Important Questions
1. List out the apparatus required to find the focal length of a concave mirror and describe the procedure of finding the focal length.
A: Required Apparatus: Concave mirror, Small Paper, Meter Scale.
Procedure:
‣ Hold a Concave mirror perpendicular to the direction of Sunlight.
‣ Take a small paper and slowly move it infront of the mirror and find out the point where you get smallest and brightest spot.
‣ This is the image of the Sun.
‣ The rays coming from Sun parallel to the concave mirror are converging at a point.
‣ This point is called focus or focal point (F) of the Concave mirror.
‣ Measure the distance of this spot from the pole of the mirror. This distance is the focal-length of the given Concave mirror.
PRECAUTION: Small paper is to be taken so that it does not obstruct the incoming Sun rays.
2. Write an experimental method in measure the distances of object and image using Concave mirror and write the table for observations. (AS3) 4 Marks.
A: Aim: Observing the types of image and measuring the object distance and image distance from the mirror.
Material required: A candle, paper, Concave mirror, (known focal length), V-stand, measuring tape or meter scale.
Procedure:
‣ Place the Concave mirror on V-stand, a candle and meter scale as shown in figure.
‣ Keep the candle at different distances from the mirror (10 cm to 80 cm) along the axis and by moving the paper (screen) find the position where you get the sharp image on paper.
‣ Note down the observations in the following table.
‣ Group your observations based on the type of image you see [e.g.: Image is bigger and inverted]. It is possible you may not get any image at some positions note down that too.
‣ We know the focal point and centre of curvature, we can classify our observations as shown in the following table.
3. Fill the table with the given ray diagrams information. (AS4) 4 Marks
A:
4. Explain where do we use the Convex and Concave mirrors in our daily life situations (AS6) 4 Marks
A: Uses of Convex mirrors:
» Side view mirrors in vehicles.
» Big Convex mirrors are used as shop security mirrors.
Uses of Concave mirrors:
» ENT Specialists used as a head mirror.
» As shaving mirrors to see a large image of the face.
» As a reflector in head lights of vehicles.
» As search light, solar furnaces, astronomical telescopes.
5. A teacher gave his student a Convex mirror and a Concave mirror and asked him to find the differences between the two Imagine and write what differences he might have given.
(or)
Compare the Concave mirror and Convex mirror. (AS1) 4 Marks
A:
|
Convex Mirror |
Concave Mirror |
|
» The Reflecting surface curved outward. » After reflection from the mirror the light rays diverge » The image formed due to Convex mirror is always virtual. » The image formed due to Convex mirror is always diminished. » This mirror produces only erect image. |
» The reflecting surface curved in ward. » After reflection from the mirror the light rays converge. » The image formed due to concave mirror is generally real. But when an object placed between vertex and focus it is virtual. » The image formed due to Concave mirror is either diminished or magnified. » This mirror produces both Erect and inverted images. |
6. Compare and contrast the real image and virtual image.
A:
|
Real Image |
Virtual Image |
|
» Real image is formed due to actual intersection of the reflected rays. |
» Virtual image is formed by extending the diverging light rays backwards. |
7. State the sign convention followed for reflection of light by spherical mirrors.
(or)
Mention the sign convention for the parameters related to the mirror equation (AS1) 4 Marks
A: » All distances should be measured from the pole.
» The distance measured in the direction of incident light, to be taken as positive.
» The direction opposited to incident light to be taken as negative.
» Height of the object (ho) and height of image (hi) are positive if measured upwards and negative if measured downwards from the axis.
8. i) What is the mirror used in car head light? Where are the bulbs kept in car head light?
ii) In the car head lights the light from bulb fall on mirror and reflects. Draw the ray diagram indicating it? (AS1) 4 Marks
A: » Concave mirror is used as a reflector in car head lights. If an electric bulb is placed at the Focus (F). A parallel intense beam of light is obtained and illuminates the objects upto a large distance.
9. Draw the ray diagram when the object is placed on the principal axis of concave mirror of the following cases. Also write the characteristics of the image formed.
i) When the object is beyond the centre of curvature.
ii) When the object is between centre of curvature and focus. (AS5) 4 Marks
A: » When the object is place beyond the centre of curvature, image is formed in between 'F' and 'C'. It is inverted and diminished. It is a real image.
» When the object is placed between 'C' and 'F' image is formed beyond 'C'. It is enlarged. The image is inverted and real.
