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7th Class CBSE Science NCERT Book

Light: 7th Class NCERT CBSE Science Chapter 15

Light 7th Class NCERT CBSE Science Chapter 15

Question: Briefly mention whether mirror change the direction of light.

Answer: Yes, when a ray of light is incident on the mirror surface, then the mirror changes the direction of light.

Question: 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: Such types of lenses which always form virtual, erect and smaller image in spite of the different positions of an object are called concave lens.

Question: Observe the given figures carefully.

The given figures show the path of light through lenses of two different types represented by rectangular boxes A and B. What is the nature of lenses A and B?

Answer: Since, in first case light rays are converging towards a point, so the lens A will be convex and in case of lens 8, light rays diverge or spread out. So, the lens will be concave lens.

Light NCERT – Question: Name the colors include in the composition of sunlight.

Answer: As we know that sunlight consists of mixture of seven colors (i.e. VIBGYOR).

Question: In the formation of a rainbow, what acts as tiny prisms?

Answer: In the formation of a rainbow, the raindrops suspended in the atmosphere acts as tiny prism.

Question: Briefly mention what makes things visible to us.

Answer: When a light ray falls on an object, then reflected light ray which comes from the object when A falls on our eys, allows us to see things around us.

Light NCERT – Question: Name the type of lens which diverges a beam of parallel rays of light.

Answer: The lens which diverges a beam of parallel rays of light is a concave lens.

Question: Is the image formed by a plane mirror is always upright?

Answer: Yes, the image formed by the plane mirror is always upright.

Question: Explain why are concave and convex mirrors called spherical mirrors.

Answer: Concave and convex mirrors are the parts of sphere whose one side is polished or silvered. So, they are called spherical mirrors.

Question: Is it possible to obtain the image on the screen when the candle is too closed to the concave mirror?

Answer: No, when the candle is too closed to the concave mirror.

Question: Can you get a real image at any distance of the object from the convex mirror?

Answer: No, we cannot get a real image for any position of an object from the mirror because convex mirror always forms virtual, erect and diminished image in spite of the position of object.

Question: Can you name the mirrors used as side mirrors in scooters?

Answer: Yes, in the side mirror of scooters, convex mirror is used to have a wider field of view of the traffic.

Light NCERT – Question: Explain why concave mirror is known as a converging mirror.

Answer: Concave mirror is known as a converging mirror because it converges a beam of parallel rays of light (reflecting from it) at one point.

Question: Name the phenomenon that is involved in the formation of image of an object by a concave mirror.

Answer: Reflection of light is the name of a phenomenon which is involved in the formation of image of an object by a concave mirror.

Question: Mention the type of image formed on a cinema screen.

Answer: Real image is an image which is formed on a cinema screen.

Question: As we know that an image is formed by the actual intersection of rays and can be obtained on the screen. So, state whether it is real or virtual image.

Answer: Since, after the actual intersection of rays, the image obtained on the screen is always real image.

Question: State in brief about the term lens.

Answer: A transparent material which is bounded by both or one spherical surface is known as a lens.

Question: Name the phenomenon that is involved in the formation of image of an object by a convex lens.

Answer: Refraction of light is the name of a phenomenon which is involved in the formation of image of an object by a convex lens.

Light NCERT – Question: Give the form of energy that produces the sensation of vision.

Answer: Light energy is the form of energy which produces the sensation of vision.

Question: What type of mirror is used as a side mirror in a scooter? Why is this type of mirror chosen?

Answer: Convex mirror is used as a side mirror in a scooter because it can form images of objects spread over a larger area.
So, this helps the driver to view the traffic over a large area behind them.

Question: Briefly mention how many types of lens are there. Explain each of them.

Answer:  There are two types of lenses. The lens which is thinner in the middle than at the edges is called a concave lens while the lens which is thicker in the middle than at the edges is called a convex lens.

Question: Consider if one half of a convex lens is covered with a black paper, then state whether this lens produces a complete image of the object.

