# 9th Standard Science Guide Light Answers in English

## Lesson 6 Light

9th Science Guide: Welcome to Lesson 6 on Light! In this lesson, we will explore the fascinating world of light and its properties. By understanding the concepts discussed in this lesson, you will gain a deeper insight into how light behaves and interacts with different mediums.

To help reinforce your understanding, we have provided the answers to the book back questions related to this lesson. These multiple-choice, true or false, fill-in-the-blanks, matching, and assertion-reason questions cover various aspects of light and its behavior.

### I. Multiple choice questions:

1. A ray of light passes from one medium to another medium. Refraction takes place when angle of incidence is

1. 0o
2. 45o
3. 90o

Ans: 45o

2. ______ is used as reflectors in the torchlight.

1. Concave mirror
2. Convex mirror
3. Plane mirror

Ans: Concave mirror

3. We can create enlarged, virtual images with

1. Concave mirror
2. Convex mirror
3. Plane mirror

Ans: Concave mirror

4. When the reflection surface is curved outwards the mirror formed will be

1. Concave mirror
2. Convex mirror
3. Place mirror

Ans: Convex mirror

5. When a beam of white light passes through a prism it gets

1. reflected
2. Deviated and dispersed
3. Only deviated

Ans: Deviated and dispersed

6. The speed of light is maximum in

1. Vacuum
2. Glass
3. Diamond

Ans: Vacuum

7. The focal length of a concave mirror is 5cm. its radius of curvature is

1. 5cm
2. 10cm
3. 2.5cm

Ans: 10cm

8. A real and enlarged image can be obtained by using a

1. Convex mirror
2. Plane mirror
3. Concave mirror

Ans: Concave mirror

9. Which of the following statements about total internal reflection is true?

1. Angle of incidence should be greater than the critical angle.
2. Light must travel from a medium of higher refractive index to a medium of lower refractive index.
3. Both (a) and (b)

Ans: Both (a) and (b)

### II. True or false – if false give the correct answer

1. The angle of deviation depends on the refractive index of the glass.

Ans: True

2. If a ray of light passes obliquely from one medium to another, it does not suffer any deviation.

Ans: False

Correct Ans: Due to the density of different medium the ray of light does not posses oblique.

3. The convex mirror always produces a virtual, diminished and erect image of the object.

Ans: True

4. When an object is at the centre of curvature of concave mirror the image formed will be virtual and erect.

Ans: False

Correct Ans: Real, inverted, equal in size at C.

5. The reason for brilliance of diamonds is mainly due to total internal reflection of light.

Ans: True

6. IF the object is at infinity in form of a convex mirror the image is formed at infinity.

Ans: False

Correct Ans: The image is at F.

7. An object is placed at a distance of 3cm from a plane mirror. The distance of the object and image is 3cm.

Ans: False

Correct Ans: The distance of the object and image is 6cm.

8. The distance from centre of curvature of the mirror to the pole is called the focal length of the mirror.

Ans: False

Correct Ans: The distance between pole and focus of a spherical mirror is called its focal length.

