper part of the lens bends the incident ray upwards and lower 
part bends the ray downwards while the central part passes the ray undeviated. 
 
  
 
Question 9: 
Define the term principal axis of a lens. 
Solution 9: 
It is the line joining the centers of curvature of the two surfaces of the lens.  
 
  
Question 10: 
Explain optical centre of a lens with the help of proper diagram(s) 
Solution 10: 
It is point on the principal axis of the lens such that a ray of light passing through this point 
emerges parallel to its direction of incidence. 
It is marked by letter O in the figure. The optical centre is thus the centre of the lens. 
 
Page 4


Class X Chapter – 05 Refraction Through A lens  Physics 
_____________________________________________________________________________ 
                     
 
EXERCISE - 5 (A) 
 
Question 1: 
What is a lens?  
Solution 1: 
A lens is a transparent refracting medium bounded by two curved surfaces which are generally 
spherical. 
 
 
Question 2: 
Name the different kinds of lenses. Draw diagrams to illustrate them. 
Solution 2: 
Lenses are of two types : 
(i) Convex or converging lens, and 
(ii) Concave or diverging lens. 
 
  
 
 
Question 3: 
State two differences between a convex and a concave lens. 
Solution 3: 
Convex lens: 
(i) It converge the incident rays towards the principal axis. 
(ii) It has a real focus. 
Concave lens: 
(i) It diverges the incident rays away from the principal axis. 
It has a virtual focus.  
 
 
Question 4: 
Which lens is converging: An equiconcave lens or an equiconvex lens. 
Solution 4: 
Equiconvex lens is converging.  
 
 
 
Class X Chapter – 05 Refraction Through A lens  Physics 
_____________________________________________________________________________ 
 
Question 5: 
Out of the two lenses, one concave and the other convex, state which one will show the divergent 
action on a light beam. 
Solution 5: 
Concave lens will show the divergent action on a light beam.  
 
 
Question 6: 
Show by a diagram the refraction of two light rays incident parallel to the principal axis on a 
convex lens by treating it as a combination of a glass block and two triangular glass prisms. 
Solution 6: 
 
 
As shown in the figure the convex lens has two glass prisms and one glass block. One of the 
glass prisms is situated above the glass block and one below the block.  
 
 
Question 7: 
Show by a diagram, the refraction of two light rays incident parallel to the principal axis on a 
concave lens by treating it as a combination of a glass block and two triangular glass prisms. 
Solution 7: 
 
As shown in the figure the concave lens has two glass prisms and one glass block. One of the 
glass prisms is situated above the glass block and one below the block.  
 
 
Question 8: 
How does the action of a convex lens differ from that of a concave lens on a parallel beam of 
light incident on them? Draw diagram to illustrate your answer. 
Class X Chapter – 05 Refraction Through A lens  Physics 
_____________________________________________________________________________ 
 
Solution 8: 
If a parallel beam of light is incident on a convex lens then the upper part of the lens bends the 
incident ray downwards. The lower part bens the ray upwards while the central part passes the 
ray undeviated. 
 
 
 
But in case of a concave lens the upper part of the lens bends the incident ray upwards and lower 
part bends the ray downwards while the central part passes the ray undeviated. 
 
  
 
Question 9: 
Define the term principal axis of a lens. 
Solution 9: 
It is the line joining the centers of curvature of the two surfaces of the lens.  
 
  
Question 10: 
Explain optical centre of a lens with the help of proper diagram(s) 
Solution 10: 
It is point on the principal axis of the lens such that a ray of light passing through this point 
emerges parallel to its direction of incidence. 
It is marked by letter O in the figure. The optical centre is thus the centre of the lens. 
 
Class X Chapter – 05 Refraction Through A lens  Physics 
_____________________________________________________________________________ 
 
  
 
Question 11: 
A ray of light incident at a point on the principal axis of a convex lens passes undeviated through 
the lens. 
(a) What special name is given to this point on the principal axis? 
(b) Draw a labelled diagram to support your answer in part (a) 
Solution 11: 
(a) This point is known as Optical centre. 
(b)   
 
  
 
Question 12: 
State the condition when a lens is called an equi-convex or equi-concave  
Solution 12: 
A lens is called an equiconvex or equiconcave when radii of curvature of the two surfaces of 
lens are equal.  
 
 
Question 13: 
Define the term principal foci of a convex lens and illustrate your answer with the aid of proper 
diagrams. 
Solution 13: 
A light ray can pass through a lens from either direction. Therefore, a lens has two principal foci. 
  
Page 5


Class X Chapter – 05 Refraction Through A lens  Physics 
_____________________________________________________________________________ 
                     
 
EXERCISE - 5 (A) 
 
Question 1: 
What is a lens?  
Solution 1: 
A lens is a transparent refracting medium bounded by two curved surfaces which are generally 
spherical. 
 
