Detailed explanations in West Bengal Board Class 8 Science Book Solutions Chapter 1.4 Light offer valuable context and analysis.
WBBSE Class 8 Science Chapter 1.4 Question Answer – Light
Multiple Choice Questions & Answers : (1 mark for each question)
Question 1.
Reflection of an object occurs due to-
i) refraction
ii) regular reflection
iii) irregular reflection
iv) all of above
Answer:
ii) regular reflection
Question 2.
If ray of light reflects perpendicular on a mirror incident angle becomes
i) 90°
ii) 0°
iii) 180°
iv) 80°
Answer:
i) 90°
Question 3.
If ray of light falls on a mirror at 30^\theta angle the reflected angle becomes-
i) 30°
ii) 90°
iii) 0°
iv) 60°
Answer:
iv) 60°
Question 4.
Virtual image –
i) ean be seen by eyes and reflected on screen ,
ii) can not be seen and can not be reflected on screen
iii) can be seen but can not be reflected on screen
Answer:
iii) can be seen but can not be reflected on screen
Question 5.
Shape of reflection created on plane mirror-
i) is of smaller size than the actual object
ii) is of larger size than the actual object
iii) is same as the object
Answer:
iii) is same as the object
Question 6.
If incident ray and reflected ray falls perpendicular on each other then refelected angle becomes-
i) 90°
ii) 45°
iii) 60°
iv) 180°
Answer:
ii) 45°
Question 7.
White light is mixer of how many colours-
i) 7
ii) 2
iii) 4
iv) 10
Answer:
i) 7
Question 8.
Which of the following series does not have lateral inversion-
i) A, B, C
ii) M, N, O
iii) X, A, O
Answer:
iii) X, A, O
Question 9.
If two plane mirrors are placed at a right angle then reflection number of that object weuld be-
i) 2
ii) 3
iii) 4
iv) 5
Answer:
ii) 3
Question 10.
What are the two marginal colours of a spectrum?
i) red and violet
ii) red and blue
iii) violet and blue
Answer:
i) red and violet
Question 11.
Spectrum occurs due to-
i) reflection
ii) refraction
iii) dispersion
Answer:
iii) dispersion
Question 12.
Distance between an object and mirror is 15 cm. If the object is brought 2 cm nearer the mirror then what will be distance between reflection and object-
i) 13 cm
ii) 4 cm
iii) 26 cm
iv) 36 cm
Answer:
i) 13 cm
Question 13.
We can see sun before sunrise and after sunset in atmosphere because-
i) refraction occurs
ii) reflection occurs
iii) dispersion occurs
Answer:
i) refraction occurs
Question 14.
In internal full reflection-
i) incident angle > reflected angle
ii) incident angle < reflected angle
iii) incident angle = reflected angle
Answer:
iii) incident angle = reflected angle
Question 15.
The optical illusion in desert is called-
i) mirage
ii) halucination
iii) imagination
Answer:
i) mirage
State whether True or False : (1 mark for each question)
1) A real source of light is sun.
2) Virtual image can be seen by eyes and reflected on screen.
3) Refrective index of diamond with respect to air is very low.
4) The image formed in a mirror is a virtual image.
5) Periscopes are used in submarine.
6) Mirrage formed in a desert can be obtained on screen.
7) In total internal reflection the image is much brighter.
8) Shape of reflaction created on plane mirror is of smaller size than the actual size .
9) Spectrum occurs due to dispersion.
10) If the ray of light falls on a mirror at 30° angle the reflected angle becomes 90<sup>6</sup>.
11) Convex lens is used in magnifying glass.
12) An unreal source of light is moon.
13) Two marginal colours of the spectrum are red and blue,
14) The angle of refraction becomes greater than the angle of incidence.
15) Critical angle of diamond with respect to air is very high.
Answer:
1) true
2) false
3) false
4) true
5) true
6) false
7) true
8) false
9) true
10) false
11) true
12) true
13) false
14) true
15) fale
Fill in the blanks : (1 mark for each question)
1. In reflection angle of incidence and angle of reflection is always ______.
2. Irregular reflection follows the two rules of ______.
3. ______ reflection does not form any image.
4. Plane mirror forms ______ image.
5. On a screen of cinema _______ images are formed.
6. An object is 15 cm away from mirror. The reflection formed will be _____ distant from mirror:
7. If an object is kept between two parallel mirrors ______ images are formet’.
8. If an object is kept between two mirrors which are inclined at right angle _______ images are fomed.
9. To form image on cinema screen ______ is used.
10. Virtual inage formed by convex lens is always ______ in respect of the object.
11. ______ object does not have any hateral invesion.
