Selina Concise Class 7 Physics Solution Chapter No. 4- ‘Light Energy’ For ICSE Board Students.
Selina Concise Physics Solutions for Class 7 Chapter 4 Light Energy with easy language, expert answers and doubt clearing sections. The book is especially designed keeping in mind the syllabus of ICSE board and the difficulty level faced by the students of ICSE boards class 7.
A) Objective Questions :
1.)
a) The image formed by a plane mirror is real.
Answer: False, 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. According to laws of reflection, incident angle is always equal to angle of reflection.
c) The image of the right hand in a plane mirror looks like that of a left hand.
Answer: True, The image of the right hand in a plane mirror looks like that of a left hand.
d) The image formed by a plane mirror is upright.
Answer: True, plane mirrors forms upright image. Upright image means erect image.
e) The image formed by a plane mirror can be obtained on a screen.
Answer: False. Plane mirror produces virtual image and we can not capture virtual image.
f) The objects around us are seen due to irregular reflection of light.
Answer: True.We get sensation of vision from the light whichreflects from objects.
g) The speed of light in vacuum is 3 x 10⁸ m/s.
Answer: True.
h) A rose appears red in light of all the colours.
Answer: False, The rose appears red when it reflects red colours.
i) A black paper absorbs light of all the colours and reflects none.
Answer: True, perfectly black colour absorbs light of all colours.
j) The primary colours are red, blue and green.
Answer: True. Light constitute from these three basic colour.
2.)
Answer:
a) Angle of incidence = Angle of reflection.
b) The incident ray, the reflected ray and the normal liein one plane.
c) The image formed by a plane mirror is at a distance behind the mirror as the object in front of it.
d) The image formed by a plane mirror is virtualand erect.
e) We are able to see the objects around us due toirregularreflection.
f) A virtualimage cannot be obtained on a screen.
g) One surface of mirror is made opaque bysilvering it followed by a thin coating of paint of lead oxide.
h) A plane mirrordoes notreflect 100 percent light falling on it.
i) The colour of an opaque object is the of light which it reflects.
j) Magenta, cyan and yellow are the secondarycolours.
3.) Match the following :
Answer:
Column A Column B
(a) A light ray passes (v) slows down from air to glass
(b) A light ray passes (i) speeds up
(c) Virtual image (iv) plane mirror
(d) Red rose (ii) reflects red light from glass to water
(e) Red, green and blue (iii) primary colours
4.) 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 :
(i) 6m (ii) 3 m
(iii) 2 m (iv) 12 m
Answer: (ii) 3 m
We know that, the distance from object is equal to distance of image for plane mirror.
As given in problem, the distance between object and image is 6 m. Thus the distance between object from mirror is half of the total distance.
Therefore,
Distance between mirror and object (man) = 3 m.
b) The angle between the incident ray and the ray reflected from the plane mirror is 70⁰. The angle of incidence will be :
(i) 70⁰ (ii) 30⁰
(iii) 35⁰ (iv) 90⁰
Answer: (iii) 35⁰
According to law of reflection incident angle is equal to angle of reflection.
Given,
Angle of incidence + angle of reflection = 70
2 × Angle of incidence = 70 (angle of incidence = Angle of reflection)
Angle of incidence = 70/2
Angle of incidence = 35⁰
c) The image formed by a plane mirror is
(i) virtual and inverted
(ii) virtual and of same size
(iii) real and inverted
(iv) real and of same size
Answer: (ii) virtual and of same size
Plane mirror produces virtual image which we can not capture. The size of that image is same so size of object.
d) The angle of incidence on a plane mirror is 30⁰. The angle of reflection will be :
(i) 30⁰ (ii) 60⁰
(iii) 15⁰ (iv) 0⁰
Answer: (i) 30⁰,
According to law of reflection incident angle is equal to angle of reflection.
