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Karnataka State Syllabus Class 8 Science Important Questions Chapter 17 Our Colourful World
Question 1.
What is light?
Answer:
Light is a form of energy which causes the sensation of sight.
Question 2.
List some of the uses of light.
Answer:
Light enables us to see the objects around us. Sunlight is essential for green plants to carry out photosynthesis. The food that organisms consume comes directly or indirectly from sunlight. Light is essential for almost all our activities. Light can also be used to produce electricity as in photocells.
Question 3.
How can we say that light is a form of energy?
Answer:
Light can do work. Light can also be converted into other forms of energy. From this we can say that light is a form of energy.
Question 4.
What is a mirror?
Answer:
Any smooth highly polished surface that reflects most of the light that falls on it is called a mirror.
Question 5.
List the various types of mirrors that you know.
Answer:
There are several types of mirrors namely plane mirror, concave mirror, convex mirror, parabolic mirror, etc.
Question 6.
What is reflection of light?
Answer:
When light falls on a surface, some or most of the light is turned back from the surface. This phenomenon of bouncing back of light from a surface is called i flection.
Question 7.
Show by a diagram, the reflection of light by a plane surface. Mark in the diagram the incident light and the reflected light.
Answer:
Question 8.
What is meant by incident light and reflected light? ..
Answer:
The incoming light is referred to as incident light. This is the light that falls on a surface. The light that is bounced away from a surface is termed the reflected light. This is the light that is sent back from a surface.
Question 9.
What is a ray of light? Represent it by a diagram.
Answer:
The path taken by light between two points is called a ray of light. A ray of light is represented by a straight line with an arrow in the middle. The arrow indicates the direction of the light ray.
Question 10.
What is a beam of light?
Answer:
A collection of light rays is called a beam of light.
Question 11.
What is incident ray and reflected ray?
Answer:
The light ray which strikes any surface is called the incident ray. The ray that comes back from the surface after reflection is known as the reflected ray.
Question 12.
What are
- convergent rays
- divergent rays
- parallel rays of light?
Show them by separate diagrams.
Answer:
- Convergent rays: Light rays which are moving towards a point are called convergent rays of light.
- Divergent rays: Light rays which are emerging from a point are called div ergent rays of light.
- Parallel rays: Light rays which are parallel to one another are called parallel rays of light. These rays neither diverge nor converge.
Question 13.
What is regular reflection? Explain.
Answer:
When a beam of light falls on a perfectly flat and well polished surface, the beam of light is reflected according to a definite pattern resulting in the formation of an image. This type of reflection is called regular reflection.
For example, when parallel rays of light fall on a flat, smooth and well polished surface, the reflected rays will also be parallel. This is an instance of regular reflection.
Question 14.
What is irregular reflection? Explain.
Answer:
When light falls on a rough and unpolished surface, the reflection occurs in an irregular manner and the reflected rays are scattered in all directions. Such type of reflection is called irregular reflection. Irregular reflection is also called diffused reflection.
For example, when parallel rays of light fall on an uneven and unpolished surface, the reflected rays are not parallel. They are found scattered in all possible directions. This is an instance of irregular reflection.
Question 15.
What is the difference between regular and irregular reflection?
Answer:
Regular reflection:
- Occurs when a beam of light falls on a smooth and polished surface.
- Reflection will be in a fixed direction.
- Reflected beams will be parallel.
- Visible only from that direction in which it reflects the light.
- Image is formed.
Irregular reflection:
- Occurs when a beam of light falls on a rough surface.
- Reflection will be in different directions and diffused.
- Reflected beams will not be parallel.
- Can be seen from any position near it.
- Image is
Question 16.
Show by a suitable diagram the reflection of a light ray from a mirror. Show in the diagram the incident ray, the reflected ray, normal at the point of incidence, angle of incidence and the angle of reflection.
Answer:
In the diagram given aside, AO is the ray that is falling on a plane mirror. It is called the incident ray. O is the point of incidence. It is also the point of reflection. OB is the ray coming back from the mirror. It is called the reflected ray.
