NCERT Exemplar Solutions- Class 7 Science- Chapter 15- Light

Question 1. Boojho and Paheli were given one mirror each by their teacher. Boojho found his image to be erect and of the same size, whereas Paheli found her image erect and smaller in size. This means that the mirrors of Boojho and Paheli are, respectively-

(a) plane mirror and concave mirror.

(b) concave mirror and convex mirror.

(c) plane mirror and convex mirror.

(d) convex mirror and plane mirror.

Answer-(c) plane mirror and convex mirror.

Explanation-

• Boojho’s Mirror- Boojho observed his image as erect and of the same size as himself. This type of image formation is characteristic of a plane mirror. Plane mirrors produce images that are virtual, erect, and of the same size as the object.
• Paheli’s Mirror- Paheli saw her image as erect but smaller in size. Such an image is typically formed by a convex mirror. Convex mirrors, which bulge outward, produce images that are virtual, erect, and diminished (smaller than the object).

Therefore, based on the nature of the images observed by Boojho and Paheli, we can conclude that Boojho was using a plane mirror, and Paheli was using a convex mirror.

Also Check Class 7 Science -Chapter 15 – Light- Complete Notes

Question2. 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.

Explanation-

• Concave Mirror- A concave mirror, which curves inward like the inside of a spoon, can form a real image. This happens when the object is placed beyond the focal point of the mirror, causing the reflected light rays to converge at a point. The real image formed is usually inverted and can be projected onto a screen.
• Plane Mirror- A plane mirror, which has a flat surface, always forms a virtual image that cannot be projected onto a screen. The image is erect and of the same size as the object.
• Convex Mirror- A convex mirror, which bulges outward, also forms only a virtual image. The image produced is erect but smaller in size than the object and cannot be projected onto a screen.
• Both Concave and Convex Mirrors- Only the concave mirror, not the convex mirror, is capable of forming a real image under certain conditions.

Therefore, among the options provided, only a concave mirror can be used to form a real image.

Also Check – What is Light? An Easy-to-Understand Guide

Question 3. If an object is placed at a distance of 0.5 m in front of a plane mirror, the distance between the object and the image formed by the mirror will be-

(a) 2 m

(b) 1 m

(c) 0.5 m

(d) 0.25 m

Answer- (b) 1 m

Explanation-

In a plane mirror, the image formed is virtual, and it appears to be at the same distance behind the mirror as the object is in front of it. Therefore, if an object is placed 0.5 metres in front of a plane mirror, the image will appear 0.5 metres behind the mirror. The total distance between the object and its image is the sum of these two distances.

• Distance from the object to the mirror = 0.5 m
• Distance from the mirror to the image = 0.5 m
• Total distance between the object and the image = 0.5 m + 0.5 m = 1 m

Thus, the distance between the object and the image formed by the mirror is 1 metre.

Question 4. You are provided with a concave mirror, a convex mirror, a concave lens, and a convex lens. To obtain an enlarged image of an object, you can use either-

(a) concave mirror or convex mirror.

(b) concave mirror or convex lens.

(c) concave mirror or concave lens.

(d) concave lens or convex lens.

Answer- (b) concave mirror or convex lens.

Explanation-

• Concave Mirror- A concave mirror can produce an enlarged image when the object is placed close to it, within the focal length. In this position, the concave mirror forms a virtual, erect, and enlarged image.
• Convex Mirror- A convex mirror always forms a virtual, erect, and diminished (smaller) image of the object, irrespective of the object’s distance from the mirror. Hence, it cannot be used to obtain an enlarged image.
• Concave Lens- A concave lens, due to its diverging nature, always forms a virtual, erect, and diminished image of the object, regardless of where the object is placed in relation to the lens’s focal length. Therefore, it cannot produce an enlarged image.
• Convex Lens- A convex lens can create an enlarged image when the object is placed very close to it, within its focal length. Under these conditions, the convex lens forms a virtual, erect, and enlarged image.

Based on these properties, the correct option to obtain an enlarged image of an object is by using either a concave mirror or a convex lens.

Question 5. A rainbow can be seen in the sky-

(a) when the sun is in front of you.

(b) when the sun is behind you.

(c) when the sun is overhead.

(d) only at the time of sunrise.

Answer-(b) when the sun is behind you.

