Light | Science 7th | Chapter 15 |
Science
– Jammu and Kashmir | jandkncert | Class 7th |
Chapter
15: Light
Q1: Fill
in the blanks:
(a)
An image that cannot be obtained on a screen is called ________.
(b) The image formed by a convex ________ is always virtual and smaller in size.
(c) An image formed by a ________ mirror is always of the same size as that of the object.
(d) An image which can be obtained on a screen is called a _______image.
(e) An image formed by a concave ________cannot be obtained on a screen.
Ans.
(b) The image formed by a convex ________ is always virtual and smaller in size.
(c) An image formed by a ________ mirror is always of the same size as that of the object.
(d) An image which can be obtained on a screen is called a _______image.
(e) An image formed by a concave ________cannot be obtained on a screen.
Ans.
(a) An image that cannot be obtained on a screen is called virtual image_.
A virtual image cannot be obtained on a screen. Only real images can be obtained on a screen.
(b) An image formed by a convex mirror is always virtual and smaller in size.
A convex mirror forms a virtual, diminished, and upright image of an object.
(c) An image formed by a plane mirror is always of the same size as that of the object.
(d) An image which can be obtained on a screen is called a real image.
Only real images can be obtained on a screen.
(e) An image formed by a concave lens cannot be obtained on a screen.
The image formed by a concave lens is always virtual. A virtual image cannot be obtained on a screen.
A virtual image cannot be obtained on a screen. Only real images can be obtained on a screen.
(b) An image formed by a convex mirror is always virtual and smaller in size.
A convex mirror forms a virtual, diminished, and upright image of an object.
(c) An image formed by a plane mirror is always of the same size as that of the object.
(d) An image which can be obtained on a screen is called a real image.
Only real images can be obtained on a screen.
(e) An image formed by a concave lens cannot be obtained on a screen.
The image formed by a concave lens is always virtual. A virtual image cannot be obtained on a screen.
Q2:Â Mark
‘T’ if the statement is true and ‘F’ if it is false:
(a) We can obtain an enlarged and erect image by a convex mirror. (T/F)
(b) A concave lens always forms a virtual image. (T/F)
(c) We can obtain a real, enlarged, and inverted image by a concave mirror. (T/F)
(d) A real image cannot be obtained on a screen. (T/F)
(e) A concave mirror always forms a real image. (T/F)
Ans.
(a) We can obtain an enlarged and erect image by a convex mirror. (T/F)
(b) A concave lens always forms a virtual image. (T/F)
(c) We can obtain a real, enlarged, and inverted image by a concave mirror. (T/F)
(d) A real image cannot be obtained on a screen. (T/F)
(e) A concave mirror always forms a real image. (T/F)
Ans.
(a)False
The image formed by a convex mirror is always virtual, diminished, and erect.
(b) True
The image formed by a concave lens is always virtual, erect, and diminished.
(c) True
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.
(d) False
The image that can be obtained on a screen is called a real image. Virtual images cannot be obtained on a screen.
(e) False
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.
The image formed by a convex mirror is always virtual, diminished, and erect.
(b) True
The image formed by a concave lens is always virtual, erect, and diminished.
(c) True
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.
(d) False
The image that can be obtained on a screen is called a real image. Virtual images cannot be obtained on a screen.
(e) False
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.
Q3:Â Match
the items, given in Column I with one or more items of Column II.
Column
I
|
Column
II
|
(a) A plane mirror
|
(i) Used as a magnifying glass.
|
(b) A convex mirror
|
(ii) Can form an image of objects spread over a large area.
|
(c) A convex lens
|
(iii) Used by dentists to see the enlarged image of teeth.
|
(d) A concave mirror
|
(iv) The image is always inverted and magnified.
|
(e) A concave lens
|
(v) The image is erect and of the same size as the object.
|
(vi) The image is erect and smaller in size than the object.
|
Ans.
Column
I
|
Column
II
|
(a) A plane mirror
|
(v) The image is erect and of the same
size as the object.
|
(b) A convex mirror
|
(ii) Can form an image of objects spread over
a large area.
|
(c) A convex lens
|
(i) Used as a magnifying glass.
|
(d) A concave mirror
|
(iii) Used by dentists to see enlarged
image of teeth.
|
(e) A concave lens
|
(vi) The image is erect and smaller in
size than the object.
|
Q4:Â State
the characteristics of the image formed by a plane mirror.