ON is the normal drawn to the mirror at the point of incidence. ∠AON is the angle of incidence and ∠NOB is the angle of reflection.
Question 17.
Define
- angle of incidence
- angle of reflection.
Answer:
- The angle formed between the incident ray and the normal at the point of incidence is called the angle of incidence.
- The angle formed between the reflected ray and the normal at the point of incidence is called angle of reflection.
Question 18.
State the laws of reflection of light.
Answer:
There are two laws of reflection of light. They are
- Angle of incidence is equal to the angle of reflection.
- The incident ray, the reflected ray and the normal drawn to the mirror at the point of incidence all lie in the same plane.
Question 19.
What is an image? Mention its types.
Answer:
When light rays coming from a point are made to fall on a mirror or passed through a lens, meet or appear to meet at some other point, then, the second point is called the image of the first. Images formed by optical systems are classified into two types
- virtual image
- real image.
Question 20.
Distinguish between virtual image and real image.
Answer:
Virtual image:
- Cannot be caught on a screen.
- Erect with respect to the object.
- Formed by divergent rays of light.
- Formed behind the mirror.
Real image:
- Can be caught on a screen.
- Inverted with respect to the object.
- Formed by convergent rays of light.
- Formed in front of the mirror.
Question 21.
List the properties of the image formed by a plane mirror.
Answer:
The images formed by a plane mirror are
- erect and virtual. They cannot be caught on a screen.
- of the same size as the object.
- formed at the same distance behind the mirror as the object is in front of the mirror.
- laterally inverted.
Question 22.
What is meant by lateral inversion? Explain with an example.
Answer:
In a plane mirror, the left hand side of the object appears as the right hand side of the image and vice-versa. This sideward inversion is known as lateral inversion. Stand in front of a plane mirror and salute with your left hand.
The image seems to salute with the right hand. Similarly, when we write the letter F on a sheet of paper and hold it in front of a mirror, we can see the image laterally inverted.
Question 23.
When can two plane mirrors form more than two images? What is this due to?
Answer:
When we place an object between two plane mirrors held at an angle to each other, we get more than two images. Multiple images are formed by multiple reflections of light.
The number of images formed will increase with the decrease in the angle between the two plane mirrors. When the two plane mirrors are parallel to each other, infinite images are formed by them.
Question 24.
Name any two devices that work on the principle of multiple reflections.
Answer:
Kaleidoscope and periscope are two devices that work on the principle of multiple reflections of light.
Question 25.
What are spherical mirrors? Mention their types.
Answer:
A mirror whose reflecting surface forms a part of a sphere is called a spherical mirror. There are two types of spherical mirrors
- concave mirror
- convex mirror.
Question 26.
What is a concave mirror? How do you represent a concave mirror in a diagram?
Answer:
A spherical mirror whose reflecting surface is curved inwards is called a concave mirror.
Question 27.
What is a convex mirror? How is a convex mirror represented in a diagram?
Answer:
A spherical mirror whose reflecting surface is curved outwards (bulged) is called a convex mirror.
Question 28.
Define the following with respect to a spherical mirror
- Pole
- Centre of curvature
- Radius of curvature
- Principal axis.
Answer:
1. Pole:
The centre of the reflecting surface of a spherical mirror is called its pole. It is represented by the letter P.
2.Centre of curvature:
The centre of the sphere of which the given spherical mirror forms a part is called its centre of curvature. It is represented by the letter C.
3. Radius of curvature:
The radius of the sphere of which the given spherical mirror forms a part is called its radius of curvature. It is represented by the letter r (r = 2f)
4. Principal axis:
An imaginary straight line passing through the pole (P) and centre of curvature (C) of a spherical mirror is called its principal axis.
Question 29.
What is meant by principal focus of a concave mirror? Show by a diagram.
Answer:
When rays of light close and parallel to the principal axis are made to fall on a concave mirror, after reflection, all the light rays actually pass through a fixed point on its principal axis. This point is known as the principal focus of the concave mirror. It is represented by the letter F.
Question 30.
What is meant by principal focus of a convex mirror? Show by a diagram.