Explanation-

A rainbow is formed when sunlight is refracted, or bent, as it passes through raindrops in the atmosphere. The sunlight is then reflected inside the raindrop and refracts again as it exits the drop, creating a spectrum of colours that we see as a rainbow. For a rainbow to be visible, the sun needs to be low in the sky behind the observer, with rain occurring in the part of the sky in front of them. This positioning ensures that the sunlight can enter the raindrops at an angle that allows for the necessary refraction and reflection to create a rainbow. Therefore, a rainbow is most commonly seen when the sun is behind the observer. This phenomenon does not depend on the time of day (such as only at sunrise) but rather on the relative positions of the sun, observer, and rain.

Also Check – Laws of Refraction- A High School Student’s Comprehensive Guide

Question 6. An erect and enlarged image can be formed by-

(a) only a convex mirror.

(b) only a concave mirror.

(c) only a plane mirror.

(d) both convex and concave mirrors.

Answer-(b) only a concave mirror.

Explanation-

• Convex Mirror- A convex mirror always forms a virtual, erect, and diminished (smaller) image, irrespective of the object’s distance from the mirror. Therefore, it cannot form an enlarged image.
• Concave Mirror- A concave mirror is capable of forming an erect and enlarged image. This happens when the object is placed very close to the mirror, specifically within the focal length of the concave mirror. In such a position, the concave mirror forms a virtual, erect, and enlarged image of the object.
• Plane Mirror- A plane mirror always forms a virtual, erect image that is of the same size as the object. It does not magnify or diminish the size of the image.
• Both Convex and Concave Mirrors- Since only the concave mirror (and not the convex mirror) can form an erect and enlarged image under certain conditions, option (d) is not correct.

Thus, among the given options, only a concave mirror has the capability to form an erect and enlarged image when the object is placed appropriately close to it.

Question 7. You are provided with a convex mirror, a concave mirror, a convex lens, and a concave lens. You can get an inverted image from-

(a) both the concave lens and convex lens.

(b) both concave mirror and convex mirror.

(c) both concave mirror and convex lens.

(d) both convex mirror and concave lens.

Answer-(c) both concave mirror and convex lens.

Explanation-

• Concave Lens and Convex Lens- A concave lens always forms a virtual, erect, and diminished image, irrespective of the object’s distance from the lens. It cannot produce an inverted image. A convex lens, however, can form an inverted image when the object is placed beyond its focal point.
• Concave Mirror and Convex Mirror- A concave mirror can form an inverted image when the object is placed beyond the focal length (real, inverted, and diminished), or between the focal point and the centre of curvature (real, inverted, and enlarged). A convex mirror, on the other hand, always forms a virtual, erect, and diminished image and cannot produce an inverted image.
• Convex Mirror and Concave Lens- Neither a convex mirror nor a concave lens can produce an inverted image under any circumstance.

Therefore, the correct answer is (c) both concave mirror and convex lens, as these are the two devices capable of producing an inverted image under specific conditions.

Question 8. An image formed by a lens is erect. Such an image could be formed by a-

(a) convex lens provided the image is smaller than the object.

(b) concave lens provided the image is smaller than the object.

(c) concave lens provided the image is larger than the object.

(d) concave lens provided the image is of the same size.

Answer- (b) concave lens provided the image is smaller than the object.

Explanation-

• Convex Lens- A convex lens can form both erect and inverted images. However, it forms an erect image only when the object is placed very close to the lens, within its focal length. In such cases, the image is erect but also larger than the object, not smaller.
• Concave Lens- A concave lens always forms a virtual, erect, and diminished image, irrespective of the object’s distance from the lens. The image formed by a concave lens is always smaller than the object. This matches the description of the image being erect and smaller than the object.
• Image Size with Concave Lens- The options (c) and (d) are incorrect for a concave lens, as it does not form images that are larger than the object or of the same size.

Therefore, the correct answer is (b) concave lens provided the image is smaller than the object, as this is consistent with the properties of a concave lens in forming images.

Very Short Answer Questions

Question- 9. 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-The lens is a concave lens.

Explanation-

• A lens that consistently forms an image that is virtual, erect, and smaller in size, regardless of the position of the object in front of it, is a characteristic of a concave lens.
• Concave lenses are thinner in the middle and thicker at the edges, causing light rays passing through them to diverge. This divergence results in the formation of a virtual image that appears to be behind the lens.
• Since the image is always smaller than the actual object, it aligns with the properties of a concave lens in optics. Therefore, based on the description of the image formation, the lens in question is identified as a concave lens.

