Detailed explanations in West Bengal Board Class 7 Science Book Solutions Chapter 1.2 Light offer valuable context and analysis.
WBBSE Class 7 Science Chapter 1.2 Question Answer – Light
(Page no. 16)
1. Fill in the blanks in the following table. Put a [✓] mark in the appropriate place.
Answer:
Class 7 Science Chapter 1.2 Question Answer West Bengal Board –Light
Multiple Choice Questions & Answers : (1 mark for each question)
Question 1.
A luminous object-
i) umbrella
ii) brick
iii) star
iv) wood
Answer:
iii) star
Question 2.
A non-luminous object-
i) sun
ii) firefly
iii) burning candle
iv) stone
Answer:
iv) stone
Question 3.
A transparent object-
i) air
ii) wood
iii) iron
iv) wall
Answer:
i) air
Question 4.
An opaque object-
i) air
ii) wood
iii) water
iv) transparent glass
Answer:
ii) wood
Question 5.
A translucent object–
i) air
ii) water
iii) glass
iv) oil paper
Answer:
iv) oil paper
Question 6.
Light can propagate-
i) without medium
ii) with medium
iii) both are true
iv) both are false
Answer:
i) without medium
Question 7.
Ultra violet rays of sun-
i) can cause skin cancer
ii) is harmful to the DNA molecules
iii) can cause harm lens of eye.
iv) all are true
Answer:
iv) all are true
Question 8.
Number of colours present in sunlight are-
i) five
ii) three
iii) seven
iv) six
Answer:
iii) seven
Question 9.
Flowering is possible in wheat if daylight length-
i) exceeds 12 hours
ii) less than 12 hours
iii) exceeds 8 hours
iv) less than 8 hours
Answer:
i) exceeds 12 hours
Question 10.
Fireflies produce light in their own body-
i) to search food
ii) to communicate between themselves
iii) to attack others
Answer:
ii) to communicate between themselves
Question 11.
When reflection occures from uneven surfaces, it is called-
i) regular reflection
ii) diffuse reflection
iii) none of the two
Answer:
ii) diffuse reflection
Question 12.
In letter M Lateral inversion-
i) will not occur
ii) will occur
iii) may occur, may not occur
Answer:
i) will not occur
Fill in the blanks : (1 mark for each question)
1. The reflection from any smooth surface is ______ reflection of light.
Answer:
regular
2. Tracing paper is an example of ______ object.
Answer:
translucent
3. Objects on the other side of ______ objects can not be seen at all.
Answer:
opaque
4. A ______ camera is a simple camera without a lens but with a tiny aperture.
Answer:
pinhole
5. The sunlight is a mixture of rays of different ______ .
Answer:
colours
6. In a plane mirror if the angle of reflection is 60° then the angle of deviation would be ______ .
Answer:
60°
7. In nature a ______ is formed due to dispersion of sunlight.
Answer:
rainbow
8. Movement of migratory birds from colder regions to warmer regions depends on the intensity of ______.
Answer:
sunlight
9. The ray that represents the light reflected by the surface is known as ______ .
Answer:
incident ray
10. The angle between the normal and incident ray is known as ______ .
Answer:
angle of incidence
11. In a pinhole camera the smaller the hole, the finer is the optical ______ .
Answer:
impression
12. When the source of light is ______ both penumbra and umbra are formed.
Answer:
large
State whether True or False : (1 mark for each question)
1. Firefly can emit light, so it is called source of light.
2. When the size of the light source is very small, we call it a point source.
3. When the source of light is large, only the umbra is formed.
4. The ray of light that bounces back from the mirror is called the index ray.
5. The angle between the normal and incident ray is known as angle of incidence
6. The angle of incidence and the angle of reflection are equal:
7. When reflection occurs from uneven surfaces, it is called regular reflection.
8. Rainbow is formed due to dispersion of sunlight.
9. The sunlight is a mixture of eight rays of different colours.
10. Sir Issac Newton discovered dispersion in 1866.
11. The process of seperation of different colours of light from a polychromatic light is called dispersion.
12. Sunlight is a polychromatic light.
13. Layer of Ozone gas prevents ultraviolet rays.
14. X-ray is one type of invisible light which can not pass through our skin and flesh.
Answer:
1. true
2. true
3. false
4. false
5. true
6. true
7. false
8. true
9. false
10. false
11. true
12. true
13. true
14. false
a. Match the column A with column B.
