Class 7 Science Chapter 15 Important Questions
Class 7 Science Chapter 15 Important Questions Answers of Light updated for academic session 2020-2021 based on latest NCERT Books. All the questions are designed in such a way it covers the entire chapter 15 of class 7 Science.These extra questions are important for school unit test as well as periodic exams. After completing the NCERT textbook chapter reading, student must do these extra questions to be confident in exams.
Class 7 Science Chapter 15 Important Questions for 2020-2021
|Contents:||Important Extra Questions|
Class 7 Science Chapter 15 Important Questions Set – 1
Two different types of lenses are placed on a sheet of newspaper. How will you identify them without touching?
On identifying the size of the letters of newspaper, we can differentiate between the two types of lenses.
If image of letters is large or magnified, then the lens is convex lens and if the image of letters is smaller or diminished in size, than the lens is a concave lens.
Suppose we wish to obtain the real image of a distant tree. Explain two possible ways in which we can do it.
In first case, we will use a concave mirror and a screen. As we know that a concave mirror forms a real image of a distant object at its focus, therefore, a real image of a distance tree will be formed on the screen placed at the focus of concave mirror.
In second case, we will use a convex lens and a screen. As convex lens forms a real image of a distant object at its focus, therefore, a real image of a distant tree will be formed on the screen placed at the focus of convex lens.
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.
If on decreasing the distance between the object and lens, the size of the image increases, then the nature of the lens is convex type.
No, if the distance between object and lens is less than the focal length of the lens, then it will form a virtual image which cannot be obtained on a screen.
You are given three mirrors of different types. How will you identify each one of them?
We can identify the mirrors by forming images of an object which are given as below:
- i) Plane mirror: In the case of plane mirror, image will be virtual, erect and of same size as that of object.
- ii) Concave mirror: In the case of concave mirror, image may be real or virtual, inverted or erect and magnified or diminished depending upon the position of the object.
- iii) Convex mirror: In case of convex mirror, image formed will always be virtual, erect and diminished in spite of the position of object.
State two differences between a convex and concave lens.
A convex lens can make images which are in large or smaller or equal to the size of the object whereas concave lens can always make smaller image.
A convex lens makes both real image and virtual images where as a concave lens always makes a virtual image.
Class 7 Science Chapter 15 Important Questions Set – 2
The side mirror of a scooter got broken. The mechanic replaced it with the plane mirror. Mention any inconvenience that the driver of the scooter will face while using it.
As we know that the side mirror of a scooter must be a convex mirror so that we can view a wide range of traffic spread over a large area.
But if plane mirror, is used, we will not be able to see large area of traffic behind us which may create a difficulty in driving vehicle and can cause accident.
How will you prove that light travels in straight line?
Take three card boards of similar size and make a hole in the centre of each. Make sure that the size and position of the hole is the same in each cardboard.
Now put the three card boards in vertical position with the help of bricks in such a way that the holes in the three card boards are in a straight line. Place a lighted candle in front of the cardboard number 1. Look through the hole of the last cardboard number 3.
The flame can be seen clearly through the holes. When one of the cardboards is slightly raised, no flame can be seen. This shows that light travels in a straight line. No flame can be seen when the holes of the three card boards are not in a straight line because light can-not bend.
Describe an activity to show that seven colours can be mixed to get white light.
Take a circular cardboard disc of about 10 cm diameter. Divide this disc into seven segments. Paint each segment with the seven rainbow colours. Make a small hole at the centre of the disc. The disc loosely on the tip of a refill of a ball pen. Rotate the disc at a high speed. All the colours get mixed up and the disc will appear whitish. Such a disc is popularly known as Newton’s disc.
Give one use of all mirrors and lenses.
- i) Plane mirror: It is used as a looking glass. It is used as reflector in solar cookers.
- ii) Concave mirror: Doctors use concave mirror for examining eyes, ears, nose and throat.
- iii) Convex mirror: Convex mirror is used as a side view mirror. These help the drivers to see the traffic behind them.
- iv) Convex lens: It is used as a magnifying glass in microscope.
- v) Concave lens: It can be used as spectacles to correct nearsightedness.
State the uses of different kinds of mirrors.
- Plane mirror are used as looking glasses. They are also used in solar cookers, periscope and kaleidoscopes.
- Concave mirrors are used as reflectors in the headlights of automobiles. As these give erect and magnified images, these are used by doctors to examine parts of body, such as ear, nose, throat etc.
