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Page No 218:

Question 1:

When an object is within the focal length of a concave mirror, the image formed is
(a) real and inverted
(b) virtual and magnified.
(c) virtual and the same size.
(d) real and magnified.

Answer:

(b) virtual and magnified.
When an object is placed between the pole and the focus (within the focal length) of a concave mirror, a virtual and magnified image is formed behind the mirror.

Page No 218:

Question 2:

An incident ray strikes a plane mirror at an angle at 15° with the mirror. the angle between the reflected ray and the incident ray is
(a) 15°
(b) 30°
(c) 150°
(d) 140°

Answer:

(c) 150°

Given:
Angle of incident ray with the mirror = 15°
Therefore,
Angle of incidence = Angle of incident ray with the normal
                                    = 90° - 15°
                                    = 75°
We know that angle of reflection is always equal to angle of incidence. Therefore, we have:
Angle of reflection = Angle of reflected ray with the normal = 75°
Hence, angle between the reflected and incident rays = 75° + 75° = 150°

Page No 218:

Question 3:

A virtual, erect and magnified image of an object is produced by a concave mirror of focal length 12 cm. At what distance should this object be placed from the mirror?
(a) 10 cm
(b) 14 cm
(c) 18 cm
(d) 24 cm

Answer:

(a) 10 cm
In a concave mirror, a virtual and erect image is formed only if the object is placed within the focus.

Page No 218:

Question 4:

A virtual image is one which
(a) we can get on a screen
(b) we cannot get on a screen
(c) is formed only by a plane mirror.
(d) is formed only b a convex mirror.

Answer:

(b) we cannot get on a screen
A virtual image cannot be obtained on a screen.

Page No 218:

Question 5:

In order to get a virtual image of the same size as the object, it can be placed any where in front of
(a) a concave mirror.
(b) a plane mirror.
(c) a convex mirror.
(d) none of these.

Answer:

(b) a plane mirror
A plane mirror always forms a virtual and erect image and the size of image is equal to that of the object.

Page No 218:

Question 6:

A small electric lamp is placed at the focus of a concave mirror. the incident rays after reflection will form a
(a) parallel beam of light.
(b) converging beam of light.
(c) diverging beam of light.
(d) bright image of the lamp.

Answer:

(a) parallel beam of light
When an object is placed at the focus of a concave mirror, the reflected rays are parallel to one another.

Page No 218:

Question 7:

A concave mirror forms a virtual and magnified image when the object is placed
(a) between 2F and F.
(b) beyond 2F.
(c) at 2F.
(d) between the lens and F.

Answer:

Correct option: between the pole of the mirror and F
A concave mirror forms a virtual and magnified image if the object is placed within the focus, i.e., between the pole and the focus.
In the book, in option (d), the word lens should be replaced by mirror.

Page No 218:

Question 8:

Which of the following correctly shows the reflection of a ray of light by spherical mirrors?

figure

Answer:

The ray diagram (c) correctly shows the reflection of a ray of light by spherical mirror.
A ray of light passing through the focus of a concave mirror becomes parallel to the principal axis after reflection.



Page No 219:

Question 9:

A real image, equal in size to the object, is obtained when the object is placed at the centre of curvature in front of
(a) a plane mirror.
(b) a convex mirror.
(c) a concave mirror.
(d) either a convex or a concave mirror.

Answer:

(c) a concave mirror
When an object is placed at the centre of curvature in front of a concave mirror, a real and inverted image is formed. Further, the size of the image is equal to that of the object.

Page No 219:

Question 10:

When is an inverted image seen in a concave mirror?
(a) never
(b) the object is very far from the mirror
(c) the object is at the centre of curvature of the mirror.
(d) the object is within the focal length of the mirror.

Answer:

An inverted image is formed in a concave mirror when:
(b) the object is very far away from the mirror
and
(c) the object is placed at the centre of curvature of the mirror

Page No 219:

Question 11:

Draw an accurate diagram to show how a ray of light incident on a plane mirror at an angle of 40° to the surface of the mirror is reflected from it. What is the angle between the incident ray and the reflected ray?