Column A | Column B |
a) Ultraviolet light very harmful | i) under our skin |
b) Fingerlings of Salmon fishes | ii) is known as spectrum |
c) Movements of migratory birds | iii) straight line |
d) Plants can not prepare food | iv) is regular reflection of light |
e) We have melanin | v) the umbra is formed |
f) The band of seven colours | vi) depends on the intensity of sunlight |
g) The reflection from smooth surface | vii) to the DNA molecules |
h) Light moves along a | viii) without any medium |
i) When we use a point source | ix) without sunlight |
j) Light can propagate | x) die if they receive less light |
Answer:
a) Ultraviolet light very harmful.to the DNA molecules.
b) Fingerlings of Salmon fishes die if they receive less light.
c) Movements of migratory birds depends on the intensity of sunlight.
d) Plants can not prepare food without sunlight.
e) We have melanin under our skin,
f) The band of seven colours is known as spectrum.
g) The reflection from smooth surface is regular reflection of light.
h) Light moves along a straight line.
i) When we use a point source the umbra is formed.
j) Light can propagate without any medium.
Very Short Questions & Answers : (1 mark for each question)
Question 1.
How many kinds of reflection are there?
Answer:
According to the nature of reflector reflection of light are of two types – a) regular reflection and b) diffuse reflection.
Question 2.
What kind of reflection helps us to see objects?
Answer:
Diffuse reflection helps us to see objects.
Question 3.
Which reflection creates image?
Answer:
Regular reflection creates image.
Question 4.
What type of image is formed in a plane mirror?
Answer:
Virtual image is formed in a plane mirror.
Question 5.
State an example of internal full reflection in nature.
Answer:
Mirage is an example of internal full reflection.
Question 6.
Is mirage a real image or virtual image?
Answer:
Mirage is a virtual image.
Question 7.
Which image cannot be formed on a screen?
Answer:
Virtual image cannot be fromed on a screen.
Question 8.
What type of reflection occurs in opaque glass?
Answer:
Diffuse reflection occurs in a opaque glass.
Question 9.
Which of these letters would have lateral inversion – D, T, A, P, B, C, N, W.
Answer:
D, P, B, C and N – these letters would have lateral inversion.
Question 10.
Give a natural example of internal full reflection.
Answer:
Air bubble in water dazzles – this is an example of internal full reflection.
Question 11.
How a ray of light changes its path when it travels from denser medium to rarer medium?
Answer:
A ray of light travelling from a denser medium to a rarer medium bends away from the normal. The angle of refraction is more than the angle of incidence.
Question 12.
Give an example of natural dispersion of light.
Answer:
A rainbow is an example of natural dispersion of light.
Question 13.
What is pure dispersion?
Answer:
In a dispersion where every colour is clearly and distinctly visible, it is called pure dispersion.
Question 14.
Give two uses of magnifying glass.
Answer:
Magnifying glass is used to read small letters in a book. It is used by watchmakers and repairers to see the minute parts of the watches.
Question 15.
At what distance an object should be placed from a convex lens to form image? What kind of image would be formed diminished or enlarged image?
Answer:
If an object is placed nearer than the length of focus of a convex lens then the object would have enlarged virtual image.
Question 16.
Is the velocity of red light and blue light same in glass? If not which one is faster?
Answer:
In glass the velocity of red and blue light are not same. In glass the velocity of red light is the greatest.
Question 17.
How we can distinguish between planet and a star?
Answer:
The heavenly bodies whose light seems to be constant are planets, and those whose lights seems to twinkle are stars.
Question 18.
What is the velocity of light in vaccum?
Answer:
The velocity of light in vaccum is = 3 × 108 m/sec.
Question 19.
What is the velocity of light in water?
Answer:
The velocity of light in water is = 2.25 × 108 m/sec.
Question 20.
What is complementary colour?
Answer:
If the two colours mixed create white colour then they are called complementary colours. As example – yellow and blue are complementary colours.
Question 21.
In which medium water or glass, velocity of light is greater?
Answer:
Light travels faster in rarer medium than in denser medium. So light travels faster in water than in glass.
Question 22.
What type of wave is light?
Answer:
Light is a form of electromagnetic wave that needs no medium to travel.
Question 23.
What is the similarity between reflection and internal full reflection?
Answer:
The similarity is both of them follow rules.
Question 24.
At which position of an object convex lens forms real image?
Answer:
From convex lens if an object is placed at the length same or greater to its focus length then the object will form real image.
Question 25.
What is reflection?
Answer:
When light falls on a mirror, it comes back, this incident is caled reflection.
Question 26.
What is ‘point of incidence’.
Answer:
The point at which the ray of light touches the mirror is known as ‘point of incidence’.
Question 27.
What is reflector?
Answer:
The line which represents the position of the mirror is known as ‘reflector’.
Question 28.
What is normal?
Answer:
The line drawn perpendicular to the reflector at the point of incidence is called the normal.
