Selina Textbook Solutions: Light Energy | Physics Class 7 ICSE PDF Download

 Page 1


 Light Energy  
Points to Remember 
?  Light is a form of energy which helps us to see objects. 
?  Light always travels in a straight line in the form of rays. 
?  Light sources are either natural or artificial. 
?  The sun and stars are natural sources of light. 
?  A bulb, a candle etc. are artificial sources of light. 
?  The bodies which emit light themsleves are called Luminous Bodies, e.g. sun, 
star. 
?  The bodies which do not emit light are called non-luminous bodies, e.g. wood, 
brick etc. 
?  Objects are of three types, transparent, translucent or opaque. 
?  The pinhole camera is a simple application of the rectilinear propagation of light. 
?  When an object blocks light, it casts a shadow. 
?  Eclipses are formed due to formation of shadows. 
?  Solar and lunar eclipses are the examples of formation of shadow in nature. 
?  An eclipse is the partial or complete hiding of one heavenly body by shadow of 
another. 
?  When light strikes a polished surface it comes back in the same medium, is called 
reflection of light. 
?  A straight highly polished, smooth and reflecting surface is known as a plane 
mirror. 
?  Mirrors are of two types (a) plane mirrors (b) spherical mirrors. 
?  Smooth and polished surface like a mirror causes reflection and is called a 
regular reflection. 
?  Rough or diffused surface causes an irregular reflection. 
?  According to first law of reflection. The incident ray, the normal and the reflected 
ray all lie in the same plane. 
?  According to the second law of relection, the angle of incidence is always equal to 
the angle of reflection. 
?  The phenomenon due to which the left side of an object appears to be the right 
side of the object and right side appears left. This is known as lateral inversion. 
?  Image is of two types (a) Real image (b) Virtual image. 
Test Yourself 
A. Objective Questions  
1. Write true or false for each statement 
(a) The image formed by a plane mirror is real. 
Answer. False. 
The image formed by a plane mirror is virtual. 
Page 2


 Light Energy  
Points to Remember 
?  Light is a form of energy which helps us to see objects. 
?  Light always travels in a straight line in the form of rays. 
?  Light sources are either natural or artificial. 
?  The sun and stars are natural sources of light. 
?  A bulb, a candle etc. are artificial sources of light. 
?  The bodies which emit light themsleves are called Luminous Bodies, e.g. sun, 
star. 
?  The bodies which do not emit light are called non-luminous bodies, e.g. wood, 
brick etc. 
?  Objects are of three types, transparent, translucent or opaque. 
?  The pinhole camera is a simple application of the rectilinear propagation of light. 
?  When an object blocks light, it casts a shadow. 
?  Eclipses are formed due to formation of shadows. 
?  Solar and lunar eclipses are the examples of formation of shadow in nature. 
?  An eclipse is the partial or complete hiding of one heavenly body by shadow of 
another. 
?  When light strikes a polished surface it comes back in the same medium, is called 
reflection of light. 
?  A straight highly polished, smooth and reflecting surface is known as a plane 
mirror. 
?  Mirrors are of two types (a) plane mirrors (b) spherical mirrors. 
?  Smooth and polished surface like a mirror causes reflection and is called a 
regular reflection. 
?  Rough or diffused surface causes an irregular reflection. 
?  According to first law of reflection. The incident ray, the normal and the reflected 
ray all lie in the same plane. 
?  According to the second law of relection, the angle of incidence is always equal to 
the angle of reflection. 
?  The phenomenon due to which the left side of an object appears to be the right 
side of the object and right side appears left. This is known as lateral inversion. 
?  Image is of two types (a) Real image (b) Virtual image. 
Test Yourself 
A. Objective Questions  
1. Write true or false for each statement 
(a) The image formed by a plane mirror is real. 
Answer. False. 
The image formed by a plane mirror is virtual. 
(b) When a light ray is reflected from a wall, the angle of incidence is not equal to the 
angle of reflection. 
Answer. False. 
When a light ray is reflected from a wall, the angle of incidence is equal to the angle of 
reflection. 
(c) The image of the right hand in a plane mirror looks like that of a left hand. 
Answer. True. 
(d) The image formed by a plane mirror is upright. 
Answer. True. 
(c) The image formed by a plane mirror can be obtained on a screen. 
Answer. False. 
The image formed by a plane mirror cannot be obtained on a screen. 
(f) The objects are seen around us due to irregular reflection of light. 
Answer. True. 
(g) The speed of light in vacuum is 3 × 10
8
 ms
-1
. 
Answer. True. 
(h) A rose appears red in light of all the colours. 
Answer. False. A rose appears red in white light. 
(i) A black paper absorbs light of all the colours and reflects none. 
Answer. True. 
(j) The primary colours are red, blue and green. 
Answer. True. 
2. Fill in the blanks 
(a) Angle of incidence = angle of reflection 
(b) The incident ray, the reflected ray and the normal lie in one plane 
(c) The image formed by a plane mirror is at a distance behind the mirror as the object 
is in front of it. 
(d) The image formed by a plane mirror is erect and virtual. 
(e) We are able to see the objects around us due to irregular reflection. 
(f) A virtual image cannot be obtained on a screen. 
(g) One surface of mirror is made opaque by silvering it followed by a thin coating of 
paint of lead oxide. . 
Page 3