Answer: If one half of a convex lens is covered with a black paper, then yes, the convex lens will produce the complete image of the object.

Question: Briefly tell that how many reflected rays can be obtained from a single incident ray in respect of a plane mirror.

Answer: For a single incident ray, there will be only one reflected ray that can be obtained in respect of a plane mirror.

Light NCERT – Question: State in brief what we can demonstrate by rotating Newton’s colour disc very fastly.

Answer: By rotating Newton’s colour disc, we can demonstrate that when seven colors are mixed together, then a white colour is formed.

Question: Mention the name of lens which can converge the sun’s rays to a point and burn a hole in the piece of paper.

Answer:
Convex lens is a lens which can converge the sun’s rays to a point and burn a hole in the piece of paper.

Question: An erect and enlarged image of an object is formed on a screen. Explain how this could be possible.

Answer: An erect and enlarged image of an object is formed only by the concave mirror or convex lens when object is inverted and placed between F and 2F.

Question: The distance between an object and a convex lens is changing. It is noticed that the size of the image formed on a screen is decreasing. Is the object moving in a direction towards the lens or away from it?

Answer: In case of convex lens, when we move the object far away from the lens, the size of image decreases and ultimately, when object is at infinity, it will form a point image at focus.

Question: Two different types of lenses are placed on a sheet of newspaper. How will you identify them without touching?

Answer: On identifying the letters of newspaper, we can differentiate the two types of lenses.
If image is large or magnified, then the lens is a convex lens and if the image is smaller or diminished in size for all the positions of object, then the lens is concave .

Question: Briefly state the characteristics of image formed by a plane mirror.

Answer: Characteristics of the Images Formed by a Plane Mirror

Now, we will describe the various characteristics of the images formed in a plane mirror by taking the example of the image of the candle.

  1. When we see the mirror, the image of candle appears to be formed behind the mirror.
  2. Now, pW a vertical screen behind the plane mirror (where the image of candle appears to be situated), then we will notice that the image of candle cannot be formed on the screen. Even if the screen is placed in front of the plane mirror, then the image of candle cannot be formed on the screen. Since, the image of candle formed in the plane mirror cannot be formed on a screen, which means that the image of candle in the plane mirror is a virtual image.
  3. If we see the figure, then we will find that the length and breadth of the image of the candle and its flame to be the same as that of the original candle and its flame. The image of candle in the plane mirror is of the same size as the original candle.
  4. Also if we see the figure, then we will find that the candle has a flame at the top and the image of candle also has a flame at the top. So, the top of candle remains at the top in the image. In the same way, the bottom of candle remains at the bottom in an image. Such an image is called an erect image (or upright image). Therefore, the image formed by a plane mirror is erect.

Light NCERT – Question: The side mirror of a scooter got broken. The mechanic replaced it with a plane mirror. Mention any inconvenience that the driver of the scooter will face while using it.

Answer: As we know that the side mirror of a scooter must be of convex mirror so that we can view a wide range of traffic spread over a large area. But if plane mirror is used, we are not able to see large area of traffic which may be difficult for driving vehicle and can cause accident.

Question: We need a shiny surface for reflection. Explain why.

Answer: Since, the extent of reflection depends upon the shine and smoothness of the surface. So, greater the shining and smoothness of the surface, greater will be the reflection. So, this is a reason why we require a shiny surface for reflection.

Question: Differentiate between a plane mirror, concave mirror and a convex mirror without touching them.

Answer: These mirrors can be differentiated by bringing our face close to each mirror turn by turn. Since, a plane mirror will produce an image of the same size as our face while a concave mirror will produce a magnified image and our face will look much bigger and on other sides, a convex mirror will produce a diminished image and our face will look much smaller like a small child.

Question: The concave reflecting surface of a torch got rusted. What effect would this have on the beam of light from the torch?