- Convex mirrors can be used to view a much larger area than is possible with a plane mirror. These are therefore used as rear view or side view mirrors in cars and scooters.
Class 7 Science Chapter 15 Important Questions Set – 3
What do you mean by “reflection of light”?
When light falls on the surface of an object, the object sends the light back. The process of sending back the rays of light which falls on the surface of an object is called reflection of light. The rays of light which falls on an object, is called incident ray. And the ray of light which is sent back by an object is called reflected ray. The object having polished, shining surfaces reflect more-light than objects having unpolished dull surfaces. In fact, any polished and shining surface can act as a mirror.
“Depending on the distance of the object, convex lens forms different kinds of images”. Write any two of them.
i) When an object is placed at a far-off distance from a convex lens, the image formed by convex lens is real, inverted and much smaller than the object.
ii) When an object is placed close to a convex lens or you can say at the less distance than its focal length, the image formed by convex lens is virtual, erect and larger than the object or magnified.
Naina wants to know about image of an object. Do you have any idea about it?
When we look into a mirror, we see our face. What we see in the mirror is actually a “reflection” of our face and it is called “image” of our face.
In this case, our face is the “object” and what we see in the mirror, is its “image”. Actually, when light from our face falls on the mirror, the mirror reflects this light. The reflected light seems to come out of the mirror and we see an image or a picture of our face in the mirror. There are two types of images that can be formed with light. They are “real images” and “virtual images”.
What do you mean by “real image and virtual image”?
Real image: The image which can be obtained on a screen is called a real image. In cinema hall, we see the images of actors and actress on the screen. So, the image formed on a cinema screen is an example of real image.
Virtual image: The image which cannot be obtained on a screen is called virtual image. Our image in a plane mirror is an example of virtual image. The virtual image is just an “illusion” or “unreal image”.
Discuss the formation of “real images” and “virtual images”.
Real image, the image which can be obtained on a screen is called a real image. In cinema hall, we see the images of actors and actress on the screen. So, the image formed on a cinema screen is an example of real image. A real image is formed when light rays coming from an object actually meet at a point after reflection from the mirror. A real image can be formed on a screen because light rays actually pass through it.
Remember, a plane mirror, a convex mirror and a concave lens can never form real images.
Virtual image, the image which cannot be obtained on a screen is called virtual image. Our image in a plane mirror is an example of virtual image. The virtual image is just an “illusion” or “unreal image”. A virtual image is formed when light rays coming from an object appears to meet after reflection from the mirror. It is not possible to obtain a virtual image on the screen because light rays actually do not pass through it. Highly polished metal objects also form virtual images. Sometimes we see the reflection of trees, buildings and other objects in the still water of a lake or pond. These reflections of objects in water are virtual images.
Remember, a type of curved mirror known as convex mirror also forms only virtual images. Similarly, a concave lens also forms only virtual images. A concave mirror and a convex lens can form a virtual image only when the object is very close to them.
Class 7 Science Chapter 15 Important Questions Set – 4
What is meant by lateral inversion?
If we stand in front of a big plane mirror, we see the image of our body in it. Though our image appears to be just as we are, but there is the difference. This is because if we lift “our right hand” then our images lift its “left hand” and if we lift “our left hand” then the image appears to lift its “right hand”. This means that the right side of our body becomes the left side in the image, whereas the left side of our body becomes right side in the image. We say that the image formed in a plane mirror is laterally inverted. In other words, when an object is placed in front of a plane mirror then the right side of object appears to become the left side of image and the left side of image appears to become the right side of image. This change of sides of “an object” and its “mirror image” is called lateral inversion.
Write short notes on concave mirror.
A spherical mirror is that mirror whose reflecting surface is the part of a hollow sphere of glass. A concave surface means “bent in” surface. A concave mirror is that spherical mirror in which the reflection of light takes place at the concave surface or bent in surface. The reflection of light takes place from this concave surface the back side of concave mirror has a silver coating which is protected by a layer of red paint.
“Convex mirror is known as diverging mirror” – Comment.
Suppose a beam of parallel rays of light falls on a convex mirror from the left side. We will see that after reflection from convex mirror, the parallel rays of light are spreading out going away from one another. When the parallel rays of light spread out, we say that the rays of light are diverging.