Ans. The image formed by a plane mirror is erect, laterally inverted, and of the same size as that of the object. The left side of the object appears on the right side of the image. Also, the distance of the image from the plane mirror is the same as the distance between the plane mirror and the object.
Ans. The image formed by a plane mirror is erect, laterally inverted, and of the same size as that of the object. The left side of the object appears on the right side of the image. Also, the distance of the image from the plane mirror is the same as the distance between the plane mirror and the object.
Q5:Â Find
out the letters of English alphabet or any other language known to you in which the image formed in a plane mirror appears exactly like the letter itself.
Discuss your findings.
Ans. 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 AÂ and U in the
middle, then we would find that the right halves are equivalent to the
left halves of the letters.Hence, even if the image interchanges sidewise, it will appear the same, as the letter.
Q6:Â What
is, a virtual image? Give one situation where a virtual image is formed.
Ans. When an image cannot be obtained on a screen, it is called a virtual image. The image formed by a plane mirror is virtual because the image cannot be obtained on a screen when placed either in front of the mirror or behind it.
Ans. When an image cannot be obtained on a screen, it is called a virtual image. The image formed by a plane mirror is virtual because the image cannot be obtained on a screen when placed either in front of the mirror or behind it.
Q7:Â State two differences between a convex and a concave lens.
Ans. (i) A Convex lens is thicker in the middle while a concave lens is thinner in the middle.
(ii) A convex lens converges the light ray falling on it, while a concave lens
diverges the light ray falling on it.
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Q8:Â Give
one uses each of a concave and a convex mirror.
Ans. Concave
mirrors can form an enlarged image of the object. Therefore, they are used by the dentist to see the enlarged image of the patient’s teeth. A Convex
mirror forms the diminished and upright image of the object. It is used as a
side-view mirror of the car because it enables the driver to view objects
spread over a large area behind him/her.
Q9:Â Which
type of mirror can form a real image?
Ans. A concave mirror can form a real image of an object. The nature of the image depends on the distance of the object from the concave mirror.
Ans. A concave mirror can form a real image of an object. The nature of the image depends on the distance of the object from the concave mirror.
Q10:Â Which
type of lens forms always a virtual image?
Ans. A concave lens always forms a virtual image of an object.
Ans. A concave lens always forms a virtual image of an object.
Q11:Â A
virtual image larger than the object can be produced by a
(i) concave lens
(ii) concave mirror
(iii) convex mirror
(iv) plane mirror
Ans. (ii) A concave mirror can form a virtual and magnified image of the object.
The concave lens and convex mirror form a virtual but diminished image of the object. The plane mirror forms the virtual and the same size image of the object.
(i) concave lens
(ii) concave mirror
(iii) convex mirror
(iv) plane mirror
Ans. (ii) A concave mirror can form a virtual and magnified image of the object.
The concave lens and convex mirror form a virtual but diminished image of the object. The plane mirror forms the virtual and the same size image of the object.
Q12:Â David
is observing his image in a plane mirror. The distance between the mirror and
his image is 4 m. If he moves 1 m towards the mirror, then the distance
between David and his image will be
(i) 3 m
(ii) 5 m
(iii) 6 m
(iv) 8 m
Ans. (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).
(ii) 5 m
(iii) 6 m
(iv) 8 m
Ans. (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).
Given,
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.
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.
Q13: The
rear-view mirror of a car is a plane mirror. A driver is reversing his car at a
speed of 2 m/s. The driver sees in his rear-view mirror the image of a truck
parked behind his car. The speed at which the image of the truck appears
to approach the driver will be
(i) 1 m/s
(ii) 2 m/s
(iii) 4 m/s
(iv) 8 m/s
Ans. (iii) 4 m/s
The speed of the car is 2 m/s which means the car is approaching the truck with a speed of 2 m per second. The distance between the car and truck will decrease at a double rate. This is because the image of the truck will travel a distance twice the distance traveled by car in equal time. Hence, the image of the truck will appear to approach the driver with, the speed of 2 ×2 = 4 m/s.
(i) 1 m/s
(ii) 2 m/s
(iii) 4 m/s
(iv) 8 m/s
Ans. (iii) 4 m/s
The speed of the car is 2 m/s which means the car is approaching the truck with a speed of 2 m per second. The distance between the car and truck will decrease at a double rate. This is because the image of the truck will travel a distance twice the distance traveled by car in equal time. Hence, the image of the truck will appear to approach the driver with, the speed of 2 ×2 = 4 m/s.
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