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Page No 186:

Question 1:

The image formed by a plane mirror is

(a) on the surface of the mirror
(b) just in front of the morror
(c) just behind the mirror
(d) as much behind the mirror as the objcet is in front of the mirror

Answer:

(d) as much behind the mirror as the object is in front of the mirror

The object distance is equal to the image distance in the plane mirror and the nature of the image is virtual and erect; hence, the image is formed behind the mirror at a distance equal to the object distance.

Page No 186:

Question 2:

Which of the following images is real?

(a) image formed by a plane mirror
(b) image formed by a convex mirror
(c) image formed by a concave mirror with object between the mirror and focus
(d) image formed by a convace mirror with object behind the focus

Answer:

(d) image formed by a concave mirror with object behind the focus

Real image is formed by a concave mirror with object behind the focus, that is beyond focus, is real.  

Page No 186:

Question 3:

A sherical mirror with its reflecting surface on the outside is a

(a) plane mirror
(b) concave mirror
(c) convex mirror
(d) either concave or convex depending on which way you look at it.

Answer:

(c) convex mirror

Convex mirror is a spherical mirror, which has its reflecting surface outside.
 

Page No 186:

Question 4:

A virtual image formed by a plane or a spherical mirror

(a) is always inverted
(b) is always erect
(c) is erect or inverted depending on the distance of the object from the mirror

Answer:

(b) is always erect 

A virtual image formed by a spherical mirror or a plane mirror is always erect. 

Page No 186:

Question 5:

If a concave mirror forms a real magnified image of an object, the object is located

(a) between pole and focus
(b) between focus and centre of curvature
(c) beyond centre of curvature
(d) at centre of curvature

Answer:

(b) between focus and centre of curvature

Image is both magnified and real if the object is placed between the centre of curvature and the pole.



Page No 187:

Question 1:

What do you mean by 'angle of incidence' of a ray of light on a plane mirror?

Answer:

Angle of incidence is the angle between the incident ray and the normal to the plane mirror at the point of incidence. 


In the figure, MN is the plane mirror, AO is the incident ray, O is the point of incidence, OX is the normal and AOX is theangle of incidence.

Page No 187:

Question 2:

What is the relationship between angle of incidence and angle of reflection in a plane mirror?

Answer:

Angle of incidence is equal to the angle of reflection.


In the figure, MN is the plane mirror, AOX is the angle of incidence XOB is the angle of reflection.

Page No 187:

Question 6:

At what position of the object does a convex lens act as a magnifying glass?

(a) between F and 2F
(b) between F and O
(c) beyond F
(d) beyond 2F

Answer:

(b) between F and O

Magnifying glass is a single lens that produces a highly magnified, erect image of the object. In concave mirror, a highly magnified erect image is produced when the object is placed between F and O. 
 

Page No 187:

Question 7:

Which of the following images can a convex lens NOT produce?

(a) real, magnified
(b) virtual, magnified
(c) erect, virtual
(d) erect, diminished

Answer:

(d) erect, diminished

Convex lens cannot produce erect and diminished image.

Page No 187:

Question 8:

Band of which of these colours is not seen in a spectrum?

(a) green
(b) yellow
(c) purple
(d) red

Answer:

(c) purple

Purple colour is not present in the spectrum.

Page No 187:

Question 9:

Which of the following is true for a convex mirror?

(a) It can only form virtual images.
(b) It can only form real images.
(c) It can form real or virtual images depending on the distance of the object from the mirror.
(d) It can form real or virtual images depending on the size of the object.

Answer:

 (a) It can only form virtual images.

A convex mirror can only produce virtual image.

Page No 187:

Question 10:

Which of the following cannot split light into its seven colours?

(a) diamond cut in a special way
(b) prism
(c) convex lens
(d) water droplets suspended in the air

Answer:

(c) convex lens

A convex lens cannot split light into its seven colours.

Page No 187:

Question 1:

If you raise your right hand, your image in a plane mirror also raises its right hand. True of false?

Answer:

False. Images formed in plane mirrors are laterally inverted. Hence, if we raise our right hand, the image will appear to raise its left hand in the mirror.