 
Question 2: 
Name the different kinds of lenses. Draw diagrams to illustrate them. 
Solution 2: 
Lenses are of two types : 
(i) Convex or converging lens, and 
(ii) Concave or diverging lens. 
 
  
 
 
Question 3: 
State two differences between a convex and a concave lens. 
Solution 3: 
Convex lens: 
(i) It converge the incident rays towards the principal axis. 
(ii) It has a real focus. 
Concave lens: 
(i) It diverges the incident rays away from the principal axis. 
It has a virtual focus.  
 
 
Question 4: 
Which lens is converging: An equiconcave lens or an equiconvex lens. 
Solution 4: 
Equiconvex lens is converging.  
 
 
 
Class X Chapter – 05 Refraction Through A lens  Physics 
_____________________________________________________________________________ 
 
Question 5: 
Out of the two lenses, one concave and the other convex, state which one will show the divergent 
action on a light beam. 
Solution 5: 
Concave lens will show the divergent action on a light beam.  
 
 
Question 6: 
Show by a diagram the refraction of two light rays incident parallel to the principal axis on a 
convex lens by treating it as a combination of a glass block and two triangular glass prisms. 
Solution 6: 
 
 
As shown in the figure the convex lens has two glass prisms and one glass block. One of the 
glass prisms is situated above the glass block and one below the block.  
 
 
Question 7: 
Show by a diagram, the refraction of two light rays incident parallel to the principal axis on a 
concave lens by treating it as a combination of a glass block and two triangular glass prisms. 
Solution 7: 
 
As shown in the figure the concave lens has two glass prisms and one glass block. One of the 
glass prisms is situated above the glass block and one below the block.  
 
 
Question 8: 
How does the action of a convex lens differ from that of a concave lens on a parallel beam of 
light incident on them? Draw diagram to illustrate your answer. 
Class X Chapter – 05 Refraction Through A lens  Physics 
_____________________________________________________________________________ 
 
Solution 8: 
If a parallel beam of light is incident on a convex lens then the upper part of the lens bends the 
incident ray downwards. The lower part bens the ray upwards while the central part passes the 
ray undeviated. 
 
 
 
But in case of a concave lens the upper part of the lens bends the incident ray upwards and lower 
part bends the ray downwards while the central part passes the ray undeviated. 
 
  
 
Question 9: 
Define the term principal axis of a lens. 
Solution 9: 
It is the line joining the centers of curvature of the two surfaces of the lens.  
 
  
Question 10: 
Explain optical centre of a lens with the help of proper diagram(s) 
Solution 10: 
It is point on the principal axis of the lens such that a ray of light passing through this point 
emerges parallel to its direction of incidence. 
It is marked by letter O in the figure. The optical centre is thus the centre of the lens. 
 
Class X Chapter – 05 Refraction Through A lens  Physics 
_____________________________________________________________________________ 
 
  
 
Question 11: 
A ray of light incident at a point on the principal axis of a convex lens passes undeviated through 
the lens. 
(a) What special name is given to this point on the principal axis? 
(b) Draw a labelled diagram to support your answer in part (a) 
Solution 11: 
(a) This point is known as Optical centre. 
(b)   
 
  
 
Question 12: 
State the condition when a lens is called an equi-convex or equi-concave  
Solution 12: 
A lens is called an equiconvex or equiconcave when radii of curvature of the two surfaces of 
lens are equal.  
 
 
Question 13: 
Define the term principal foci of a convex lens and illustrate your answer with the aid of proper 
diagrams. 
Solution 13: 
A light ray can pass through a lens from either direction. Therefore, a lens has two principal foci. 
  
Class X Chapter – 05 Refraction Through A lens  Physics 
_____________________________________________________________________________ 
 
For a convex lens, the first focal point is a point F1 on the principal axis of the lens such that the 
rays of light starting from it or passing through it, after refraction through lens, become parallel 
to the principal axis of the lens. 
 
 
The second focal point for a convex lens is a point F2 on the principal axis such that the rays of 
light incident parallel to the principal axis, after refraction from the lens, pass through it. 
 
  
 
Question 14: 
Define the term principal foci of a concave lens and show them with the help of proper diagrams. 
Solution 14: 
A light ray can pass through a lens from either direction. Therefore, a lens has two principal foci. 
For a concave lens, the first focal point is a point F1 on the principal axis of the lens such that 
the incident rays of light appearing to meet at it, after refraction from the lens become parallel 
to the principal axis of the lens. 
 
 
The second focal point for a concave lens is a point F2 on the principal axis of the lens such that 
the rays of light incident parallel to the principal axis, after refraction from the lens, appear to 
be diverging from this point.