 Light Energy  
Points to Remember 
?  Light is a form of energy which helps us to see objects. 
?  Light always travels in a straight line in the form of rays. 
?  Light sources are either natural or artificial. 
?  The sun and stars are natural sources of light. 
?  A bulb, a candle etc. are artificial sources of light. 
?  The bodies which emit light themsleves are called Luminous Bodies, e.g. sun, 
star. 
?  The bodies which do not emit light are called non-luminous bodies, e.g. wood, 
brick etc. 
?  Objects are of three types, transparent, translucent or opaque. 
?  The pinhole camera is a simple application of the rectilinear propagation of light. 
?  When an object blocks light, it casts a shadow. 
?  Eclipses are formed due to formation of shadows. 
?  Solar and lunar eclipses are the examples of formation of shadow in nature. 
?  An eclipse is the partial or complete hiding of one heavenly body by shadow of 
another. 
?  When light strikes a polished surface it comes back in the same medium, is called 
reflection of light. 
?  A straight highly polished, smooth and reflecting surface is known as a plane 
mirror. 
?  Mirrors are of two types (a) plane mirrors (b) spherical mirrors. 
?  Smooth and polished surface like a mirror causes reflection and is called a 
regular reflection. 
?  Rough or diffused surface causes an irregular reflection. 
?  According to first law of reflection. The incident ray, the normal and the reflected 
ray all lie in the same plane. 
?  According to the second law of relection, the angle of incidence is always equal to 
the angle of reflection. 
?  The phenomenon due to which the left side of an object appears to be the right 
side of the object and right side appears left. This is known as lateral inversion. 
?  Image is of two types (a) Real image (b) Virtual image. 
Test Yourself 
A. Objective Questions  
1. Write true or false for each statement 
(a) The image formed by a plane mirror is real. 
Answer. False. 
The image formed by a plane mirror is virtual. 
(b) When a light ray is reflected from a wall, the angle of incidence is not equal to the 
angle of reflection. 
Answer. False. 
When a light ray is reflected from a wall, the angle of incidence is equal to the angle of 
reflection. 
(c) The image of the right hand in a plane mirror looks like that of a left hand. 
Answer. True. 
(d) The image formed by a plane mirror is upright. 
Answer. True. 
(c) The image formed by a plane mirror can be obtained on a screen. 
Answer. False. 
The image formed by a plane mirror cannot be obtained on a screen. 
(f) The objects are seen around us due to irregular reflection of light. 
Answer. True. 
(g) The speed of light in vacuum is 3 × 10
8
 ms
-1
. 
Answer. True. 
(h) A rose appears red in light of all the colours. 
Answer. False. A rose appears red in white light. 
(i) A black paper absorbs light of all the colours and reflects none. 
Answer. True. 
(j) The primary colours are red, blue and green. 
Answer. True. 
2. Fill in the blanks 
(a) Angle of incidence = angle of reflection 
(b) The incident ray, the reflected ray and the normal lie in one plane 
(c) The image formed by a plane mirror is at a distance behind the mirror as the object 
is in front of it. 
(d) The image formed by a plane mirror is erect and virtual. 
(e) We are able to see the objects around us due to irregular reflection. 
(f) A virtual image cannot be obtained on a screen. 
(g) One surface of mirror is made opaque by silvering it followed by a thin coating of 
paint of lead oxide. . 
(h) A plane mirror does not reflect 100 percent light falling on it. 
(i) The colour of an opaque object is the colour of light which it reflects. 
(j) Magenta, cyan and yellow are the secondary colours. 
3. Match the following 
 