12. In splitting of white light, ______ light deviates the least.
13. Prism does not create any _______, only separates the band of colours present in white light.
14. A _____ is formed by the dispersion of sunlight by tiny drops of water present in the air:
15. To create virtual image by convex lens the object should be kept _____ the focus length.
16. Convex lens is used in ____.
17. Dispersion of white light is done by _______.
18. When a ray of light travels from denser medium to a rerer medium, the angle of regration is _____ than the angle of incidence.
19. When a ray of light strikes at the surface of seperation of the two media at right angle, the angle If refraction is ______.
20. Velocity of light in water _______.
Answer:
1) same
2) reflection
3) irregular
4) virtual
5) numerous
6) 15 cm
7) numerous
8) three
9) convex
10) inverted
11) Symmetrical
12) red
13) colour
14) rainbow
15) outside
16) magnifying glass
17) prism
18) more
19) zero
20) 3 × 108 m / s.
Very Short Questions & Answers : (1 mark for each question)
Qustion 1.
What is light?
Answer:
Light is a kind of energy, which after rearhing our eyes creates a sensation to view things.
Qustion 2.
How much time it takes light to reach from sun to earth?
Answer:
It takes 8.3 minutes to reach light from sun to earth.
Qustion 3.
Name a real and unreal source of light.
Answer:
A real source of light is sun and an unreal source of light is moon.
Qustion 4.
Name an instrument which is made using the principal of reflection of light.
Answer:
By using the principal of reflection of light periscope instrument is made.
Qustion 5.
In a plane mirror if a ray of light reflects at 35° angle then what will be the value of reflected angle?
Answer:
The value of reflected angle would be 35°.
Qustion 6.
If a ray of light strikes a mirror perpendicularly what will be the value of reflected angle?
Answer:
The value of reflected angle would be 0°.
Qustion 7.
Which property of light is proved in a pin-bole camera?
Answer:
The property that light travels in a straight line is proved in a pin-hole camera.
Qustion 8.
In what kind of surface regular reflection occurs?
Answer:
Regular reflection occurs in smooth and polished surface.
Qustion 9.
In what kind of surface irregular reflection occurs?
Answer:
Irregular reflection occurs in rough surface.
Qustion 10.
What will be the critical angle of diamond with respect to air?
Answer:
The critical angle of diamond with respect to air is 24.5°.
Qustion 11.
In internal reflection light reflects from which medium to which medium?
Answer:
In internal reflection light reflects from denser medium to rarer medium.
Qustion 12.
Name a natural phenomena which is created due to internal reflection?
Answer:
‘Mirage’ is a natural phenomena which is created due to internal reflection.
Qustion 13.
What is absolute refractive index?
Answer:
The absolute refractive index of a medium is difined as the ratio of the speed of light in vacuum (or air) to the speed of light in that medium.
Qustion 14.
Is Mirage a virtual reflection or a real reflection?
Answer:
Mirage is a virtual reflection.
Qustion 15.
When light enters a denser medium from a rarer medium how it changes its direction?
Answer:
Light moves to wards the normal.
Qustion 16.
When light enters a rarer medium from a denser medium how it changes its direction?
Answer:
Light moves aways from the normal.
Qustion 17.
What type of image is created in a plane mirror?
Answer:
Virtual image is created in a plane mirror.
Qustion 18.
What type of reflection occurs on the page of a book?
Answer:
Irregular reflection occurs on a page of a book.
Qustion 19.
In what kind of reflection image is formed?
Answer:
In regular reflection image is formed.
Qustion 20.
What type of reflection can not be taken on a screen?
Answer:
Virtual reflection can not be taken on a screen.
Short Questions & Answers : (2 marks for each question)
Qustion 1.
What are the essential conditions for the total internal reflection?
Answer:
There are two necessary conditions for the total internal reflection :
i) The light must travel from a denser to a rarer medium.
ii) The angle of incidence must be greater than the critical angle for the pair of media.
Qustion 2.
What is meant when it is said that the critical angle of water with respect to air is 49° ?
Answer:
The critical angle of water with respect to air is 49° then value of refracted angle would be 90° or one right angle.
Qustion 3.
What is meant when it is said that the critical angle of diamond with respect to air is 24.4° ?
Answer:
The critical angle of diamond with respect to air is 24.4° means when ray of light travels from diamond to air then if value of incident angle is 24.4° then value of refracted angle would be 90° or one right angle.
Qustion 4.
What is critical angle?
Answer:
Critical angle is the angle of incidence in the denser medium corresponding to which the angle of refraction in the rarer medium is 90°.
Qustion 5.
On what factors the value of critical angle depends?
Answer:
The value of critical angle depends on two following factors:-
i) nature of the two media that means the refractive index of the media
ii) the colour of light.