Answer:


Given:
Angle of incident ray with the mirror = 40°
Therefore,
Angle of incidence = Angle of incident ray with the normal = 90°- 40°
                                                                                        = 50°

We know that angle of reflection is always equal to angle of incidence. Therefore, we have:
Angle of reflection = Angle of reflected ray with the normal = 50°
Hence, angle between the reflected ray and the incident ray = 50° + 50° = 100°

Page No 219:

Question 12:

State the Laws of Reflection.

Answer:

The laws of reflection state that:
1. The incident ray, the reflected ray and the normal, all lie in the same plane.
2. The angle of incidence is always equal to the angle of reflection.

Page No 219:

Question 13:

Copy and complete the following ray diagram by drawing the reflected ray.
figure

Answer:



A ray of light passing through the focus of a concave mirror becomes parallel to the principal axis after reflection.

Page No 219:

Question 14:

Two plane mirors PQ and QR are kept perpendicular (at 90°) to each other. The incident beam AB falls on one mirror PQ at an angle of 20° to the mirror. Complete the diagram to show the path of the reflected ray.

Answer:



Using reflection law, it can be observed that the given ray of light will reflect from the second mirror at an angle of 20°.

Page No 219:

Question 15:

Ajay was looking at the mirror image of the clock in his living room. The clock on the wall was showing 2:50. What time was the clock in his mirror showing?
Draw the positions of the hands of the real clock and its image in the mirror.

Answer:

The clock in the mirror will show 9:10.

Page No 219:

Question 16:

Draw a diagram to explain the following terms.
pole of a mirror, principal axis, concave surface, focus, centre of curvature

Answer:


Pole:
The centre of a mirror is known as its pole (P).

Principal axis:
The straight line passing through the centre of curvature and pole of a mirror is called principal axis.

Concave surface:
A bent-in surface of concave mirror at which reflection of light takes place is called concave surface.

Focus:
Focus (F) is a point in front of a mirror at which all the parallel rays of light converge after getting reflected from the concave mirror.

Centre of curvature:
Centre of curvature (C) is the centre of the hollow sphere of glass of which the mirror is a part.

Page No 219:

Question 17:

Which would be a better mirror on your cycle: convex or concave? Explain.

Answer:

Convex mirror can be used as a rear view mirror to see the traffic at the back side. Therefore, it is a better mirror for any vehicle.

Page No 219:

Question 18:

Draw ray diagrams to show how a concave mirror can be used
(a) to obtain a real, enlarged image.
(b) to obtain an inverted image of the same size.
(c) to obtain an erect, magnified image.

Answer:


(a) A real and enlarged image is formed when the object is placed between focus and centre of curvature.



(b) To obtain an inverted image of same size, the object is placed at the centre of curvature.



(c) To obtain an erect and magnified image, the object is placed within the focus.

 

Page No 219:

Question 19:

List the differences between the following:
(a) regular and diffuse reflection
(b) real and virtual image
(c) concave and convex mirror

Answer:

(a)

Regular reflection Diffused reflection
Regular reflection takes place from a smooth or regular surface. Irregular or diffused reflection takes place from an irregular surface.
In regular reflection, all reflected rays are parallel to each other for parallel incident rays. In diffused reflection, the reflected rays are not parallel to each other for parallel incident rays.


(b)
            Real image          Virtual image
It can be captured on a screen. It cannot be captured on a screen.
It is always inverted. It is always erect.
Light rays actually meet to form a real image. Light rays don't actually meet to form a virtual image.

(c)
Concave mirror Convex mirror
In a concave mirror, the reflecting surface is on the inner part of the sphere of which the mirror is a part. In a convex mirror, the reflecting surface is on the outer side of the sphere of which the mirror is a part.
In a concave mirror, the image formed can be either real and inverted or virtual and erect. In a convex mirror, the image is always virtual and erect.
The magnification of a concave mirror can be either greater, equal or less than one. In a convex mirror, the magnification is always less than one.