Page No 187:

Question 2:

The image formed by a plane mirror can be obtained on a screen. True of false?

Answer:

False. The image formed by a plane mirror is virtual and erect in nature and virtual images cannot be obtained on a screen.

Page No 187:

Question 3:

The image formed by a plane mirror is as much __________ the mirror as the object is in front of it.

Answer:

The image formed by a plane mirror is as much behind the mirror as the object is in front of it.

This is because object distance is equal to image distance in a plane mirror.

Page No 187:

Question 4:

Through which point of all rays parallel to the principal axis of a spherical mirror pass after reflection by the mirror?

Answer:

All rays parallel to the principal axis of a spherical mirror pass through the focus after reflection by the mirror.

Page No 187:

Question 5:

Which of these mirrors converges a parallel beam of light-plane, concave or convex?

Answer:

A concave mirror converges a parallel beam of light-plane.

Page No 187:

Question 6:

A real image formed by a mirror is always inverted. True of false?

Answer:

True.

Real images formed by the mirrors are always inverted.

Page No 187:

Question 7:

What kind of spherical mirror would you use if you want to see the magnified image of your face in the mirror?

Answer:

We will need a concave mirror to get the magnified image of our face in the mirror.

Page No 187:

Question 8:

A convex lens _________ (converges/diverges/dos not affect) a parallel beam of light incident on it.

Answer:

A convex lens converges a parallel beam of light incident on it.

Page No 187:

Question 9:

Which of these has smaller focal length-a thick lens or a thin lens?

Answer:

Focal length of a thick lens is smaller than that of a thin lens.

Page No 187:

Question 10:

If the distance of an object from a convex lens is equal to twice the focal length of the lens, where is the image formed?

Answer:

Image will be formed at twice the focal length of the lens on the opposite side.
 
If an object is placed at twice the focal length of the lens, it is said be placed at 2F position. In this case, the image will be formed at 2F position on the opposite side of the lens.
 

Page No 187:

Question 11:

A diverging lens forms a real image of an object placed beyond F, and a virtual image of an object placed between O and F. True of false?

Answer:

False.
 
A diverging lens always forms a virtual image irrespective of the placement of the object.

Page No 187:

Question 12:

Name the triangular piece of glass that splits white light into different colours.

Answer:

The name of the triangular piece of glass that splits white light into different colours is prism.



Page No 188:

Question 3:

What is a real image?

Answer:

An image that can be projected on a screen is called a real image. It is formed by the actual meeting of the rays of light from the object. Image formed by a cinema projector on the screen is an example of real image.

Page No 188:

Question 4:

What do you mean by lateral inversion?

Answer:

In the image formed by a plane mirror, there is an interchange of the left and the right side. In other words, the left side of the object appears to be the right side of the image and vice versa. This property is known as the lateral inversion. 

(Right hand appears as left hand.)

Page No 188:

Question 5:

A convex rear view mirror is preferred over a plane mirror in a car. Why?

Answer:

Convex mirrors are used as rear view mirror in cars because they form small images of the objects. This helps us to have a wide field of view of the objects behind the car which is not possible with plane mirror.

Page No 188:

Question 6:

A convex lens is said to have a real focus and a concave lens a virtual focus. Why?

Answer:

In a convex lens, the parallel rays of light passing through the lens actually meet at the focus in front of the lens. This is why it is called real focus. But in a concave lens, the rays of light passing through the lens diverge and when these diverging rays are produced backward, they appear to meet at a point behind the lens. This point is the focus of the lens. Because the light rays do not actually meet at the focus, it is called virtual or imaginary focus.
 

Page No 188:

Question 7:

What is a 'spectrum'?

Answer:

Ordinary white light is made of seven colours, namely violet, indigo, blue, green, yellow, orange and red. The pattern formed by these colors is called spectrum.

Page No 188:

Question 8:

What does a convex lens form a virtual image? Is the virtual image bigger of smaller than the object?

Answer:

A convex lens forms a virtual image when the object is placed between the optical centre and the focus of the lens.
The virtual image formed by a convex lens is bigger than the object.