e) The angle of incidence on a plane mirror is 30⁰. The angle between the incident ray and the reflected ray is
(i) 30⁰ (ii) 15⁰
(iii) 60⁰ (iv) 90⁰
Answer: (iii) 60⁰
Given,
Angle of incidence = 30⁰
Angle between incidence ray and reflected ray = angle of incidence + Angle of reflection
But we know that,
Angle of incidence = Angle of reflection
Therefore
Angle between incidence ray and reflected ray = 2 × angle of incidence
Angle between incidence ray and reflected ray = 2 × 30⁰
Angle between incidence ray and reflected ray = 60⁰
f) The property due to which a light ray striking a surface is returned back into the same medium is called
(i) refraction (ii) reflex action
(iii) reflection (iv) regression
Answer: (iii) reflection
Bending of light at the surface in same medium is called reflection of light.
g) A ray of light after reflection from a mirror is known as
(i) reflected ray (ii) normal
(iii) incident ray (iv) refracted ray
Answer: (i) reflected ray.
h) The speed of light is maximum in
(i) glass (ii) water
(iii) air (iv) wood
Answer: (iii) air.
Light travels fastest in air than liquid or ssolid.
i) A red rose is seen in green light. It will appear
(i) red (ii) blue
(iii) yellow (iv) black
Answer: (iv) black
Red surface does not reflect green light so rose appears black in green light.
j) The primary colours are
(i) Red, Blue and Yellow
(ii) Magenta, Yellow and Cyan
(iii) Red, Blue and Cyan
(iv) Blue, Green and Red.
Answer:
(iv) Blue, Green and Red.
Blue, Green and Red are three basic colours. Light constitute from these basic colours.
B.) Short/Long Answer Questions :
1) What do you mean by the term reflection of light ?
Answer:
We can see this beautiful world because of reflection phenomenon.
When light falls on surface then it bends in same medium, this property of light is called reflection.
Definition : Bending of light in same medium is called reflection.
2) How is a plane mirror made?
Answer:
Plane mirror consists of a glass. One side of this glass is coated with silver. Another side of this glass is polished with some transparent material whish works as a perfectly reflecting surface. We illustrate plane mirror using following diagram.
3) Explain the following terms :
Incident ray, Reflected ray, Angle of incidence,
Angle of reflection, Normal.
Answer:
- Incidence Ray:
The light ray which emitted from source or sometimes objects and fall on the surface of mirror is called incidence ray.
- Reflected Ray:
The light ray which bends backs in same medium after falling on reflecting surface is called Reflected ray.
- Angle of incidence:
The angle made between incidence ray and perpendicular side of the mirror(normal)is called angle of incidence.
Angle
- Angle of reflecting:
The angle made between reflected ray and perpendicular side of the mirror is called angle of reflection.
- Normal:
The perpendicular side drawn to mirror is called normal. Normal does not part of the images it is not real thus It is denoted by dotted lines always.
4) Draw a diagram showing the reflection of a light ray from a plane mirror. Label on it the incident ray, the reflected ray, the normal, the angle of incidence i and the angle of reflection r.
Answer: The diagram below shows that reflection of light rays from a plane mirror.
5) State the two laws of reflection of light.
Answer: Light bends back in same medium is called reflection of light. Reflection of light holds some laws. These laws are as follows-
- The angle between incident ray and normal is called angle of incidence and the angle between reflected ray and normal is called angle if reflection. Angle of incidence is equal to angle of reflection.
∠I = ∠ r
- Incidence ray, reflected ray and normal lies in a same plane.
- Incidence Ray and reflected ray are lies opposite side to each other.
6) Describe an experiment to verify the laws of reflection of light.
Answer:
7) A ray of light falls normally on a plane mirror. What is the angle of incidence ?
Answer:
We know that, angle of incidence is always equal to angle of reflection. When incident ray falls normally on a plane mirror then it made angle of 0⁰ . Thus according to this law, angle of reflection must be 0⁰.
So reflected ray goes back along normally.
8) Draw a diagram to show the reflection of a light ray incident normally on a plane mirror.
Answer:
When a ray incident along normally to mirror then it reverse back along same path.