Answer: If the reflecting surface of a torch got rusted, it produces diffused light with lower intensity and the objects are not clearly visible in this diffused and lower intensity of light.

Question: Mention the type of image

  1. which can be formed on the screen.
  2. which cannot be formed on the screen.

Answer:

  1. The image which can be formed on the screen is real image.
  2. The image which cannot be formed on the screen is virtual image.

Light NCERT – Question: Mention any two letters of English alphabet whose image formed in a plane mirror appears exactly similar to the letters.

Answer: Since, image formed by the plane mirror shows lateral inversion, i.e. left seems to right or vice-versa. But in the case of alphabets A and H, these letters show the same image in the plane mirror.

Question: 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: If a shopkeeper wanted to fix a mirror which will give him maximum view of his shop, he should use convex mirror. In case of convex mirror, it will give a wider field of view, i.e. it can collect light from a large area spread over them.

Question: Suppose we wish to obtain the real image of a distant tree. Explain two possible ways in which we can do it.

Answer:

  1. In first case, we will use a concave mirror, as we know that concave mirror can form real image, i.e. image on screen. The image of distant tree will be at focus in case of concave mirror.
  2. In second case, we can use a convex lens as it forms real image of a distant object at focus, i. e. image of distant tree in case of convex lens will be formed at focus.

Light NCERT – Question: Explain the term lateral inversion.

Answer: When an image is formed by the mirror in such a way that the left side of image is object’s right side and right side of image is object’s left side. So, this process of change in the sides of the object as well as of image is known as lateral inversion.

Question: State the characteristics of images formed by a concave mirror.

Answer: The characteristics of image formed by a concave mirror are:

  1. The image formed by a concave mirror is not always the same size as the object. It can be either larger or smaller depending on the distance of the object to the mirror.
  2. The image formed by a concave mirror is inverted when the object is kept far from the mirror. So, when it is brought very close to the concave mirror, then an erect and enlarged image is formed.

Question: Boojho made light from a laser torch to fall on a prism. Will he be able to observe a band of seven colors? Explain with a reason.

Answer: No, he will not observe band of seven colors because laser light consists of monochromatic light of single colour. But seven colors of bands are only observed, when a white light is passed through a prism and dispersion takes place.

Question: Briefly explain why in ambulance words are written laterally inverted.

Answer: In case of mirror, the image formed by it is laterally inverted, i.e. left seems to be right and right seems to be left. So, in ambulance there is emergency for patients to reach hospital and by viewing in rear mirror, it can be easily identified of a vehicle and everyone give him a way.

Long Answer Type Questions

Question: Explain with the help of an activity that when seven colors are mixed together, then white colour is formed.

Answer: Firstly, take a circular cardboard disc of about 10 cm diameter, then divide this disc into seven equal segments and paint the seven colors of white light (red, orange, yellow, green, blue, indigo and violet) on these segments (as shown in figure given). Instead of painting the segments, we can also paste seven colored papers on these segments. So, this disc painted with the seven colors of spectrum is called Newton’s disc. Now, make a small hole at the center of the disc.


(a) A disc with seven colors (b) It appears white on rotating

Fix the disc loosely on the tip of a refill of a ball pen. Be confirm that the disc can rotate freely. Now, rotate the disc quickly in day light. So, when the disc is rotated very fast, then all the seven colors mix together due to which the disc appears to be white (see figure). Therefore, the rapid rotation of Newton’s colour disc tells us that mixing of seven colors of the spectrum can give us white light.

Light NCERT – Question: It was observed that when the distance between an object and a lens decreases, the size of the image increases. What is the nature of this lens? If you keep on decreasing the distance between the object and the lens, will you still able to obtain the image on the screen? Explain.

Answer: On decreasing the distance between the object and lens, the size of the image increases, the nature of the lens will be convex type.

If the distance between object and lens is less than the focus of the lens, then it forms a virtual image and this image cannot be obtained on a screen but formed on the same side of the object.