10. Write the rules to trace an image formed by a convex mirror. (AS1) 4 Marks
A: Rule 1:
A ray, running parallel to axis, an meeting the convex mirror, will get reflected so as to appear as if it is coming from the focal point.
Rule2:
A ray going in the direction of the focal point, after reflection will come parallel to main axis.
Rule 3:
A ray going in the direction of the centre of curvature will on reflection, get back in opposite direction and looks that it is coming from the centre of curvature.
11. Write the rules for image tracing formed by a concave mirror with the help of ray diagrams. (AS1) 4 Marks
A: Rule 1: All the rays, that are parallel to the axis, get reflected such that they pass through the focal point of the mirror.
Rule 2: A ray that passes through the focal point of the mirror will, travel parallel to the axis after reflection.
Rule 3: A ray, passing through the centre of curvature of the mirror and incidents on the mirror, after reflection will travel along the same line, but in opposite direction.
12. We choose three rays R1, R2 and R3 to determine easily the positions and characteristics of the images of the objects placed at different position on the principal axis infront of a spherical mirrors. Give reasons. (AS1) 4 Marks
A: To draw ray diagrams more easily, we will always take rays that come from the tip of the object. We know that all the rays that are parallel to the axis get reflected such that they pass through the focal point of the mirror. So, for drawing any diagram the most suitable ray to draw will be the one that comes from the object and goes parallel to the axis of the mirror. The reflected ray will be the line drawn from the point of incidence, on the mirror and passes through the focal point of the mirror. See the ray R1 in the figure given below.
The conversion situation of previous one is also true, that is a ray originated from the tip of the object and passing through the focal point of the mirror will travel parallel to the axis after reflection by the mirror. This is the ray that we have to draw as a second ray. See R2 in the following figure.
We know that a line drawn from the centre of curvature to the mirror is perpendicular to the tangent at the point where the line meets the curve. So, a ray coming from the tip of the object and going through the centre of curvature to meet the mirror, will get reflected along the same line. In general, a ray travelling along normal retraces its path. This ray is shown as R3 in the figure given below.
13. Explain the nature of images formed with the help of ray diagrams, when an object is placed at different points before a concave mirror. (AS1) 4 Marks
A: i) When an object is at infinity:
When an object is at infinity. The image is formed at focal point (F) of the mirror.
The image is real and point sized.
ii) When the object is between infinity and centre of curvature:
When the object in between infinity and centre of curvature (C), the image is formed between F and C. This is a real inverted and smaller image.
iii) When the object is at C:
When the object is at C, the image also forms at C. This is a real, inverted and same sized image.
iv) When the object is in between C and F:
When the object is in between C and F, the image is formed beyond C. This is a real, inverted and bigger image.
v) When the object is at F:
When the object is at F, the image is formed at infinity
vi) When the Object is between F and Pole:
When the object is in between F and pole, the image forms behind the mirror.
This is a virtual, enlarged and erect image.
14. What are the consequences occur when the spherical mirrors were not known to human beings? (AS2) 2 Marks
A: » If would be unsafe to drive cars, especially in the night.
» Automobile head lights, torch lights, search lights can not give lighting for long distances.
» Dentists may not have proper diagnosis of teeth.
» Making of reflecting telescopes would not be possible.
15. Write about different points related to mirrors. (AS1) 2 Marks
A: The different points related to mirrors are:
Vertex (p): The point where the central axis touches the Mirror is called Vertex.
Focus or Focal Point (F): The rays coming from a distinct object meet at point on principal axis after reflection. That Point is known as focus or focal point.
Centre of curvature (C): It is the centre of sphere to which the mirror belongs.
16. Write about various distances related to mirrors. (AS1) 2 Marks
A: The various distances related to mirrors are
Focal length (F): The distance between vertex and focus is called Focal length.
Radius of curvature (R): The distance between radius of curvature.
17. In the given ray diagram mention the pole of mirror, Focus, centre of curvature, position of image. (AS5) 2 Marks
A: 
P - Pole of mirror
F - Focus
C - Centre of curvature
Image position between F and C
18. For the formation a virtual image in a concave mirror, where should we place the object? Draw a ray diagram to show this (AS5) 2 Marks
A: 
19. Which property of Concave mirror is used by dentists? (AS1) 2 Marks
A: They enlarge the image when the object is held close to the mirror (less than the focal length). Also the image is erect. So, dentists and ENT Specialists use Concave mirrors to see small inner parts of throat, nose etc.