4. Select the correct alternative 
(a) A man standing in front of a plane mirror finds his image to be at a distance of 6 
metre from himself. The distance of man from the mirror is 
1. 6 m 
2. 3 m 
3. 2 m 
4. 12 m 
Page 4


 Light Energy  
Points to Remember 
?  Light is a form of energy which helps us to see objects. 
?  Light always travels in a straight line in the form of rays. 
?  Light sources are either natural or artificial. 
?  The sun and stars are natural sources of light. 
?  A bulb, a candle etc. are artificial sources of light. 
?  The bodies which emit light themsleves are called Luminous Bodies, e.g. sun, 
star. 
?  The bodies which do not emit light are called non-luminous bodies, e.g. wood, 
brick etc. 
?  Objects are of three types, transparent, translucent or opaque. 
?  The pinhole camera is a simple application of the rectilinear propagation of light. 
?  When an object blocks light, it casts a shadow. 
?  Eclipses are formed due to formation of shadows. 
?  Solar and lunar eclipses are the examples of formation of shadow in nature. 
?  An eclipse is the partial or complete hiding of one heavenly body by shadow of 
another. 
?  When light strikes a polished surface it comes back in the same medium, is called 
reflection of light. 
?  A straight highly polished, smooth and reflecting surface is known as a plane 
mirror. 
?  Mirrors are of two types (a) plane mirrors (b) spherical mirrors. 
?  Smooth and polished surface like a mirror causes reflection and is called a 
regular reflection. 
?  Rough or diffused surface causes an irregular reflection. 
?  According to first law of reflection. The incident ray, the normal and the reflected 
ray all lie in the same plane. 
?  According to the second law of relection, the angle of incidence is always equal to 
the angle of reflection. 
?  The phenomenon due to which the left side of an object appears to be the right 
side of the object and right side appears left. This is known as lateral inversion. 
?  Image is of two types (a) Real image (b) Virtual image. 
Test Yourself 
A. Objective Questions  
1. Write true or false for each statement 
(a) The image formed by a plane mirror is real. 
Answer. False. 
The image formed by a plane mirror is virtual. 
(b) When a light ray is reflected from a wall, the angle of incidence is not equal to the 
angle of reflection. 
Answer. False. 
When a light ray is reflected from a wall, the angle of incidence is equal to the angle of 
reflection. 
(c) The image of the right hand in a plane mirror looks like that of a left hand. 
Answer. True. 
(d) The image formed by a plane mirror is upright. 
Answer. True. 
(c) The image formed by a plane mirror can be obtained on a screen. 
Answer. False. 
The image formed by a plane mirror cannot be obtained on a screen. 
(f) The objects are seen around us due to irregular reflection of light. 
Answer. True. 
(g) The speed of light in vacuum is 3 × 10
8
 ms
-1
. 
Answer. True. 
(h) A rose appears red in light of all the colours. 
Answer. False. A rose appears red in white light. 
(i) A black paper absorbs light of all the colours and reflects none. 
Answer. True. 
(j) The primary colours are red, blue and green. 
Answer. True. 
2. Fill in the blanks 
(a) Angle of incidence = angle of reflection 
(b) The incident ray, the reflected ray and the normal lie in one plane 
(c) The image formed by a plane mirror is at a distance behind the mirror as the object 
is in front of it. 
(d) The image formed by a plane mirror is erect and virtual. 
(e) We are able to see the objects around us due to irregular reflection. 
(f) A virtual image cannot be obtained on a screen. 
(g) One surface of mirror is made opaque by silvering it followed by a thin coating of 
paint of lead oxide. . 
(h) A plane mirror does not reflect 100 percent light falling on it. 
(i) The colour of an opaque object is the colour of light which it reflects. 
(j) Magenta, cyan and yellow are the secondary colours. 
3. Match the following 
 
