KSEEB Solutions for Class 8 Science Chapter 17 Our Colourful World

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Karnataka State Syllabus Class 8 Science Chapter 17 Our Colourful World

KSEEB Class 8 Science Our Colourful World Textbook Exercise Questions and Answers

I. Four alternatives are given to each of the following incomplete statements/questions. Choose the right answer:

Question 1.
Rear view mirror of motor vehicles is a.
(a) plane mirror
(b) convex mirror
(c) concave mirror
(d) convex lens
Answer:
(b) convex mirror

Question 2.
If a ray of light is traveling from a denser medium to a rarer medium and if the angle of incidence is greater than critical angle, then the following takes place
(a) total internal reflection
(b) refraction
(c) dispersion
(d) multiple refractions
Answer:
(a) total internal reflection

Question 3.
A concave mirror forms a magnified inverted image when the object is placed at.
(a) F
(b) C
(c) between F and C
(d) beyond C
Answer:
(c) between F and C

Question 4.
On a new stainless steel spoon, if you see the image of your face upside down, then that part of the spoon acts like,
(a) convex lens
(b) concave mirror
(c) convex mirror
(d) concave lens
Answer:
(b) concave mirror

Question 5.
You can see the image of your face on the surface of still water. But of the water is disturbed your image will not be clear, because of
(a) total internal reflection
(b) refraction
(c) irregular reflection
(d) dispersion
Answer:
(c) irregular reflection

II. Fill in the blanks with suitable words:

Question:
1) The letter ‘p’ looks like _______________ in a plane mirror.
2) The type of lens used by watch repairers is _______________.
3) Bending of light when it passes from one transparent medium to another is called _______________.
4) Formation of a rainbow is due to _______________.
Answer:
1. 9 nine (lateral inversion),
2. convex lens,
3. refraction,
4. dispersion of light

III. Answer the following:

Question 1.
Give one reason to prove that light is a form of energy.
Answer:
a) Plants can carry out photosynthesis with the help of light energy and prepare food. Since light has the ability to do work it is a form of energy.
b) One form of energy can be transferred into other forms. Since a photoelectric cell can change light into electrical energy, it is a form of energy.

Question 2.
What is the difference between regular and irregular reflection?
Answer:
Regular reflection:

• happens on a perfectly flat surface or polished surface.
• image formed will be clear.
• the pattern of incident light is present in reflected light.
  

Irregular reflection:

• happens if the reflecting surface is rough. » image will not be clear.
• since the light gets scattered, the pattern of incident light is not found in reflected light
  

Question 3.
State the laws of reflection.
Answer:

1. The angle of incidence is equal to the angle of reflection.
2. The incident ray, the normal drawn to the mirror at the point of incident and the reflected ray all lie on the same plane.

Question 4.
Mention the uses of concave mirrors.
Answer:
Concave mirrors are used by dentists and in the solar furnace and in headlights of motor vehicles.

Question 5.
Why is the convex mirror used as a rearview mirror in motor vehicles?
Answer:
Convex mirrors always produce an erect diminished virtual image of the object. The driver gets a clear image of the vehicles coming from behind. Therefore it is used as a rearview mirror on motor vehicles.

Question 6.
Mention any two effects of refraction of light in daily life.
Answer:

1. The bottom of the swimming pool appears to be raised.
2. A coin in a cup that is just invisible when seen at an angle can be made visible by pouring water in the cup.

Question 7.
What are the conditions for total internal reflection to take place?
Answer:
The conditions for total internal reflection ‘ to take place are ‘

• Lightray should pass from a denser medium to a rarer medium.
• The angle of incidence must be higher than the critical angle.

Question 8.
Write diagrams to show the refraction of parallel rays of light in
(i) Convex lens
(ii) Concave lens.
Answer:

Refraction of parallel rays in a convex lens.

Refraction of parallel rays in the concave lens.

Question 9.
Mention any four devices that contain a convex lens.
Answer:
A convex lens is used in a simple microscope, compound microscope, telescope, and also in the spectacles used by people who have hypermetropia.