Qustion 6.
What is the refraction of light?
Answer:
The change in direction of the path of light, when it passes from one transparent medium to another transparent medium, is called refraction. The refraction of light is essentially a surface phenomenon.
Qustion 7.
What is real image?
Answer:
The images that can be cast on screen are said to be real image. Rays of light coming from a certain point after reflection or refraction if meet at a point, then the second point is known as the real image of the first one.
Qustion 8.
What is virtual image?
Answer:
The images that can not be cast on screen are virtual images. Rays of light coming from a certain point after reflection or refraction if it seems to come from another point, then the second point is said to be the virtual image of the first point.
Qustion 9.
Write down the laws of reflection.
Answer:
i) The angle of incidence is equal to the angle of reflection.
ii) The incident ray, the reflected ray and the normal at the point of incidence. lie in the same plane.
Qustion 10.
Write down the laws of refraction.
Answer:
i) The incident ray, the refracted ray and the normal at the point of incidence, all lie in the same plane.
ii) For refraction of an obliquely ray of incidence of light of any given colour in a given pair of optical media irrespective of the magnitude of angle of incidence and angle of refraction, the refractive index is a constant.
Qustion 11.
What is total internal reflection of light?
Answer:
When a ray of light travels obliquely from a denser medium to a rarer medium, if the angle of incidence becomes greater than the critical angle of the media, the ray of light returns to the first media getting reflected to the surface of separation of the two media. The incident is known as total internal reflection of light.
Long Questions & Answers : (3 marks for each question)
Qustion 1.
Write the properties of real image?
Answer:
i) Real image is inverted with respect to the object.
ii) A photograph of a real image can be taken.
Qustion 2.
Write the properties of virtual image.
Answer:
- A virtual image is seen with eyes but can not be obtained on a screen.
- A virtual image is erect with respect to the object.
- Virtual image formed on a plane mirror had lateral inversion.
Qustion 3.
Why do water on an aurum leaf appear glittering?
Answer:
From air light enters into the drop of water that moves on the aurum leaf. When rays of light try to come out again in air, it appears on the surface of separation of water and air with angle of incidence, more than the critical angle of the two media. As a result, total internal reflection takes place there. The emergent rays then reach the viewer’s eyes and the viewer finds the area glittering.
Qustion 4.
Why does diar ond look shiny?
Answer:
Refractive index of diamond with respect to air is very high. So, eritical angle of diamond with respect to air is very low only 24.5°. Diamond is cut in such a way that when a ray of light attempts to emerge for diamond, the angle of incidence exceeds the magnitude of eritical angle of diamond in air. As a result, total internal reflection occurs and diamond look shiny.
Qustion 5.
Why stars twinkle?
Answer:
Stars are very far away from earth. So the light which emits from star passes through different layers of air. Continuous change in density and refractive index of the air take place due to continuous change of temperature. As a result, the direction of light changes every moment while passing through the succesive air layers. As a result the star seems twinkling to the eye of viewer.
Qustion 6.
Distinguish between real image and virtual image.
Answer:
| Real image | Virtual image |
| 1) A real image is formed due to actual intersection of the reflected rays. | 1) A virtual image is formed when the reflected rays meet if they are produced back wards. |
| 2) A real image can be obtained on a screen. | 2) A virtual image can not be obtained on a screen. |
| 3) A real image is inverted with respect to the object. | 3) A virtual image is erect with respect to the object. |
Qustion 7.
Which of the following reflections are real image or virtual image, Give reasons.
i) Image on cinema screen,
ii) Image formed on still water of pond.
iii) Image formed on photography plate.
iv) Image formed by concave lens.
Answer:
i) Image on cinema screen is real image as it is obtained on screen.
ii) Image formed on still water of pond is virtual as it cannot be obtained on screen.
iii) Image formed on photographic plate is real as it can be seen as photograph.
iv) Image formed by concave lens is virtual as it cannot be obtained on screen.
Qustion 8.
Distinguish between the total internal reflection and reflection from a plane mirror.
Answer:
| Total internal reflection | Reflection from a plane mirror |
| 1) It takes place only when light passes from a denser medium to a rearer medium at an angle of incidence greater then the critical angle for that pair of media. | 1) It takes place when light is incident on a plane mirror from any medium at any angle of medium. |
| 2)The entirfe light is reflected. | 2) Only a part of light is reflected while rest is refracted and absorbed. |
| 3) There is no loss of energy. The energy of reflected ray is same as that of incident ray. | 3) There is a loss of energy. The energy of reflected ray is less than that of the incident ray. |
| 4)The image is much brighter. | 4) The image is less bright and the bright ness as the silvering on miror becomes old and rough. |