Answer:
When rays of light close and parallel to the principal axis are made to fall on a convex mirror, after reflection, all the light rays appear to diverge from a fixed point on its principal axis. This point is known as the principal focus of the convex mirror. It is represented by the letter F.
Question 31.
What is meant by focal length of a spherical mirror?
The distance between the pole (P) and principal focus (F) of a spherical mirror is called its focal length. It is represented by the letter f.
Question 32.
What is the relationship between focal length and radius of curvature of a spherical mirror?
Answer:
In a spherical mirror, the principal focus (F) lies exactly at the centre of the line joining P and C.
PC = PF + FC
r = f + f
r = 2f
Therefore, the radius of curvature of a spherical mirror is equal to twice its focal length.
Question 33.
What is the focal length of a concave mirror whose radius of curvature is 40 cm?
Answer:
The focal length of a concave mirror is equal to half its radius of curvature
\(f=\frac{r}{2}=\frac{40 \mathrm{cm}}{2}=20 \mathrm{cm}\)
Question 34.
What is meant by ‘object distance’ and ‘image distance’ with respect to a spherical mirror?
Answer:
1. Object distance:
The distance of the object from the pole of the mirror is known as ‘object distance’ It is represented by the letter ‘u’.
2. Image distance:
The distance of the image from the pole of the mirror is known as ‘image distance’ It is represented by the letter V.
Question 35.
Describe a simple experiment to determine the focal length of a concave mirror.
Answer:
Take a concave mirror, a mirror stand, and a screen. Place the given concave mirror on the mirror stand. Direct the mirror towards a bright distant object, say, a tree. Place the screen in front of the mirror.
Move the screen back and forth until a sharp, well defined image is formed on the screen. Measure the distance between the mirror and the screen. This distance gives the approximate focal length of the concave mirror.
Question 36.
Summarise the nature, position and relative size of the image formed by a concave mirror for various positions of the object.
Answer:
Question 37.
For what positions of the object does a concave mirror give a real image?
Answer:
A concave mirror gives a real image when the object is placed beyond its principal focus. The various positions of the object for which a concave mirror gives real image are:
- when the object is between F and C
- when the object is at C
- when the object is beyond C
- when the object is far beyond C.
Question 38.
For what position of the object does a concave mirror give a virtual enlarged image?
Answer:
A concave mirror gives a virtual enlarged image when the object is placed between P and F.
Question 39.
Where should an object be placed in front of a concave mirror to get a diminished image on a screen?
Answer:
In a concave mirror the object should be placed beyond C in order to get a diminished image on a screen.
Question 40.
Where should a source of light be placed in front of a concave mirror to get parallel beam of light?
Answer:
A source of light should be kept at the principal focus (F) of a concave mirror to get a parallel beam of light.
Question 41.
For what position of the object does a concave mirror give an image equal to the size of the object?
Answer:
A concave mirror gives a real image equal to the size of the object when the object is placed at C.
Question 42.
The image formed by a lens is always virtual, erect and smaller in size for an object kept at different positions in front of it. Identify the nature of the lens.
Answer:
Concave lens.
Question 43.
What is the nature, position and relative size of the image formed by a convex mirror?
Answer:
A convex mirror always gives a virtual, erect and a diminished image irrespective of the position of the object.
Question 44.
Why is a convex mirror used as rear view mirror in motor vehicles?
Answer:
A convex mirror is used as rear view mirror in vehicles because
- it always gives a virtual, erect, diminished image of the object.
- it has a wide range of view
- images of nearing objects grow in size and those of receding objects diminish.
Question 45.
Write the mirror formula with respect to spherical mirrors.
Answer:
The mirror formula is \(\frac{1}{f}=\frac{1}{u}+\frac{1}{v}\)
where u → object distance
v → image distance
f → focal length
\(f=\frac{u v}{u+v}\)
Or
focal length \(=\frac{\text { Product of image distance and object distance }}{\text { Sum of image distance and object distance }}\)
Question 46.
List four uses of concave mirrors.
Answer:
Concave mirrors are used
- as background mirrors in torches, search lights and headlights of motor vehicles.