9) The diagram in fig 4.18 shows an incident ray AO and the reflected ray OB from a plane mirror. The angle AOB is 30⁰. Draw normal on the plane mirror at the point O and find
(a) the angle of incidence
(b) the angle of reflection
Answer:
Answer:
Normal is missing in above diagram. We draw normal in figure and calculate angle of incidence and angle of reflection.
Given,
Angle of incidence + angle of reflection = 30
According to law of reflection, angle of incidence = Angle of reflection
2 × Angle of incidence =30
Angle of incidence = 30/2
Angle of incidence = 15⁰
Thus, angle of incidence = Angle of reflection = 15⁰
10) In the following diagrams (Fig 4.19), measure and write the angle of incidence and draw the reflected ray in each case.
Answer:
We know that, angle of incidence is always equals to angle of reflection. We can calculate angle of reflection using this law for above diagram.
a) The incident ray and reflected rays lies on normal. Thus the angle reflection will be 0⁰.
b) incidence ray is close to normal. Thus the incident angle may be approximately 30⁰. Therefore Angle of reflection will be 30⁰.
c) The incident ray goes away from normal in figure c. Thus the incidence angle may be greater than 30⁰. Therefore Angle of reflection will also greater than 60⁰.
11) The diagram in Fig 4.20 shows an incident rayAO and the normal ON on a plane mirror. Draw the reflected ray. State the law you use to draw the direction of the reflected ray.
Answer: we know that,
Law of reflection: The angle of incidence is always equals to angle of reflection.
The angle of incidence in above figure is 60⁰. Thus angle of reflection is also 60⁰.
12) The following diagram (Fig 4.21) shows an incident ray AO and the normal ON on a plane mirror. Find the angle of incidence and angle of reflection.
Answer: Given,
The angle between incident ray and surface of mirror = 30⁰.
But we know that, angle between surface and normal will be 90⁰.
Therefore,
Incident angle = 90 -30
Incidence angle = 60⁰.
According to law of reflection,
Incident angle = angle of reflection.
Incidence angle = angle of reflection = 60⁰.
13) State in words, how do you find the location of image of an object formed by a plane mirror.
Answer: If the object is placed in front of mirror then image will be produced behind the mirror. When the object moves away from mirror then image also moves behind the mirror or vice versa.
14) Draw a ray diagram showing the formation of image of a point object by a plane mirror.
Answer: The image of point object placed in front of mirror will be produced as,
15) The following diagram (Fig. 4.22) shows a point object O placed in front of a plane mirror. Take two rays from the point O and show how the image of O is formed and seen by the eye.
Answer:
16) State four characteristics of the image formed by a plane mirror.
Answer:
Characteristics of the image formed by a plane mirror-
- The image formed by plane mirror will be virtual. We do not capture such images.
- The image will be erect.
- The image formed by plane mirror also be laterally inverted. If you move your right hand in front of mirror then left hand of image will moves.
- Size of the image will be same as size of the object. Mirror does not amplify size of image.
- Location of image formed by plane mirror is behind the mirror. When the object moves away from mirror then image also moves behind the mirror or vice versa.
17) How is the position of image formed by a plan mirror related to the position of the object ?
Answer:
Position of image formed by plane mirror is behind the mirror. When the object moves away from mirror then image also moves behind the mirror or vice versa.
18) You are standing at a distance 2 metre from plane mirror.
(a) What is the distance of your image from mirror ?
(b) What is the distance between you and image ?
Answer:
Plane mirror forms image behind the it. The distance between object and mirror is equal to distance between image and mirror.
Given, the distance of object from mirror = 2 m.
(a) The distance of our image from mirror will also be 2 m behind the mirror.
(b) As we know that,
Distance between image and object = distance of image + distance of object.
But for plane mirror,
Object distance = image distance,
Therefore,
Distance between image and object = 2 × distance of image.
Distance between image and object = 2 × 2
Distance between image and object = 4 m.
The distance between me and image will be 4 m.
19) What is meant by lateral inversion of an image in a plane mirror ? Explain it with the help of a diagram.