Question 10.
What is the dispersion of light?
Answer:
The splitting up of white light into its constituent colours is called dispersion.

IV. Match the following:-
A – B
1. Dispersion – (a) plane mirror
2. Diminished image – (b) convex lens
3. laterally inverted – (c) mirage image
4. Total internal – (d) convex mirror reflection, (e) rainbow, (f) multiple reflections, (g) glass slab
Answer:
1. e,
2. d,
3. b,
4. c

KSEEB Class 8 Science Our Colourful World Additional Questions and Answers

I. Answer the following:

Question 1.
Mention the sources of light.
Answer:
Major source of light is the sun. Light can also be produced with the help of candles, bulbs, fluorescent lamps, gas lamps etc.

Question 2.
Write the characters of the image formed by a plane mirror.
Answer:

• The image is laterally inverted.
• It is virtual.
• The image on the plane mirror is erect.
• The image is as far behind the mirror as the object is in front of it.

Question 3.
What is meant by lateral inversion?
Answer:
Lateral inversion means left of the object will become right of the image and right of the object will become left of the object,

Question 4.
Draw a ray diagram to show the laws of reflection by a plane mirror.
Answer:

Question 5.
What is a spherical mirror and mention the types?
Answer:
A spherical mirror is a part of a sphere with a polished surface on one side. The two types are concave mirror and convex mirror.

Question 6.
Draw diagrams to show the reflection of parallel rays of light in (1) concave mirror and (2) convex mirror.
Answer:

Question 7.
What is meant by the following terms?
Answer:
a) Pole of the mirror: Pole of the mirror is the center of the reflecting surface. The ray of light along the principal axis which is incident on the pole of the mirror retraces its path, along the principal axis.

b) Principal focus: It is the point, of convergence of a parallel beam of light which is also parallel to the principal axis after reflection. It is denoted by ‘F’.

c) Focal length: The distance between the pole of the mirror and the point of focus of Flo reflected parallel rays of light which are incident on the mirror is called the focal length. It is denoted by ‘f,

d) Centre of curvature: it is the center of the sphere of which the mirror is a part. It is denoted by ‘o’.

e) The radius of curvature: The radius of the sphere of which the mirror is a part is called the radius of curvature. It is denoted by ‘r’. The radius of curvature is twice the focal length of 2 f.

f) Principal axis: The imaginary extended line on which the pole of the mirror, the point of focus and the centre of the curvature is located is called the principal axis.

g) Object distance: The distance of the object from the pole of the mirror is called object distance. It is denoted by ‘u

h) image distance: The distance between the mirror and the image is called image distance, it is denoted by V.

Question 8.
Write the expression to show the relationships among focal length, object length, and image length.
Answer:
\(\mathrm{f}=\frac{\mathrm{u} \mathrm{v}}{\mathrm{v}+\mathrm{u}} \quad \text { OR } \quad \frac{1}{\mathrm{P}}=\frac{1}{\mathrm{u}}+\frac{1}{\mathrm{v}}\)

Question 9.
What is refraction?
Answer:
The bending of light when it passes from one transparent medium to another is called refraction.

Question 10.
What happens when an oblique ray of light travels from air to water?
Answer:
Water is a denser medium than air. When an oblique ray of light travels from a rarer medium to a denser medium, it bends towards the normal. That means the angle of incidence is greater than the angle of refraction.

Question 11.
What happens when an oblique ray of light travels from water to air?
Answer:
Water is a denser medium than air When an oblique ray of light passes from a denser medium to a rarer medium it bends away from the normal. That means angle of incidence will be smaller than the angle of refraction.

Question 12.
What is total internal reflection?
Answer:
The phenomenon of reflection of light when a ray of light travelling in a denser medium gets incident at the interface of two media at an angle greater than critical angle is called total internal reflection.

Question 13.
Under what conditions does the total internal reflection.takes place?
Answer:
Total internal reflection takes place.

• If the light ray travels from a denser medium to a rarer medium.
• The angle of incidence must is greater than the critical angle for the two media.