20. How do you conduct an experiment to prove the mirror formulae
(AS 3) 2 Marks
A: AIM: To prove that
Material required: Acandle, paper, Concave mirror, V - stand, measuring tape, meter scale.
Procedure:
» Place the Concave mirror on V - stand, a candle and meter scale as shown in the figure.
» Keep the candle at different lengths from the mirror and by moving the paper along with axis.
» Find the position where we get sharp image on the paper.
» Note down the observations in the given table
|
Observation |
Distance of candle |
Distance of paper (image) from the mirror (v) |
 |
 |
 |
|
1 2 3 4 5 6 |
» From the above table we can notice that 
in all observations.
21. Draw a diagram to show the place of vessel to be heated in a solar cooker. (AS5) 2 Marks
A:
22. Which mirrors are called spherical mirrors? (AS1) 1 Mark
A: The mirrors which have curved surface are known as spherical mirrors. They can be thought of as being made from a portion cut off from a hollow glass sphere.
23. Write the mirror formula. (AS1) 1 Mark
A: The mirror formula is 
Where: f - focal length
u - object distance
v - image distance
24. What is the relation between focal length and radius of curvature? (AS1) 1 Mark
A: The radius of curvature of a spherical mirrors is twice to its focal length.
Radius of curvature = 2 × focal length
R = 2 × f
(or)
25. Can a Convex mirror burn a paper? Why? (AS2) 1 Mark
A: The rays coming parallel to principal axis of a Convex mirror after reflection diverge from the mirror. So, we cannot burn a paper by using a Convex mirror.
26. Which mirror has wider field of view? (AS6) 1 Mark
A: A Convex mirror have wider field of view.
27. Why do we prefer a Convex mirror as a rear-view mirror in the vehicles. (AS1) (AS6) 1 Mark
A: It gives erect and small image and covers long distances.
28. Where do you place the vessel in solar cooker? (AS1) (AS6) 1 Mark
A: We place the vessel in solar cooker at the focal point.
A: A ray parallel to the axis on meeting the convex mirror will get reflected. So, as to appear as if it is coming from the focal point.
30. Draw the path of ray after reflection in diagram (AS5) 1 Mark
A: 
PROBLEMS - SOLUTIONS
1. If the focal length of a concave mirror is 15 cm. What is its radius of curvature?
(AS1) 1 Mark
Sol: Focal length (f) = 15 cm
Radius of curvature (R) = ?
R = 2f
R = 2 × 15 = 30 cm.
2. The focal length of a convex mirror is 10 cm. What is its radius of curvature? (AS1) 1 Mark
Sol: Focal length (f) = 10 cm
Radius of curvature (R) = ?
R = 2f
R = 2 × 10 = 20 cm.
3. If the radius of curvature of a spherical mirror is 16 cm. What is its focal length? (AS1) 1 Mark
Sol: Radius of curvature (R) = 16 cm
Focal length (f) = ?
4. The distance between vertex and radius of curvature is 20 cm, the focal length of mirror is how much? (AS1) 2 Marks
Sol: Radius of curvature (R) = 20 cm
Focal length (f) = ?
R = 2f
5. An object is placed at a distance of 20 cm from a convex mirror, of focal length 30 cm then find the position and nature of the image? (AS1) 1 Mark
Sol: u = - 20 cm
f = 30 cm
The image is erect and diminished.
6. An object is kept at a distance of 20 cm, infront of a concave mirror. The focal length of mirror is 30 cm, then
a) What is image distance?
b) What is the magnification in this case? (AS1) 4 Marks
Sol: a) Object distance (u) = -20 cm
Image distance (v) = ?
Focal length (f) = -30 cm
7. There is an object infront of convex mirror at a distance of 5 cm. If its focal length is 10 cm. Then
a) What is the image distance?
b) What is its magnification? (AS1) 4 Marks
Sol: a) In the case of convex mirror
Object distance (u) = -5 cm
Focal length (f) = + 10 cm
8. What is the distance of the image, when an object is placed on the principal axis at a distance of 15 cm infront of a concave mirror whose radius of curvature is 10 cm. (AS1) 4 Marks
Sol: Radius of curvature (R) = 10 cm
Object distance (u) = 15 cm
image distance (v) = ?
... v = 7.5 cm.