Question 10 – Fill in the blank

Question 10(a)-The inner surface of a steel spoon acts as a ___________ mirror.

Answer-Concave

Explanation-The inner surface of a steel spoon curves inward, similar to the shape of a concave mirror. This curvature causes the light rays to converge, or come together, when they reflect off the surface, a characteristic behaviour of concave mirrors.

Question 10(b)-The outer surface of a flat steel plate acts as a ___________ mirror.

Answer-Plane

Explanation-The outer surface of a flat steel plate is smooth and flat, resembling the characteristics of a plane mirror. A plane mirror reflects light without carving it, producing images that are virtual, erect, and of the same size as the object.

Question 10(c)-The outer shining surface of a round bottom steel bowl acts as a __________ mirror.

Answer-Convex

Explanation-The outer surface of a round bottom steel bowl bulges outward, similar to a convex mirror. Convex mirrors cause light rays to diverge, or spread out, a defining feature of convex mirrors, which produce images that are virtual, erect, and smaller than the object.

Question 10(d)-The inner surface of the reflector of a torch acts as a __________ mirror.

Answer-Concave

Explanation-The inner surface of the reflector of a torch is curved inward, resembling the characteristics of a concave mirror. This design helps to converge the light rays into a focused beam, enhancing the torch’s illumination efficiency, a typical use of concave mirrors.

Question 11- State whether the following statements are True or False.

Question 11(a)-A concave lens can be used to produce an enlarged and erect image.

Answer-False

Explanation-A concave lens always forms a virtual, erect, and diminished (smaller) image, irrespective of the object’s position relative to the lens. It diverges light rays, so it cannot produce an enlarged image. Therefore, the statement that a concave lens can produce an enlarged and erect image is false.

Question 11(b)-A convex lens always produces a real image.

Answer-False

Explanation-While a convex lens can produce a real image, it is not always the case. A convex lens forms a real image when the object is placed beyond its focal length. However, if the object is placed very close to the lens, within its focal length, it forms a virtual and erect image. Therefore, the statement that a convex lens always produces a real image is false.

Question 11(c)-The sides of an object and its image formed by a concave mirror are always interchanged.

Answer-True

Explanation-When an image is formed by a concave mirror, it undergoes lateral inversion, meaning the left side of the object appears as the right side in the image and vice versa. This lateral inversion results in the sides of the object and its image being interchanged. Thus, the statement is true.

Question 11(d)-An object can be seen only if it emits light.

Answer-False

Explanation-Objects can be seen not only when they emit light but also when they reflect light. Most objects we see in our daily life are visible because they reflect light from a light source, like the Sun or a lamp, to our eyes. Emission of light is not the only way for an object to be visible. Hence, the statement is false.

Short Answer Questions

Question 12-What type of mirror is used as a side mirror in a scooter? Why is this type of mirror chosen?

Answer-The type of mirror used as a side mirror in a scooter is a convex mirror. This type of mirror is chosen because it provides a wider field of view. Convex mirrors bulge outward, allowing the rider to see a larger area and more of the road behind and to the side of the scooter, which is crucial for safety while driving. This wider field of view helps the rider to be more aware of their surroundings, including other vehicles, without the need to turn their head significantly. However, it is important to note that while convex mirrors provide a wider view, they also make objects appear smaller, which can affect the perception of distance.

13. Observe the figures given in Figure 15.1 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 –A- Convex lens, B- Concave lens

Question 14-Boojho made light from a laser torch fall on a prism. Will he be able to observe a band of seven colours? Explain with a reason.

Answer-No, Boojho will not be able to observe a band of seven colours when light from a laser torch falls on a prism. This is because a laser emits light of a single colour (monochromatic light), which means it has only one wavelength. When this monochromatic light passes through a prism, it does not split into a spectrum of colours. The dispersion of light into its component colours (like the band of seven colours in a rainbow) occurs only when white light, which contains multiple wavelengths corresponding to different colours, passes through a prism. Since laser light contains only one colour, it will only bend slightly due to refraction but will not disperse into a spectrum of colours.

15. State the correct sequence (1-7) of colours in the spectrum formed by the prisms A and B, shown in Figure 15.2.

Answer 

Question 16-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-The driver of the scooter will face the inconvenience of a reduced field of view. Unlike convex mirrors, plane mirrors do not provide a wider view. They only reflect objects in a limited area directly in front of them. This limited field of view can make it difficult for the driver to see traffic and obstacles behind and to the sides of the scooter, potentially increasing the risk of accidents.