4. Select the correct alternative 
(a) A man standing in front of a plane mirror finds his image to be at a distance of 6 
metre from himself. The distance of man from the mirror is 
1. 6 m 
2. 3 m 
3. 2 m 
4. 12 m 
(b) The angle between the incident ray and the ray reflected from the plane mirror is 
70°. The angle of incidence will be : 
1. 70° 
2. 30° 
3. 35° 
4. 90° 
(c) The image formed by a plane mirror is 
1. virtual and inverted 
2. virtual and of same size 
3. real and inverted 
4. real and of same size 
(d) The angle of incidence on a plane mirror is 30°.The angle of reflection will be: 
1. 30° 
2. 60° 
3. 15° 
4. 0° 
(e) The angle of incidence on a plane mirror is 30°. The angle between the incident ray 
and the reflected ray is 
1. 30° 
2. 15° 
3. 60° 
4. 90° 
(f) The property due to which a light ray striking a surface is returned back into the same 
medium is called 
1. refraction 
2. reflex action 
3. reflection 
4. regression 
(g) A ray of light after reflection from a mirror is known as 
1. reflected ray 
2. normal 
3. incident ray 
4. refracted ray 
Page 5


 Light Energy  
Points to Remember 
?  Light is a form of energy which helps us to see objects. 
?  Light always travels in a straight line in the form of rays. 
?  Light sources are either natural or artificial. 
?  The sun and stars are natural sources of light. 
?  A bulb, a candle etc. are artificial sources of light. 
?  The bodies which emit light themsleves are called Luminous Bodies, e.g. sun, 
star. 
?  The bodies which do not emit light are called non-luminous bodies, e.g. wood, 
brick etc. 
?  Objects are of three types, transparent, translucent or opaque. 
?  The pinhole camera is a simple application of the rectilinear propagation of light. 
?  When an object blocks light, it casts a shadow. 
?  Eclipses are formed due to formation of shadows. 
?  Solar and lunar eclipses are the examples of formation of shadow in nature. 
?  An eclipse is the partial or complete hiding of one heavenly body by shadow of 
another. 
?  When light strikes a polished surface it comes back in the same medium, is called 
reflection of light. 
?  A straight highly polished, smooth and reflecting surface is known as a plane 
mirror. 
?  Mirrors are of two types (a) plane mirrors (b) spherical mirrors. 
?  Smooth and polished surface like a mirror causes reflection and is called a 
regular reflection. 
?  Rough or diffused surface causes an irregular reflection. 
?  According to first law of reflection. The incident ray, the normal and the reflected 
ray all lie in the same plane. 
?  According to the second law of relection, the angle of incidence is always equal to 
the angle of reflection. 
?  The phenomenon due to which the left side of an object appears to be the right 
side of the object and right side appears left. This is known as lateral inversion. 
?  Image is of two types (a) Real image (b) Virtual image. 
Test Yourself 
A. Objective Questions  
1. Write true or false for each statement 
(a) The image formed by a plane mirror is real. 
Answer. False. 
The image formed by a plane mirror is virtual. 
(b) When a light ray is reflected from a wall, the angle of incidence is not equal to the 
angle of reflection. 
Answer. False. 
When a light ray is reflected from a wall, the angle of incidence is equal to the angle of 
reflection. 
(c) The image of the right hand in a plane mirror looks like that of a left hand. 
Answer. True. 
(d) The image formed by a plane mirror is upright. 
Answer. True. 
(c) The image formed by a plane mirror can be obtained on a screen. 
Answer. False. 
The image formed by a plane mirror cannot be obtained on a screen. 
(f) The objects are seen around us due to irregular reflection of light. 
Answer. True. 
(g) The speed of light in vacuum is 3 × 10
8
 ms
-1
. 
Answer. True. 
(h) A rose appears red in light of all the colours. 
Answer. False. A rose appears red in white light. 
(i) A black paper absorbs light of all the colours and reflects none. 
Answer. True. 
(j) The primary colours are red, blue and green. 
Answer. True. 
2. Fill in the blanks 
(a) Angle of incidence = angle of reflection 
(b) The incident ray, the reflected ray and the normal lie in one plane 
(c) The image formed by a plane mirror is at a distance behind the mirror as the object 
is in front of it. 
(d) The image formed by a plane mirror is erect and virtual. 
(e) We are able to see the objects around us due to irregular reflection. 
(f) A virtual image cannot be obtained on a screen. 
(g) One surface of mirror is made opaque by silvering it followed by a thin coating of 
paint of lead oxide. . 
(h) A plane mirror does not reflect 100 percent light falling on it. 
(i) The colour of an opaque object is the colour of light which it reflects. 
(j) Magenta, cyan and yellow are the secondary colours. 
3. Match the following 
 