- by dentists to examine teeth.
- in solar furnaces to generate high temperatures.
- in solar cookers.
- as shaving mirrors.
Question 47.
List any two uses of convex mirrors.
Answer:
Convex mirrors are used as rear view mirrors in vehicles. They are also used in laboratory to carry out experiments.
Question 48.
What is refraction of light?
Answer:
Refraction is the bending of light when it passes from one transparent medium to another of different density.
Question 49.
How will a light ray that travels obliquely bend (a) when it travels from a rarer medium to a denser medium, (b) when it travels from a denser medium to a rarer medium?
Answer:
(a) A ray of light that enters obliquely from a rarer medium to a denser medium, on entering the second medium, will bend towards the normal. In this case, the angle of refraction is smaller than the angle of incidence.
(b) A ray of light that enters obliquely from a’denser medium to a rarer medium, on entering the second medium, will bend away from the normal. In this case, the angle of refraction is larger than the angle of incidence.
Question 50.
When will a light ray not change its direction despite changing the medium of propagation?
Answer:
A light that enters from one medium to another along the normal to the interface of the two media will not change its direction.
Question 51.
Mention any two applications of refraction of light in daily life.
Answer:
Two instances of refraction in daily life are
- A coin placed inside a bucket filled with water appears to be raised. This is due to refraction of light.
- A pencil partially immersed in water appears bent due to refraction of light.
Question 52.
Explain the twinkling of stars.
Answer:
Twinkling of stars is the apparent shifting of the position of a star when seen from the earth. This occurs due to refraction of light by the atmosphere of the earth. Different layers of the earth have different densities.
A light ray from a star on entering the atmosphere of the earth undergoes a series of refractions thus changing the apparent position of the star. This causes twinkling of stars.
Question 53.
Why does a pencil partially immersed in water taken in a glass tumbler appear to be bent?
Answer:
A pencil immersed partially in an oblique manner at an angle in water appears bent due to refraction of light. The light coming from the part of the pencil inside water will bend away from the normal on entering air.
As a result, the a Angle of incidence position of the pencil inside water appears to get shifted to a new position. This causes the pencil to appear bent.
Question 54.
Why does a swimming pool appear shallower than what it actually is?
Answer:
The bottom of a swimming pool appears to be raised due to refraction of light. When light rays enter from water to air, they bend away from the normal. This causes an apparent upward shift of the bottom of the pool. This is why swimming pool appears shallower.
Question 55.
What is meant by total internal reflection? Explain.
Answer:
A ray of light travelling from a denser medium to a rarer medium bends away from the normal on entering the rarer medium. When the angle of incidence in the denser medium exceeds a certain minimum, the light ray gets completely reflected back into the same denser medium. This phenomenon is known as total internal reflection.
Question 56.
Whats is the critical angle for a pair of media?
Answer:
When a ray of light travels from a denser medium to a rarer medium, that particular angle of incidence in the denser medium for which the angle of refraction is 90° is called critical angle for the given pair of media.
Question 57.
What are the conditions under which total internal reflection occurs?
Answer:
Total internal reflection occurs when the following two conditions are satisfied:
- The light ray must be travelling from a denser medium to a rarer medium.
- The angle of incidence must be greater than the critical angle.
Question 58.
What is mirage? Why does it happen?
Answer:
Mirage is an optical illusion caused by total internal reflection of light. In desert areas, the air nearer to the surface of the earth is hotter than air at upper levels. The light that falls on a tall object such as a tree will undergo total internal reflection. This gives an apparent image of the object just below it. This creates an illusion of water just below the tree.
Question 59.
Why a does a diamond sparkle?
Answer:
Light travels quite slowdy through diamond. When the faces of a diamond are cut to have a low critical angle, the light that enters through it will undergo total internal reflection many times. This makes the diamond to sparkle.
Question 60.
Mention one application of total internal reflection.
Answer:
The principle of total internal reflection is used in optic fibres which is used in communication.
Question 61.
What is a lens? Mention various types of lenses.