Page No 188:

Question 1:

Draw a labelled diagram of an experiment to show rectilinear propagation of light.

Answer:


Let us take three rectangular pieces of cardboard A, B, and C and make holes in them at the same horizontal level. With the help of a wooden stand as shown in the diagram, we make them stand parallel to each other with holes in the same straight line. We place a source of light, for example, a candle at the other side of cardboard C and adjust the cardboard pieces such that the light is visible through the hole at A. Now, we move the cardboard B in such a way that the hole no more stays in the same straight line with A and C.

We will find that when the hole on B is moved away from the straight line, the light will not be visible from A’s end. This proves that light moves in a straight line.

Page No 188:

Question 2:

What are the characteristics of the image formed by a plane mirror?

Answer:

Following are the characteristics of the image formed by a plane mirror:
i) Images are virtual and erect: This means that the images are formed at the backside of the mirror and the images are erect.
ii) Images are laterally inverted: In the image formed by a plane mirror, there is an interchange of left and right side.
iii) Images are of the same size of the object.
iv) The image appears to be as much as behind the mirror as the object in front of it.

Page No 188:

Question 3:

State three uses of concave mirrors.

Answer:

Following are the uses of a concave mirror:
1) Use in trapping solar energy: Since concave mirror can gather light and heat and direct the rays to focus, the focus becomes very hot. This property of the concave lens can be used to collect sun’s heat and this heat can be converted to electricity.  
2) Use as shaving mirror: Since concave mirrors can produce magnified images of the object when object is placed between pole and focus, this property can be used for making shaving mirrors to prevent cuts and have a better view while shaving.
3) Use in reflecting telescopes: Telescopes are used to study distant stars. Some form of telescopes known as reflecting telescopes uses concave mirror to gather light from distant stars, galaxies, etc, and produce magnified images that help astronomers and scientists to study the structure of the universe.
4) Use as dentist’s mirror: Dentists use small hand held concave mirror to produce a magnified image of the teeth and gum.
 5) Use in reflectors: When a source of light is placed at the focus of a concave mirror, the rays become parallel after getting reflected from the mirror and this allows a powerful beam of light to be focused to a long distance. So, concave mirrors are used as reflectors in headlights of cars, torches etc.

Page No 188:

Question 4:

Explain the difference between real and virtual images.

Answer:

Following are the differences between real and virtual images:
 

Real Image Virtual Image
i) Real images are formed when rays of light actually meet each other at a point. i) Virtual images are formed when rays of light appear to be coming from the image which does not actually exist.
ii) Real images being actual intersection of rays can be projected on a screen. ii) Virtual images being non-existent cannot be projected on a screen.
iii) Real images are inverted. iii)Virtual images are erect. 

Page No 188:

Question 5:

State the position and nature of the image formed by a concave mirror for the following positions of the object:

(a) between O and F
(b) between F and C
(c) beyond C

Answer:

(a) Between O and F:  Position of the image is behind the mirror; nature of the image is virtual, erect and highly magnified.  

(b) Between F and C: Position of the image is beyond C; nature of the image is real, inverted and magnified.

(c) Beyond C: Position of the image is between C and F; nature of the image is real, inverted and diminished.

Page No 188:

Question 6:

State the position and nature of the image formed by a convex lens for the following positions of the objcet:

(a) between O and F
(b) between F and 2F
(c) at 2F

Answer:

(a)  between O and F:  Position of the image is on the same side as the object; nature of the image is virtual, erect and highly magnified.  

(b) between F and 2F: Position of the image is beyond 2F; nature of the image is real, inverted and magnified.

(c) at 2F: Position of the image is at 2F on the opposite side of the object; nature of the image is real, inverted and of the same size as the object.


(d) beyond 2F: Position of the image is between F and 2F on the opposite side of the object. The nature of the image is real, inverted and diminished.

Page No 188:

Question 7:

Explain the following with the help of a ray diagram, in relation to the two main types of spherical lenses:

(a) principal focus
(b) focal length

Answer:

Spherical lenses are mainly of two types: convex lens and concave lens.