4. Select the correct alternative 
(a) A man standing in front of a plane mirror finds his image to be at a distance of 6 
metre from himself. The distance of man from the mirror is 
1. 6 m 
2. 3 m 
3. 2 m 
4. 12 m 
(b) The angle between the incident ray and the ray reflected from the plane mirror is 
70°. The angle of incidence will be : 
1. 70° 
2. 30° 
3. 35° 
4. 90° 
(c) The image formed by a plane mirror is 
1. virtual and inverted 
2. virtual and of same size 
3. real and inverted 
4. real and of same size 
(d) The angle of incidence on a plane mirror is 30°.The angle of reflection will be: 
1. 30° 
2. 60° 
3. 15° 
4. 0° 
(e) The angle of incidence on a plane mirror is 30°. The angle between the incident ray 
and the reflected ray is 
1. 30° 
2. 15° 
3. 60° 
4. 90° 
(f) The property due to which a light ray striking a surface is returned back into the same 
medium is called 
1. refraction 
2. reflex action 
3. reflection 
4. regression 
(g) A ray of light after reflection from a mirror is known as 
1. reflected ray 
2. normal 
3. incident ray 
4. refracted ray 
(h) The speed of light is maximum in 
1. glass 
2. water 
3. air 
4. wood 
(i) A red rose is seen in green light. It will appear. 
1. red 
2. blue 
3. yellow 
4. black 
(j) The primary colours are 
1. Red, Blue and Yellow 
2. Magenta, Yellow and Cyan 
3. Red, Blue and Cyan 
4. Blue, Green and Red 
B. Short/Long Answer Questions 
Question 1. 
What do you mean by the term reflection of light ? 
Answer: 
Reflection of light— When light strikes a polished surface it comes back in the same 
medium, is called reflection of light. 
Question 2. 
How is a plane mirror made ? 
Answer: 
To make a plane mirror, a thin piece of glass is taken. One surface of the mirror is made 
opaque by silvering it. On the top of that, another thin coating of red lead oxide is given 
which protects the silvering of the mirror. 
Question 3. 
Explain the following terms: 
Incident ray, Reflected ray, Angle of incidence, Angle of reflection, Normal. 
Answer: 
Incident ray— The ray of light falling on the surface AB is called the incident ray. In 
figure PN is the incident ray. 
Reflected ray— The incident ray bouncing back in the same medium after striking the 
reflecting ourface is called reflected ray. In figure NQ is the reflected ray.