Answer:
20) Write down the letter C and I as seen in a plane mirror.
21) What is irregular reflection ? Give an example.
Answer: When light falls on smooth plane mirror then it reflects back in same medium. But when the reflecting surface does not plane or smooth perfectly then light rays bends according to that surface. Each light ray incident on surface with different value. Therefore Angle of reflection also different for each ray. Thus image produced after reflection from such surfaces will be blur. This reflection of light from rough surfaces us called irregular reflection.
Examples-
- Reflection of light on rigid rough wall.
- Reflection of light from surface of wood.
22) How do we see objects around us ?
Answer:
We can see this beautiful world because of light. Light gives sensation of vision to us.
The process of seeing objects is as-
- Light fall on objects and reflects.
- This reflected rays reach to our eyes.
- Our eyes get sensation of different colours and we can see this object.
- The image formation in our eyes is a beautiful and complex process.
23) State two uses of a plane mirror.
Answer:
Uses of a plane mirror:
- Plane mirrors are widely used for domestic purposes to see the image of ours.
- Plane mirrors are used in barber’s shop. The angle between to mirrors places in barber’s shop is critical angle. At this angle, multiple images will be produced.
24) Can light travel in vacuum ?
Answer: Yes, light can travel in vacuum also. Light is a transverse wave so it can travels in vacuum.
25) State the speed of light in (a) air, (b) glass.
Answer:
Speed of light is depend on medium. It will be faster in rarer medium than denser.
- Speed of light in air = 3 × 10⁸ m/s. It will be fastest in vacuum. The speed is approximately 3 lacs kilometer per second.
- The speed of light in glass is 2 × 10⁸ m/s.
26) State whether light slows down or speeds up in the following cases :
(a) Light going from air to glass.
(b) Light going from glass to water.
(c) Light going from water to air.
Answer: Speed of light is depend on medium. It will be faster in rarer medium than denser.
(a) Light going from air to glass- glass is denser than air thus speed of light decreases when traveling from air to glass.
(b) Light going from glass to water- glass is denser than water thus speed of light increases when traveling from glass to water.
(c) Light going from water to air- water is denser than air thus speed of light increases when traveling from water to air.
27) What are primary colours ? Name the three primary colours.
Answer: Primary colours are the basic components of white light. When we mix these colours in equal proportions then we get white colour.
There are three primary colours.
- Red
- Blue
- Green
28) What are secondary colours ? Name the three secondary colours.
29) Fill in the blanks with the appropriate colour :
(a) Blue + ——— = Cyan
(b) Red + Blue + ——— = White
(c) Red + Blue = ——–
(d) Green + Red = ———
Answer:
(a) Blue + Green = Cyan
(b) Red + Blue + Green = White
(c) Red + Blue = Magenta
(d) Green + Red = Yellow.
30) The leaves appear green when seen in white light. Give a reason.
Answer: The colour of the image will depends on reflecting surface. White light consists of seven basic colours. The leaves of plants are made up from chlorophyll pigment which reflects green light only. Thus all six colours absorbs this surface of leaves and reflect only green colour. Therefore leaves appears green in white light.
31) A rose appears red in white light. How will it appear in (i) green light, (ii) red light ? Give a reason for each of your answers.
Answer:
A red rose appears red in white light because, it reflects only red light from seven colours of light.
It appears differently according to incident light.
(i) In green light- rose appears black in green light because it absorbs green light and does not reflect it.
(ii) In red light- Red rose appears red in red light. It reflects red colour which incident on it and appear more reddish.
32) Why does a piece of paper appear white in sunlight ? How would you expect it to appear when viewed in red light ?
Answer: The colour of object will depends on reflecting surface and light incident on it.
White paper reflecting all the colours of white light so it appears white.
But when we incident red light on white paper then it reflects red colour and appears reddish.
33) A piece of paper appears black in sunlight. What will be its colour when seen in red light ?
Answer: Black colour surface have the ability to absorb all the colours. If the paper appears black in white light of sunlight then it must be black. So it does not reflects red colour thus it appear black in red light also.