(a)  Principal focus:
Convex lens: In a convex lens, the parallel rays of light passing through the lens actually meet at a point in front of the lens known as focus. In the following figure, F is the focus.


Concave lens: In a concave lens, the rays of light passing through the lens diverge and when these diverging rays are produced backward, they appear to meet at a point behind the lens. The point is called the focus of concave lens. In the following figure, F is the focus.



(b) Focal length
Convex lens:
Focal length of a convex lens is the distance between the focus F and the optical centre O. In the figure, OF is the focal length. The focal length is on the right side in a convex lens.
Concave lens:
Focal length of a concave lens is the distance between the focus F and the optical centre O. In the figure, FO is the focal length. The focal length is on the left side in a convex lens. 

Page No 188:

Question 8:

Explain the set-up used by Newton to obtain a spectrum.

Answer:

Sir Isaac Newton showed that when a thin ray of light passed through a triangular piece of glass known as prism, the light split into seven colors called spectrum. He allowed a thin beam of light to fall on a prism kept in a dark room. After passing through the prism, the light split into seven different colors, namely violet, indigo, blue, green, yellow, orange and red, collectively known as spectrum.

Page No 188:

Question 1:

What will be the difference in the size of the image (of the same object) formed by a small and a large plane mirror?

Answer:

There will be no difference in the size of the image. This is because in a plane mirror, size of the image is equal to the size of the object. Since the size of the image does not depend upon the size of the mirror, the image size will remain the same whether it is formed by a small or a large mirror

Page No 188:

Question 2:

Identify three letters of the English alphabet or of your mother tongue whose mirror images are exactly the same as the letters.

Answer:

Three letters of the English alphabet whose mirror images are exactly the same as the letters are A, O and I.

Page No 188:

Question 3:

Car rear view mirrors carry a warning message that 'objects in the rear view mirror are closer than they appeer'. Why do you think this is so?

Answer:

Car rear view mirrors are convex mirrors.  In a convex mirror, the image of an object appears small, therefore, our brain recognises the size of the image and tries to calculate the distance of the actual object on this basis. As we know, objects far away appear smaller, therefore, when we see smaller images of the objects in the mirror, our brain thinks that the objects are far away than they actually are. In other words, 'objects in the rear view mirror are closer than they appear'.

Page No 188:

Question 4:

A concave mirror is said to have a real focus whereas a convex mirror is said to have a virtual foucs. apply the same logic to concave and convex lenses and say which of them has a real focus and which has a virtual focus.

Answer:

The logic behind the first case is not applicable in the second case. This is because in the first case, the mirror works on the principle of reflection of light, whereas in the second case, the lens works on the principle of bending of light.  
 
The correct logic is that converging systems have a real focus while diverging systems have a virtual focus.
 
In the first case, a concave mirror acts as a converging instrument. Therefore, it has a real focus. On the other hand, a convex mirror acts as a diverging instrument. Therefore, it has a virtual focus.
 
In the second case, a convex lens acts as a converging instrument. Therefore, it has a real focus. On the other hand, concave lens acts as a diverging instrument. Therefore, it has a virtual focus.

Page No 188:

Question 5:

Two different materials-glass and diamond-are used to make convex lenses of the same thickness. We know that diamond can bend light more than glass. Which lens will have a greater focal length?

Answer:

The glass lens will have a greater focal length than the diamond lens.
Diamond being able to bend light rays more, a lens made of diamond will make rays of light passing through it, meet at a point nearer than a lens made of glass. Thus, focal length of a diamond lens will be smaller than that of a glass lens.



Page No 189:

Question 6:

In which mirror are all normals at the surface parallel to each other-plane or spherical?

Answer:

Normals at the surface are parallel to each other in plane mirrors.
This is because perpendiculars on the same plane are parallel.

Page No 189:

Question 7:

How many images do you think will be formed if an object is kept between two plane mirrors parallel to each other?

Answer:

Infinite number of images will be formed if an object is kept between two plane mirrors parallel to each other.
This happens because rays of light from the object will fall infinite number of times on the mirror by getting reflected from the two mirrors and each time it is reflected back, images are formed. Thus, infinite number of images of the object is formed. 



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