Answer:
Any transparent material bound by at least one spherical surface is called a lens. The various types of lenses are biconvex lens, biconcave lens, plano-concave lens, plano-convex lens and concavo- convex lens.
Question 62.
Which are the two major types of lenses?
Answer:
The two major types of lenses are
- convex lens
- concave lens.
Question 63.
What is a convex lens? Show by a diagram the convergence of light by a convex lens.
Answer:
A lens that is bound by two bulged spherical surfaces is called a convex lens. It is also called a converging lens. A convex lens is thicker in the middle and thinner at the edges.
Question 64.
What is a concave lens? Show by a diagram diverging action of a concave lens.
Answer:
A lens that is bound by two shallow spherical surfaces is called a concave lens. It is also called diverging lens. A concave lens is thinner in the middle and thicker at the edges.
Question 65.
Name the devices that make use of convex lenses.
Answer:
Convex lenses are used in camera, binoculars, spectacles, telescope, projectors, simple microscopes and compound microscopes.
Question 66.
What is the type of lens present in our eyes?
Answer:
Our eyes contain convex lens.
Question 67.
A shopkeeper wanted to fix a mirror which will give a maximum view of his shop. What type of mirror should he use? Give reason.
Answer:
The shopkeeper should fix a convex mirror because it can form images of objects spread over a large area.
Question 68.
What is meant by dispersion of light?
Answer:
The phenomenon of splitting of composite light into its constituent colours is called dispersion of light.
Question 69.
Show by a neat diagram, the dispersion of light through a triangular glass prism.
Answer:
Question 70.
Describe an activity to show that white light consists of seven colours.
Answer:
White light is composite light. This means that white light consists of more than one colour. When a ray of white light is passed through a triangular glass prism, we get a band of seven colours.
When these lights are passed through another identical prism held in inverted position, we get back white light. This shows that white light is made up of seven colours.
Question 71.
What is spectrum of light? Name the colours present in the spectrum of white light.
Answer:
The band of colours obtained by the dispersion of composite light is called spectrum. The spectrum of white light consists of violet, indigo, blue, green, yellow, orange and red colours.
Question 72.
What is formation of rainbow due to? Why can’t we see all the colours in a rainbow?
Answer:
A rainbow is formed due to dispersion of sunlight by droplets of water hanging in air. We cannot see all the colours of sunlight in a rainbow because the colours overlap.
Question 73.
Describe an activity to demonstrate dispersion of light.
Answer:
Take clean water in a shallow trough. Place a plane mirror in inclined position inside the trough such that most part of the mirror is immersed inside water. Allow sun rays to fall directly on the mirror. Get the reflection on a white wall or any other suitable screen. You will see the dispersion of sunlight in the form of a spectrum on the wall.
Multiple Choice Question
Question 1.
A ray of light strikes a plane mirror making an angle of 40° with the surface of the mirror. The angle of reflection for this ray would be
(a) 22°
(b) 30°
(c) 40°
(d) 50°
Answer:
(d) 50°
Question 2.
The rear view mirror in motor vehicles is a
(a) plane mirror
(b) convex mirror
(c) concave mirror
(d) convex lens.
Answer:
(b) convex mirror
Question 3.
Which one of the following can take place when a ray of light is travelling from a denser medium to a rarer medium?
(a) refraction
(b) reflection
(c) dispersion
(d) multiple refraction.
Answer:
(a) refraction
Question 4.
A concave mirror forms a magnified inverted image when the object is placed at.
(a) F
(b) C
(c) beyond C
(d) between F and C
Answer:
(c) beyond C
Question 5.
The mirror formula for a spherical mirror is
(a) \(\frac{1}{f}=\frac{1}{u}+\frac{1}{v}\)
(b) \(\frac{1}{f}=\frac{1}{2 u}+\frac{1}{v}\)
(c) \(\frac{1}{f}=\frac{1}{u}-\frac{1}{v}\)
(d) \(\frac{1}{f}=\frac{1}{u}+\frac{1}{2 v}\)
Answer:
(a) \(\frac{1}{f}=\frac{1}{u}+\frac{1}{v}\)
Question 6.
