NCERT Solutions for Class 7 Science Chapter 15 - Light
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Chapter 15 - Light Exercise 189
(a) Virtual image
Concept insight: A virtual image cannot be obtained on a screen. Only real images can be obtained on a screen.
Concept insight: A convex mirror forms a virtual, diminished and upright image of an object.
Concept insight: Only real images can be obtained on a screen.
Concept insight: The image formed by concave lens is always virtual. A virtual image cannot be obtained on a screen.
Concept insight: The image formed by a convex mirror is always virtual, diminished and erect.
Concept insight: The image formed by a concave lens is always virtual, erect and diminished.
Concept insight: The image formed by a concave mirror can be real or virtual, enlarged or diminished, and erect or inverted, depending upon the distance of the object from the mirror.
Concept insight: A real image can be obtained on a screen. Virtual images cannot be obtained on a screen.
Concept insight: A concave mirror can form both real and virtual images. The nature of the image depends on the distance of the object from the mirror.
(a) - (v)
A plane mirror forms an image which is erect and of the same size as the object.
(b) - (ii) and (vi)
A convex mirror forms a virtual, diminished, and upright image and therefore, covers a large area of view.
(c) - (i)
A convex lens can form a magnified image of the object. Therefore, it is used as a magnifying glass.
(d) - (iii)
A concave mirror forms an enlarged image of the object. Therefore, it is used to see an enlarged image of teeth by the dentist.
(e) - (vi)
The image is erect and smaller in size than the object.
Characteristics of image formed by a plane mirror are:
i. Image is erect.
ii.Image is laterally inverted.
iii. Image is of same size as the object.
iv. Distance between the object and mirror is equal to the distance between mirror and image.
If the letters of English alphabet A, H, I, M, O, T, U, V, W, X, Y are kept in front of a plane mirror, then they would form images which exactly look like the original letters of the alphabet. These letters are vertically symmetric. For example, if we divide letters A and U in the middle, then we would find that the right halves are equivalent to the left halves of the letters.
Chapter 15 - Light Exercise 190
A concave lens always forms a virtual image of an object.
Concept insight: A concave lens always forms virtual image of an object, while a convex lens forms real as well as virtual image depending on the position of the object.
(ii) Concave mirror
Concept insight: Concave mirror forms virtual and enlarged image of the object when the object is placed close to it. Concave lens and convex mirror form virtual but diminished image of the object. Plane mirror forms virtual and same sized image of the object.
(iii) 6 m
In the case of a plane mirror, the distance between the object and the mirror (d1) is same as the distance between the image and the mirror (d2).
Distance between the mirror and David's image, d2 = 4 m
Therefore, d1 = d2 = 4 m
If David moves 1 m towards the mirror, then d1 = 4 ? 1 = 3 m
Again, d1 = d2 = 3 m
Therefore, the distance between David and his image is d1 + d2 = 3 + 3 = 6 m.
Concept insight: In case of an image formed by a plane mirror, distance between object and mirror is equal to the distance between mirror and image.
(iii) 4 m/s
Concept insight: In a plane mirror, the object and its image always remain at the same distance from the mirror. So, when the car reverses at a speed of 2 m/s, then the image will also appear to move towards the mirror at the same speed of 2 m/s. So, the speed at which the image of truck appears to approach the car driver will be 2 m/s + 2 m/s = 4 m/s.
An image which cannot be obtained on a screen is called a virtual image.
The image formed by a plane mirror is virtual, because the image cannot be obtained on a screen placed either in front of the mirror or behind it.
Concept insight: A virtual image cannot be obtained on a screen.
Concave mirrors: They are used by dentists to see enlarged images of patients' teeth so that it becomes easier to locate the defect in the tooth.
Concept insight: When an object is placed close to a concave mirror, then an enlarged image of the object is seen in the mirror.
Convex mirrors: They are used as rear view mirrors in vehicles to see the traffic coming from behind.
Concept insight: Convex mirror forms erect and very small images of the things due to which we can see a large number of things spread over a large area. It has a wide field of view.
A concave mirror can form a real image.
Concept insight: Concave mirror can form real as well as virtual image depending on the position of the object, while a convex mirror always forms virtual image for all positions of the object.
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