Page No 219:

Question 20:

State whether the following statements are true or false. Rewrite the false statements correctly.
(a) The laws of reflection hold good only for plane mirrors and not for spherical mirrors.
(b) The radius of curvature is double the focal length of a concave mirror.
(c) A ray passing through the focus cuts through the principal axis of a concave mirror after reflection.
(d) When an object is placed between a concave mirror and its focus, the image formed is virtual, erect and larger than the object.
(e) The image formed on a cinema screen is virtual.
(f) A convex mirror is used by stage artists.
(g) A convex mirror always gives real images.
(h) A pinhole camera gives a real image.
(i) A convex surface bulges inwards.
(j) Incident rays parallel to the principal axis of a curved mirror pass through the focus after reflection.
(k) A concave mirror always forms a real image.
(l) There can be many reflected rays for a single incident ray falling on a reflecting surface.

Answer:

(a) False.
The laws of reflection of light apply to all mirrors including plane and spherical mirrors.

(b) True.

(c) False.
A ray passing through the focus of a concave mirror becomes parallel to the principal axis after reflection.

(d) True.

(e) False.
The image formed on a cinema screen is real.

(f) False.
A concave mirror is used by stage artists.

(g) False.
A convex mirror always forms virtual image.

(h) True.

(i) False.
A convex surface bulges outwards.

(j)True.

(k) False.
In a concave mirror, the image formed can be either real or virtual.

(l) False.
For one incident ray, there is only one reflected ray.



Page No 220:

Question 21:

A man standing very close to a special mirror sees his image with a very small head, a fat body and legs of the same size. What different types of reflecting surfaces is the mirror made of?

Answer:

The upper part of the mirror is convex.

The middle part of the mirror is concave.

The lower part of the mirror is plane.

Page No 220:

Question 22:

Both a mirror and a page of this book reflect light. You can see an image in a mirror but not in the book. Explain.

Answer:

The reflection from a mirror is regular reflection, whereas the reflection from a page of the book is diffused reflection. Regular reflection allows us to see our image, whereas diffused reflection allows us to see the object on which the light falls.
Therefore, we cannot see our image in the page of the book.

Page No 220:

Question 23:

The figure below shows a trick in which a lighted candle can be made to appear to be burning in a glass of water.
figure
(a) What must be the distance of the lighted candle from the sheet of glass for this trick to work?
(b) What does the glass do to the light to get this effect?
(c) Which fact about a plane mirror is shown by this trick?

Answer:

(a) The distance of water glass from the glass sheet must be equal to the distance of lighted candle from the glass sheet.

(b) Here, though the glass is transparent, it acts as a mirror to get this effect.

(c) Reflection of light is shown here. Some light falling on the glass is reflected because transparent objects do reflect some light.

Page No 220:

Question 24:

Match the words in column I to the phrases in column II.
 

Column I Column II
a. Real image i. cannot be obtained on a screen
b. concave mirror ii. normal at the centre of the curved mirror
c. focus of a concave mirror iii. enlarged
d. principal axis iv. distance from centre of mirror to focus
e. focal length v. smaller
f. virtual image vi. can be obtained on a screen
g. inverted vii. bulging inwards
h. diminishing viii. bulging outwards
i. magnified ix. point at which rays parallel to the axis meet after reflection
j. convex surface x. upside down

Answer:

.

Column I Column II
a. Real image  vi. can be obtained on a screen
b. Concave mirror  vii. bulging inwards
c. Focus of a concave mirror  ix. point at which rays parallel to the axis meet after reflection
d. Principal axis  ii. normal at the centre of the curved mirror
e. Focal length  iv. distance from centre of mirror to focus
f. Virtual image  i. cannot be obtained on a screen
g. Inverted  x. upside down
h. Diminishing  v. smaller
i. Magnified  iii. enlarged
j. Convex surface  viii. bulging outwards



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