Question 14.
Mention the applications of total internal reflection.
Answer:
The applications of total internal reflection are

• To explain the sparkling nature of the diamond.
• To explain the formation of mirages.
• It is used in optic fiber cable:
• It is used in endoscopy.

Question 15.
What do you mean by endoscopy?
Answer:
Endoscopy is an instrument by which doctors can observe the inside of the stomach. This instrument works using the principle of total internal reflection.

Question 16.
What is a mirage? How is it formed?
Answer:
Mirage is an optical illusion created by the total internal reflection of light. Under normal circumstances, the density of air is uniform on tar roads and hence light travels in a straight line. The air layer in contact with Earth gets heated up and becomes rare when compared to the upper strata of the atmosphere. The light coming from the distant object undergoes total internal reflection. This creates an illusion like a reflection in the water. The inverted image caused due to the hot air column below is responsible for his. This phenomenon is rare in case of positions above the level of the observer.

Question 17.
What is the lens?
Answer:
Any curved piece of the transparent medium is called a lens. The two types are convex lens and concave lens.

Question 18.
What is the spectrum?
Answer:
A band of seven colours [VIBGYOR] formed by the splitting of white light is called a spectrum.

Question 19.
Draw a neat diagram to show the refraction of light in a prism.
Answer:

Question 20.
Give scientific reasons for the following:-
Answer:

Question 1.
The bottom of a water tank appears to be raised:
Answer:
The bottom of the water tank appears to be raised because the light gets refracted when it enters a different medium.

Question 2.
A pencil partially immersed in an oblique manner in water appears to be bent:
Answer:
A pencil partially immersed in an oblique manner in water appears to be bent due to the refraction of light.

Question 3.
A coin is a cup that is just invisible, can be made visible by pouring water in the cup.
Answer:
When water is poured, the light gets refracted as it enters a different medium. Therefore a coin in a-cup which is just invisible can be made visible by pouring water in the cup.

Question 4.
Stars appear to be twinkling:
Answer:
Stars appear to be twinkling as-the beams of light deviate from their path due to refraction.

Question 5.
Desert travellers some times see the inverted image of palm trees .it as if there is water near the tree:
Answer:
The light coming from the distant object undergoes is total internally reflection as there will be the difference in the density of air in Ho upper strata of the atmosphere due to heat,

Question 6.
In the rainbow, we may not see all the seven colours distinctly:
Answer:
In the rainbow, we may not see all the seven colours due to the overlapping of colours.

Question 7.
Diamond sparkles:
Answer:
Diamond sparkles due to the effect of total internal reflection.

Question 8.
When white light passes through a glass prism the constituent colours get dispersed:
Answer:
All the constituent colors do not bend equally du^ to different refractive indices of the medium for different colours.

II. Fill in the blanks:

Question:
1. Light is a form of ______________.
2. The ability to do work is called ______________.
3. Solar energy can be converted into electrical energy by ______________.
4. The major source of light is ______________.
5. Sun and stars are but the moon is not ______________.
6. The moon appears to be bright by ______________ the of sun.
7. In the motor vehicles, ______________ motors help in viewing the vehicles coming from back.
8. The light ray which strikes any surface is called the ______________.
9. The ray which comes back from the surface is known as ______________.
10. A collection of rays is called a ______________.
11. The image which cannot be caught on a screen is known as ______________.
12. In a plane mirror, the image is ______________ image.
13. The image which can be caught bn a screen is known as ______________ image.
14. In an irregular reflection, a clear image is not formed because reflected rays are ______________.
15. Multiple reflections of light is also called ______________ of light.
16. In Kaleidoscope and periscope, several images are formed due to ______________.
17. ______________ mirrors are used as rearview mirrors.
18. The concave lens never forms ______________ images.
19. The concave lens forms ______________ images When compared to the size of the object.
20. The type of spectrum depends upon the ______________ of the source of light.
21. In a spectrum produced by a composite light, ______________ colour is least refracted.
22. In a spectrum produced by a composite light, ______________ colour is maximum refracted.
23. During a refraction, the red light bends the ______________ and violet bends the ______________.
24. Air is a medium while the glass is a ______________ medium.
25. Mirage is an optical illusion created by ______________ of light.
26. A rainbow is a ______________ of seven colors.
27. Convex mirrors always produce ______________ and ______________ images of an object.
Answer:
1. energy,
2. energy,
3. photocells,
4. sun,
5. self-luminous,
8. reflected light,
7. rearview,
8. incident ray,
9. reflected ray,
10.beam,
11. virtual,
12. laterally inverted,
13. real,
14. scattered,
15. echo,
16. multiple reflections,
17. convex,
18. real,
19. diminished,
20. nature,
21. red,
22. violet,
23. least, maximum,
24. rarer, denser,
25. total internal reflection,
26. spectrum,
27. diminished and virtual.