Question: Give the definition dispersion of light. Also, explain why does it take place? Draw a diagram to show the dispersion of white light by a glass prism.

Answer: In the year 1665, Newton discovered by his experiments with glass prisms that white light (like sunlight) consists of a mixture of lights of seven colors. Newton found that if a beam of white light is passed through a glass prism, then the white light splits to form a band of seven colors on a white screen . The band of seven colors formed on a white screen, when a beam of white light is passed through a glass prism, is known as spectrum of white light. The seven colors of the spectrum are Red, Orange, Yellow, Green, Blue, Indigo and Violet .


A prism splits a beam of sunlight into seven colors

So, dispersion of light is phenomenon of splitting up of white light into seven colors on passing through a transparent medium like a glass prism. The formation of spectrum of seven colors indicates that white light is a mixture of seven colors. White light can be sunlight. So, now we can say that the sunlight consists of seven colors.
We can mix these colors to get white light. This can be done by using Newton’s disc, let us try this.

Light NCERT – Question: You are given three mirrors of different types. How will you identify each one of them?

Answer: We can identify the mirrors by forming image of an object.

  1. Plane mirror In case of plane mirror, image will be virtual, erect and of same size as that of object.
  2. Concave mirror In case of concave mirror, image may be real, virtual and magnified or diminished.
  3. Convex mirror In case of convex mirror, image formed will always be virtual, erect and diminished.

Question: Namit was driving a car and suddenly became aware of a loud sound coming from behind. He looked through his rear view mirror and saw an ambulance. He recalled reading that such emergency vehicles often have their name written in the mirror (AMBULANCE) or writing, i. e.
He quickly made way for the ambulance, murmuring a quick prayer for the speedy recovery of the patient inside the ambulance.

  1. Name the type of mirror which is used as a rear view mirror and why?
  2. Mention values did Namit exhibit.

Answer:

  1. Convex mirror is a mirror which is used as a rear view mirror. It is used to get a wide view of traffic behind.
  2. The values exhibit by Namit here is courtesy concerned for other, sympathy, knowledgeable.

Question: One day, Ramesh’s friend was performing their respective experiments given by their teacher. While sitting in the practical lab instead of performing experiment, Ramesh was playing with his meter scale. All the sudden, he held the scale in his hand and started moving in front of tubelight, then he observed the seven colors of white light. After sometimes, he got scolded by his lab teacher for not performing the experiment.

  1. Give the name of phenomenon that leads to the formation of band of seven colors.
  2. Name the constituent colors of white light.
  3. Mention the values shown by Ramesh here.

Answer:

  1. Dispersion of white light is the name of phenomenon which leads to the formation of band of seven colors.
  2. There are seven constituent colors of white light, they are Violet, Indigo, Blue, Green, Yellow, Orange, Red (i.e. VIBGYOR).
  3. Ramesh seems to be quite innovative with respect to the scientific methods and also very eager to learn science regarding phenomena.

Light NCERT – Question: Last Sunday Girish’s mother, Priyanka was preparing the dinner in her kitchen for Girish and his father. Both of them were feeling so much hungry and they sat ideally on their respective chairs and having empty plates and spoons on their dinning table. They were shouting like a 5 years old child that they wanted the dinner so much eagerly in order to fulfill their stomach. At the sometime, Girish’s mother served the cooked food to both of them and the moment, when Girish was just about to eat the rice with the help of spoon, he noticed that the spoon has such a shining surface that it reflected the light rays incident on it.

  1. Explain the importance of shining surface in the reflection of light.
  2. Name the type of spherical mirror represented by the shining steel spoon.
  3. Mention the values shown by Priyanka here towards her family.

Answer:

  1. As we know that the extent of reflection depends upon the shining of surface. So, greater the shining of the surface, more will be the reflection.
  2. Both convex and concave mirrors are represented by the shining steel spoon.
  3. The values shown by Priyanka here is concern and a satisfaction that she gets after serving the cooked food to her son and her husband.

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