Question 29.
What is angle of incidence?
Answer:
The angle between the normal and incident ray is known as angle of incidence.
Question 30.
What is angle of reflection?
Answer:
The angle between the normal and the reflected ray is known as angle of reflection.
Question 31.
Write the names of few flowers that bloom during summer days.
Answer:
Sunflower, Balsam, Tulip.
Question 32.
Write the names of few flowers that bloom during winter days.
Answer:
Marrigold, Dahlia, Rose.
Question 33.
Write the names of two plants whose process of flowering – requires day-length more than 12 hours.
Answer:
Wheat. Radish.
Question 34.
Write the names of two plants whose process of flowering requires day length less than 12 hours.
Answer:
Sunflower, Tomato.
Short Questions & Answers : (2 marks for each question)
Question 1.
What is luminous object? Give example.
Answer:
The object which smits light from its own source internally is called luminous object. Sun. stars. glowworm etc. are luminus objects.
Question 2.
What is non-luminous object?
Answer:
The objects which emit light from other source externally or through reflection of the luminous body is called non-luminous object. Bricks. wood, stone etc. are non-luminous object
Question 3.
What is extended luminous source?
Answer:
When the size of a luminous source is larger than the ‘point source’, the source is known as extended luminous source, like – Sun, torch, electric bulb etc.
Question 4.
What is point source?
Answer:
When the size of the light source is very small, we call it point source, like – light passing through a pinhole in black cardboard.
Question 5.
What is transparent medium?
Answer:
The medium through which light can pass easily is called transparent medium. Air, transparent glass. water etc. are known as ‘transparent objects”.
Question 6.
What is ‘Incident Ra’ and what is ‘Reflected Ray’?
Answer:
The ray of the light approaching the mirror is known as ‘Indcident Ray’.
The ray of light that leaves the mirror is called ‘Reflected Ray’.
Question 7.
What is opaque medium?
Answer:
The objects or medium through which light cannot pass at all are called opaque medium. Objects like wood, wall, iron etc. are known as opaque objects as light cannot pass through them.
Question 8.
What is Translucent media?
Answer:
Through some objects light couldn’t pass well. it passes in a mild and faded way, those sorts of objects or media are called translucent objects or translucent media. Ground glass. fog. tracing paper etc are translucent media.
Question 9.
Write down a property of light.
Answer:
Light moves along a straight line. This is one of the properties of light.
Question 10.
What is light ray?
Answer:
The imaginary rectilinear path with an arrow representing a path of light is known as light ray. Light ray does not exist in reality.
Question 11.
What is Beam of light? How many types are they?
Answer:
A collection of light rays is called as beam of light. Beam of light are of three types –
a. Parallel beam of light.
b. Divergent beam of light.
c. Convergent beam of light.
Question 12.
When umbra and penumbra is formed?
Answer:
When the source of light is large, then both the penumbra and umbra are formed, but when we use a point source only the umbra is formed. The darker portion of a shadow is umbra and the partially dark portion is penumbra.
Question 13.
State the laws of reflection.
Answer:
a) The angle of incidence and the angle of reflection are equal.
b) Incident ray, reflected ray and the normal on the reflector at the point of incidence must lie on the same plane.
Question 14.
What is Regular reflection?
Answer:
When reflection occurs on the polished surface it is called Regular reflection. The reflection from any smooth surface is regular reflection of light. The reflection on a plane mirror or curved mirror is an example of Regular reflection.
Question 15.
What is the surface of the separation of the two media?
Answer:
When two liquids of different density is mixed, then the surface of contact of the two liquidshaving different density, is called surface of separation of the two media.
Question 16.
What is the refraction of light?
Answer:
When light ray pases from one medium to another medium, it changes its path from the surface of separation of two media. This phenomenon of light is called refraction of light.
Question 17.
Which letter from A to Z suffer lateral inversion?
Answer:
The letters which suffer lateral inversion are – B, C, D, E, F, G, J, K, L, N, P, Q, R, S, Y and Z.
Question 18.
Why is word ‘AMBULANCE’ written in a reverse way on the car?
Answer:
The letters on the front of an ambulance are written laterally inverted like. The driver of the vehicle moving on road ahead of the ambulance reads these words laterally inverted as AMBULANCE in his rear view mirror and so he gives side to pass the ambulance first.
Question 19.
What is compound light?
Answer:
The Sunlight is actually a compound of number of rays of different colours. This type of light is called compound light.
Question 20.
What is ‘Spectra’? What is ‘dispersion’?
Answer:
When the Sunlight passes through a prism, the lights of different colours are separated from each other. We distinguish the light in seven colours. This band of seven colours is known as ‘Spectra’. The process of separation of different colours of light from a compound light is called as “dispersion”.