The parallel rays of light diverge they cannot meet at a point in front of the convex mirror. The focus of a convex mirror is a point behind the mirror from which the parallel rays of light falling on convex mirror appear to diverge after reflection from convex mirror.
The distance from focus up to the convex mirror is called “focal length” of convex mirror. The focus of a convex mirror is behind it. The focus of a convex mirror is virtual because the light rays do not actually pass through it. Since, a convex mirror diverges a parallel beam of light rays, therefore, a convex mirror is known as diverging mirror.
Write down the characteristic of images by plane mirror.
- i) The image formed in a plane mirror is virtual.
- ii) The image formed in a plane mirror is behind the mirror.
- iii) The image formed in a plane mirror cannot be received on screen.
- iv) The image in a plane mirror is of the same size as the object.
- v) The image formed in a plane mirror is erect or upright.
- vi) The image formed in a plane mirror is at the same distance behind the mirror as the object is in front of the mirror.
- vii) The image in a plane mirror is laterally inverted or sideways reversed with respect to the object.
“Convex lens is known as converging lens” – Comment.
Suppose a beam of parallel rays of light falls on a convex lens from the left side, but we will see that after passing through the convex lens, the beam of parallel rays of light converges to one point call “focus” on the other side of convex lens. The focus of a convex lens is a point where parallel rays of light converge after passing through the convex lens.
The distance from focus up to the centre of convex lens is called focal length of convex lens. A convex lens converges the light rays falling on it, therefore, it is also called a converging lens. In this respect, a convex lens is similar to a “concave mirror”.
Class 7 Science Chapter 15 Important Questions Set – 5
What do you mean by “magnifying glass”?
The word “magnify” means “to make something look bigger” then, it actually is. A convex lens gives a magnified image of a closed object. Used in this way, a convex lens is commonly known as magnifying glass. A magnifying glass is a convex lens which is used to make small things look bigger or by producing their magnified images. To use a convex lens as a magnifying glass, the object to be viewed should be placed close to the convex lens so that a magnified and erect image of the small object can be seen on looking through the convex lens. In other words, the object should be at a less distance than the focal length of convex lens then we can see the larger view of the object.
With the help of an activity shows the creation of “Newton’s disc”.
Take a circular cardboard disc of about 10 cm diameter. Divide this disc into seven segments. Paint the seven rainbow colours on these segments. You can also paste, coloured papers on these segments. Make a small hole at the centre of the disc. Fix the disc loosely on the tip of a refill of a ball pen. Ensure that the disc rotates freely. Rotate the disc in the daylight. When the disc is rotated fast, the colours get mixed together and the disc appears to be whitish. Such a disc is popularly known as Newton’s disc.
“The rainbow is produced by the dispersion of sunlight by tiny rain-drops suspended in the atmosphere” – Discuss.
Just after rain, there are a large number of small water drops suspended in the atmosphere. Each of these water drops act like a small prism. As a white sun light enters and leaves these tiny water drops, the various coloured rays of sunlight are deviated by different amounts and a long arch of seven colours called “rainbow” is formed in a sky.
The formation of rainbow having seven colours shows that white sunlight consists of seven colours. The seven colours are red, orange, yellow, green, blue, indigo and violet.
“Depending on the distance of the object, concave mirror forms different kinds of images”. Discuss.
- When an object is placed at a far-off distance from concave mirror, the image formed by a concave mirror is real, inverted and much smaller than the object.
- When an object is placed at a distance greater than twice the focal length of a concave mirror, the image formed by concave mirror is real, inverted, and smaller than the object or diminished.
- When an object is placed at a distance between focal length and twice the focal length of a concave mirror, then the image formed by concave mirror is real, inverted and larger than the object or magnified.
- When an object is placed close to a concave mirror (at a less distance than its focal length), the image formed by a concave mirror is virtual, erect, and larger than the object or magnified.
Write the uses of “concave mirror”.
- i) Concave mirror are used as shaving mirror to see a larger image of the face.
- ii) Concave mirrors are used by dentists to see the large images of teeth of patients.
- iii) Concave mirrors are used as reflectors in torches, headlights of vehicles and search lights to get a strong, straight beam of light.
- iv) Doctors use concave mirror reflectors to produce a parallel beam of light for examining the body parts such as eyes, ears, nose and throat.
- v) Concave mirror shaped “dishes” of dish antennas are used to receive TV signals from satellites which are high up in the sky.