... Image distance = 7.5 cm
9. A concave mirror produces 4 times magnified real image of an object placed at 10 cm infront of it. Where is the image located? (AS1) 2 Marks
Sol: As the image is real, so m is -ve
Magnification (m) = -4
Object distance (u) = -10 cm
Image distance (v) = ?
10. An object is in 2 cm. size, is place at 25 cm infront of a concave mirror of focal length 15 cm. At what distance from the mirror would a screen be placed in order to obtain a sharp image? Find the nature and the size of the image. (AS1) 4 Marks
Sol: According to sign convention
Focal length (f) = -15 cm
Object distance (u) = -25 cm
Object height (ho) = + 2 cm
Image distance (v) = ?
Image height (hi) = ?
... v = -37.5 cm
So, the screen should be placed at 37.5 cm from the pole of the mirror, the image is real.
So, the image is inverted and enlarged.
Short Questions
1. Mention two situations when a light ray does not deviate after incident on a spherical mirror. (AS1) 1 Mark
A: When the ray travels along the normal or through centre of curvature, along the principal axis, it does not deviate.
2. Imagine what would have happened if the concave mirrors were not found? (AS2) 1 Mark
A: Solar cookers, head lights of vehicles, torch lights, search lights etc., would not have been found.
3. What would have happened if convex mirrors were not found? (AS2) 1 Mark
A: The rear view mirrors in vehicles might not be fitted.
4. A mirror is broken when if fell down. Will there be any change in its focal length?
(AS2) 1 Mark
A: No, it does not change.
5. What happens if we use concave mirror as a rear view mirror? (AS2) 1 Mark
A: Image can not be identified properly. Inverted images will be formed in some situations.
6. What happens in a concave mirror if the light rays after reflection will not converge at a point? (AS2) 1 Mark
A: Real images will not be formed
7. Write the list of apparatus required to find the focal length of a concave mirror?
(AS3) 1 Mark
A: Concave mirror, a small piece of paper.
8. Write the preventive measures to be taken while finding the focal length of a concave mirror. (AS3) 1 Mark
A: The paper should be small so that it does not obstruct the incoming sun rays.
9. Write the uses of a spherical mirrors in our daily life. (AS6) 2 Marks
A: i) Concave mirrors are used as shaving mirrors, used by dentists, used in solar cookers,
head light of vehicles etc.,
ii) Convex mirrors are used as rear view mirrors in vehicles.
10. Where the normals drawn at different points for a Convex mirror meet? (AS1) 1 Mark
A: At centre of curvature.
11. Which ray behave similarly when it passes through a Concave and Convex mirrors? (AS1) 1 Mark
A: The ray which passes along the principal axis.
12. To convert the watch glass as a convex mirror, on which side of it the silver coating is done? (AS1) 1 Mark
A: On its concave surface.
13. In which situation, a magnified real image is formed in concave mirror?
(AS1) 1 Mark
A: When the object is placed between C and F.
14. In which instance a magnified virtual image is formed in a concave mirror?
(AS1) 1 Mark
A: When object is placed between F and P.
15. How are relative distances measured in spherical mirrors? (AS1) 1 Mark
A: Pole of the mirror.
16. What is minimum distance between the object and real image in a concave mirror?
(AS1) 1 Mark
A: Zero.
17. How does the height of object (ho) and height of image (hi) considered according to sign convention system when they are below the principal axis?
(AS1) 1 Mark
A: Considered as '-ve'.
18. How are distances considered when they are measured in a direction opposite to the direction of propagation of light? (AS1) 1 Mark
A: Negative
19. Can the dentist use a convex mirror! Predict the reasons. (AS2) 1 Mark
A: Image formed will be small. So can't be used.
20. What kind of mirror can have a focal length of -10 cm? (AS1) 1 Mark
A: Concave mirror.
21. What kind of a mirror can have a focal length of +12 cm?(AS1) 1 Mark
A: Convex mirror.
22. If an object is placed at the focus of a concave mirror, where is the image formed?
(AS1) 1 Mark
A: At infinity.
23. If an object is at infinity (very large distance) in front of a concave mirror, where is the image formed? (AS1) 1 Mark
A: At focus.
24. Name the spherical mirror which can produce a virtual and diminished image of an object? (AS1) 1 Mark
A: Convex mirror.
25. A Scientist burnt the ships by using mirrors. Who was the scientist? (AS6) 1 Mark
A: Archimedes
26. Which mirror has a wider field of view? (AS6) 1 Mark
A: Convex mirror.