The laws of reflection hold good for
(a) plane mirrors only
(b) convex mirrors only
(c) concave mirrors only
(d) all types of mirrors
Answer:
(d) all types of mirrors
Question 7.
The apparent bending of a pencil in a glass of clear water is caused by
(a) the reflection of light.
(b) the diffraction of light,
(c) the refraction of light.
(d) the dispersion of light.
Answer:
(c) the refraction of light.
Question 8.
Rays of light from the Sun converge at a point 20 cm in front of a concave mirror. At what distance from the mirror should an object be placed to obtain a real image equal to the size of the object?
(a) 10 cm
(b) 15 cm
(c) 20 cm
(d) 40 cm
Answer:
(d) 40 cm
Question 9.
A convex mirror always gives
(a) A real, inverted, enlarged image of the object
(b) A real, diminished, erect image of the object
(c) A virtual, erect, diminished image of the object
(d) A virtual, inverted, enlarged image of the object.
Answer:
(c) A virtual, erect, diminished image of the object
Question 10.
On a new stainless steel spoon, if you see the image of your face upside down, the part of the spoon acts like
(a) convex lens
(b) concave mirror
(c) convex mirror
(d) concave lens
Answer:
(b) concave mirror
Question 11.
Different colours of light correspond to different light
(a) polarities.
(b) velocities
(c) intensities.
(d) wavelengths.
Answer:
(d) wavelengths.
Question 12.
You can see the image of your face clearly on the surface of still water but not on the surface of disturbed water because of
(a) total internal reflection
(b) refraction
(c) irregular reflection
(d) dispersion
Answer:
(c) irregular reflection
Question 13.
Which of the following can be used to form a real image?
(a) Concave mirror only
(b) Plane mirror only
(c) Convex mirror only
(d) Both concave and convex mirrors.
Answer:
(a) Concave mirror only
Question 14.
If a ray of light is travelling from denser medium to 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 refraction
Answer:
(a) Total internal reflection
Question 15.
In a plane mirror, the image of the letter ‘P looks like,
(a) 
(b) 
(c) 
(d) 
Answer:
(b) 
Question 16.
In the headlights of motor vehicles the bulb is usually placed at the principal focus of the concave mirror. This is done to
(a) make the light rays parallel
(b) make the light rays converge
(c) get real image of the bulb at short distance
(d) get virtual image of the bulb
Answer:
(a) make the light rays parallel
Question 17.
Rainbow is an example for this property of light.
Answer:
(a) refraction
(b) rectilinear propagation
(c) dispersion
(d) reflection
Answer:
(c) dispersion
Question 18.
In a compound microscope, there will be
(a) one convex lens and one concave lens
(b) two convex lenses
(c) two concave lenses
(d) one plane glass and one concave lens
Answer:
(b) two convex lenses
Question 19.
You can see the objects greater in size than your eye because on the retina image formed will be
(a) inverted
(b) straight
(c) laterally inverted
(d) smaller in size than the object
(e) interpreted by the brain appropriately
Answer:
(e) interpreted by the brain appropriately
Fill In The Blanks
1. In a plane mirror, the letter ‘P’ looks like 
2. The type of lens commonly used by watch repairers is convex lens
3. Bending of light as it passes from one transparent medium to another is called refraction
4. Formation of rainbow is due to dispersion
5. A diverging lens is also called concave lens
6. The centre of the reflecting surface of a spherical mirror is called its pole
7. The type of lens used as a hand lens is convex lens
8. The inner surface of a steel spoon acts as a mirror, concave
9. The outer surface of a flat steel plate acts as a mirror, plane
10. The outer shining surface of a round bottom steel bowl acts as a mirror, convex
Match The Following
| A | B |
| 1. Dispersion | (a) Plane mirror |
| 2. Diminished image | (b) Concave lesns |
| 3. Laterally inverted image | (c) Mirage |
| 4. Total internal reflection | (d) Convex Mirror |
| (e) Rainbow | |
| (f) Multiple reflections | |
| (g) Glass slab |
Answer:
1 – e, 2 – d, 3 – a, 4 – c.