Question 17-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 concave reflecting surface of a torch gets rusted, the quality of the reflected light beam will deteriorate. Rusting can cause the surface to become irregular and less reflective, which will lead to a less focused and weaker beam of light. This decrease in reflectivity and focus can significantly reduce the torch’s effectiveness in illuminating objects at a distance.

Question 18-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 formed on a screen is generally not possible through standard optical means, as erect images are usually virtual and cannot be captured on a screen. However, this could be achieved through the use of a projecting system, such as an overhead projector or a camera obscura setup, where lenses or mirrors are used to project the image onto a screen. These systems can manipulate the light rays to create an enlarged, erect image on the screen.

Question 19-Two different types of lenses are placed on a sheet of newspaper. How will you identify them without touching them?

Answer-You can identify the two types of lenses by observing the effect they have on the text of the newspaper. A convex lens (thicker at the centre) will magnify the text, making it appear larger. In contrast, a concave lens (thinner at the centre) will make the text appear smaller and slightly distorted. These visual effects can be observed by looking through the lenses at the newspaper’s text.

Question 20-A shopkeeper wanted to fix a mirror which would give a maximum view of his shop. What type of mirror should he use? Give a reason.

Answer-The shopkeeper should use a convex mirror. Convex mirrors curve outward and provide a wider field of view compared to plane or concave mirrors. This characteristic would allow the shopkeeper to see more of the shop in the mirror, making it easier to monitor the entire area and enhancing security.

Question 21-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-If the size of the image formed on a screen by a convex lens is decreasing, it indicates that the object is moving away from the lens. As the object moves further from the lens, the image formed by the lens becomes smaller. This behaviour aligns with the properties of a convex lens in image formation.

Long Answer Questions

Question 22-Suppose we wish to obtain the real image of a distant tree. Explain two possible ways in which we can do it.

Answer-To obtain a real image of a distant tree, two possible methods involve using either a concave mirror or a convex lens-

• Using a Concave Mirror-
  • Place a concave mirror such that its reflective surface faces the tree.
  • Adjust the position of the mirror so that the distant tree is beyond the focal point of the concave mirror.
  • The light rays coming from the distant tree will converge after reflecting off the concave mirror.
  • Place a screen at the point where the light rays converge to capture the real image of the tree.
• Using a Convex Lens-
  • Position a convex lens between the tree and a screen.
  • Adjust the distance between the lens and the screen until a clear image of the tree is formed on the screen.
  • As the tree is distant, its light rays will be nearly parallel when they reach the lens. The lens will then converge these rays to form a real image on the screen.

In both cases, the real image of the distant tree will be inverted and can be captured on a screen.

Question 23-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 be able to obtain the image on the screen? Explain.

Answer-The nature of the lens in this scenario is a convex lens. When the object is brought closer to a convex lens, the image formed initially increases in size and remains real and inverted as long as the object is outside the focal length of the lens.

• If the distance between the object and the convex lens continues to decrease and the object comes within the focal length of the lens, the nature of the image changes.
• Inside the focal length, the convex lens will form a virtual, erect, and enlarged image.
• In this case, you will not be able to obtain the image on a screen anymore because virtual images cannot be projected onto a screen.

Question 24-You are given three mirrors of different types. How will you identify each one of them?

Answer-To identify the three different types of mirrors – plane, concave, and convex – you can perform simple observations and tests-

• Plane Mirror-
  • Look for a flat and smooth reflective surface without any curvature.
  • Observe the image formed in the mirror. A plane mirror forms an image that is virtual, erect, and of the same size as the object.
  • Check if the image formed undergoes lateral inversion (left appears right and vice versa).
• Concave Mirror-
  • Look for a reflective surface that curves inward (like the inside of a spoon).
  • Bring an object close to the mirror and observe the image. A concave mirror can form an enlarged and erect image when the object is very close to it.
  • Move the object farther away and observe the formation of a real, inverted image that can be projected on a screen.
• Convex Mirror-
  • Look for a reflective surface that bulges outward (like the back of a spoon).
  • Observe the image formed in the mirror. A convex mirror always forms a virtual, erect, and diminished image, regardless of the object’s distance.
  • Notice the wider field of view offered by the convex mirror compared to the other two types.

By conducting these observations, you can distinguish between the three types of mirrors based on their physical properties and the nature of the images they form.