III. Choose the best alternative.

Question 1.
The band of seven colours obtained by dispersion is
(a) Spectrum
(b) Mirage
(c) Reflection
(d) Refraction
Answer:
(a) Spectrum

Question 2.
Mirage is an optical illusion created by ______________ of light.
(a) Reflection
(b) Refraction
(c) Dispersion
(d) Total internal reflection
Answer:
(d) Total internal reflection

Question 3.
Droplets of water split sunlight to form a spectrum known as
(a) Mirage
(b) Rainbow
(c) dispersion
(d) echo
Answer:
(b) Rainbow

Question 4.
The splitting of white light into seven constituent colours is called
(a) reflection
(b) refraction
(c) dispersion
(d) mirage
Answer:
(c) dispersion

Question 5.
The laws of reflection are true for
(a) plane mirrors only
(b) convex mirrors only
(c) concave mirrors only
(d) all reflecting surfaces
Answer:
(d) all reflecting surfaces

Question 6.
The mirror used in searchlight is
(a) concave
(b) convex
(c) plane
(d) biconvex
Answer:
(a) concave

Question 7.
A ray of light is travelling from a rarer medium to a denser medium then,
(a) i = 90°
(b) i = r
(c) i < r
(d) i > r
Answer:
(d) i > r

Question 8.
Which of the following factors is responsible for the refraction of light?
(a) frequency of light
(b) mass density
(c) optical density
(d) angle of incidence on the medium
Answer:
(c) optical density

Question 9.
Which of the following indicates the correct path of ray when it enters from air to glass?

Answer:

Question 10.
When a ray of light is incident normally on a rectangular piece of glass the value of angle of refraction will be
(a) 180°
(b) 90°
(c) 45°
(d) 0°
Answer:
(d) 0°

Question 11.
In refraction of light through a rectangular glass slab, the incident ray and the emergent ray are
(a) perpendicular to each other
(b) parallel to each other
(c) converging rays
(d) diverging rays
Answer:
(b) parallel to each other

Question 12.
The cause of twinkling of the star is
(a) refraction
(b) blowing of air
(c) density of air
(d) all the above
Answer:
(a) refraction

Question 13.
If the angle of incidence is equal to the critical angle the angle of refraction will be
(a) 45°
(b) 90°
(c) 180°
(d) 0°
Answer:
(b) 90°

Question 14.
The principle used In the optic fibre is
(a) reflection
(b) refraction
(c) dispersion
(d) total internal reflection
Answer:
(d) total internal reflection

Question 15.
The physicist who proved that light consists of seven colours were
(a) Newton
(b) Galileo
(c) Plank
(d) Einstein
Answer:
(a) Newton

Question 16.
During sunset and sunrise, the sky appears red due to
(a) reflection of light
(b) refraction of light
(c) scattering of light
(d) total internal reflection of light
Answer:
(c) scattering of light

Match the following:
A – B
1) Solar furnace – a) prism
2) rearview mirror – b) convex lens
3) spectrum – c) concave lens
4) Rainbow – d) convex mirror
5) telescope – e) water drops
6) myopia – f) concave mirror
Answer:
1. f,
2. d,
3. a,
4. e,
5. b,
6. c