Question 21.
Who discovered ‘dispersion’? What are seven colours in Sunlight?
Answer:
The famous scientist Sir Issac Newton discovered ‘dispersion’ in 1666. The seven colours in sunlight are 1. Violet
2. Indigo
3. Blue
4. Green
5. Yellow
6. Orange
7. Red
Question 22.
How the living world is saved from the ultra-violet ray of Sun?
Answer:
in the atmosphere high above the Earth, there is a layer of Ozone gas. This is known as Ozonosphere. This layer absorbs Ultra-violet rays otherwise the living world would have been in great danger.
Long Questions & Answers : (3 marks for each question)
Question 1.
What is Deffuse reflection?
Answer:
When reflection occurs in uncven surfaces. then it is called diffuse reflection of light. In diffuse reflection, when a parallel beam of light falls on a rough surface. they donot remain parallel after reflection. The diffused rays, after reflection goes in different directions. For example, the reflection on the wall, trees, soil, screen of the cinema hall etc.
Question 2.
What is a pinhole camera and how does it work?
Answer:
A pinhole camera is a simple camera without a lens but with a tiny apperature. A pinhole camera – effectively a light-proof box with a small hole in one side. Light frpom a scene passes through the aperture and projects an inverted image on the opposite side of the box, which is known as the camera obscure effect.
Question 3.
Why the ozone layer is being progressively destroyed? State its ill-effect.
Answer:
Various human activities produce such gaseous compound that reach the ozone layer and destroy ozone molecules and prevent their formation. As a result of this. the ozone layer is being progressivly destroyed. Progressive destruction of the Ozone layer leads to the rise in the number of skin cancer patients all over the world day by day.
Question 4.
Write a short note on X-ray.
Answer:
X-ray is one type of an invisible light. It can pass through our skin and flesh, but it cannot pass through the bones. So we can have a photograph of the damaged portion of the bone. But prolonged use of X-ray can also cause cancer. For that reason, a baby in the womb must not be exposed to X-ray.
Question 5.
Why cannot a plant survive in dark conditions?
Answer:
It is not possible for plants to prepare food with sunlight. The green parts of plants absorb solar energy and convert it into energy of food. Different parts of plant body are able to respond to external stimuli by utilising the energy derived from food. That is why, plants cannot survive when placed in dark conditions.
Question 6.
What is Rainbow? How is it formed?
Answer:
The Rainbow is formed in nature by the process of natural dispersion of sunlight. Rainbow is generally seen in the afternoon or evening sky after a shower. There are some floating water droplets in the sky. The sunrays while passing through them undergoes dispersion and finally it produce rainbow.
Question 7.
Name one harmful invisible light. Explain how this light is harmful.
Answer:
One harmful invisible light is Ultra-Violet light. It is extremely harmful to cells of living organism. It can cause harm to lens of eye and also to the photosensitive cells in the inner layer of eyeball (ratina). Besides, the ultra-violet ray of sun can cause skin canser.
Question 8.
Distinguish between reflection of light and refraction of light.
Answer:
When light rays move from one medium to another medium. a part of the light after undergoing reflection on the surface goes back to the former medium. This phenomenon is known as reflection of light. The remaining part of the light rays enter into the second medium and changes its path. Then each ray of light moves along a different straight line. This phenomenon is called refraction of light.
Question 9.
When a pencil or stick is placed in a glass of water, it appears to be bent, why?
Answer:
This is due to refraction of light in passing from water to air. The rays of light coming from tip of pencil or stick inside water, bend away from the normal at the surface of separation of the two media. So the immersed part of the pencil or stick appears to be bent on the surface of water.
Question 10.
When water is filled in a bucket and seen from above the base of the bucket seems to have risen, why this happen?
Answer:
This happens due to refraction of light. When the light ray from the base of the bucket after passing through water (the denser medium) enters into air (the rarer medium), at that time it deviates from the normal at the surface of separation. When it reaches our eye we can see the image of the base at a height above its original position.
Question 11.
Now try to find the examples of following influences in different animals –
a. Movement of animals under the influence of light.
Answer:
Locust stop their movement when sun remains covered by dark cloud.
b. Laying of eggs of animals under the influence of light.
Answer:
Salman fishes lay eggs in sufficient light as the Fingerlings of Salman fishes die if they receive less light.
c. Changing of colour of eye under the influence of light.
Answer:
The eyes of dog, cat. tiger glow at night in darkness.
d. Changing of colour of skin under the influence of light.
Answer:
If some cave-dwelling animals are brought into sunlight, coloured pigments start to be synthesized in their skin.