9. Light is one of the slowest travelling energy with a speed of 3×108 ms-1

Ans: False

Correct Ans: Fastest travelling energy with a speed of 3×108 ms-1

10. The angle of incidence at which the angle of refraction is 0o is called the critical angle.

Ans: False

Correct Ans: Angle of refraction is 90o

### III. Fill in the blanks/complete the sentence

1. In going from a rarer to denser medium, the ray of light bends _________

Ans: Towards normal

2. The mirror used in search light is _________

Ans: Concave mirror

3. The angle of deviation of light ray in a prism depends on the angle of _________

Ans: incidence

4. The radius of curvature of a concave mirror whose focal length is 5cm is _________

Ans: 10cm

5. Large _________ mirrors are used to concentrate sunlight to produce heat in solar furnaces.

Ans : Concave

6. A spherical mirror whose reflecting surface is curved outwards is called _________ mirror.

Ans: Convex

7. The ration of sine of the angle of incidence to the sine of _________ is a constant.

Ans: Angle of refraction

8. All distances parallel to the principle axis are measured from the _________ of the mirror.

Ans: Pole

9. A negative sign in the value of magnification indicates that the image is _________

Ans: real

10. Light is refracted or bent while going from one medium to another because its _________, _________ changes.

Ans: speed, wavelength

### IV. Match the following

 List I List II 1. Ratio of height of image to height of object. concave mirror 2. Used in hairpin bends in mountains total internal reflection 3. Coin inside water appearing slightly raised magnification 4. Mirage convex mirror 5. Used as Dentist’s mirror refraction Ans : 1 – C, 2 – D, 3 – E, 4 – B, 5 – A

ii) Position of object Position of image Size and nature of image

 1. Within focus Between F and C Magnified , Real, inverted 2. At focus F At C Magnified, virtual, erect 3. Between F and C Behind the mirror Diminished, Real, inverted 4. At C Infinity Highly Diminished, Real, inverted 5. Beyond C At F Highly Magnified , Real, inverted 6. At infinity Beyond C Same size, Real, inverted Ans: 1 – C – B, 2 – D – E, 3 – F – A, 4 – B – F, 5 – A – C, 6 – E – D

### V. Assertion & Reason

1. Assertion: For observing the traffic at a hairpin bend in mountain paths a plane mirror is preferred over convex mirror and concave mirror.
Reason: A convex mirror has a much larger field of view than a plane mirror or a concave mirror

1. If both assertion and reason are true and reason is the correct explanation.
2. If assertion is true but reason is false.
3. If Assertion is false but reason is true.

Ans : If Assertion is false but reason is true.

2. Assertion: Incident ray is directed towards the centre of curvature of spherical mirror. After reflection it retraces its path.
Reason: Angle of incidence I = Angle of reflection r = 0o

1. If both assertion and reason are true and reason is the correct explanation.
2. If assertion is true but reason is false.
3. If Assertion is false but reason is true.

Ans : If both assertion and reason are true and reason is the correct explanation.

1. According to Cartesion sign convention which mirror and which lens has negative focal length?

Concave, mirror is having negative focal length.

2. Name the mirror(s) that can give (i) an erect and enlarged image, (ii) same sized, inverted image.

Concave mirror

3. If an object is placed at the focus of a concave mirror, where is the image formed?

Infinity as real and inverted.

4. Why does a ray of light bend when it travels from one medium to another?

A ray of light bend when it travels from one medium to another due to the change in velocity of light in two different medium.

5. What is speed of light in vacuum? Who first measured the speed of light?

The only medium in which speed of light is equal to that in vacuum is air. Speed of light in vacuum is 3 × 108 m/s. Armand Fizeau first measured the speed of light.

6. Concave mirrors are used by dentists to examine teeth. Why?

As concave mirror produces virtual, erect and magnified images when an object is placed in between focus and pole. So he can clearly gets a magnified view of cavities.

7. A coin in a glass beaker appears in rise as the beaker is slowly filled with water, why?

Due to the refraction

8. Name the spherical mirror(s) that has/have

(i) virtual principle focus

Convex mirror

(ii) Real principle focus

Concave mirror

9. Pick Out the concave and convex mirrors from the following and tabulate them.

(Rear-View, mirror, Dentists mirror, Torch-light mirror, Mirrors in shopping malls, Make-up mirror Concave mirror Convex mirror

10. Give two examples of transparent medium that are denser than air.

1. Water
2. Glass

1. a) Complete the diagram to show how a concave mirror forms the image of the object. b) What is the nature of the image? Real, inverted, and magnified.

2. Pick out the concave and convex mirrors from the following and tabulate them. Rear-view mirrors, Dentist’s mirrors, Torch-light mirrors, Mirrors in shopping malls, and Make-up mirrors.

 Concave Mirror Convex Mirror Dentist’s mirror Rearview mirror Torchlight mirror Mirrors in shopping malls Makeup mirror

3. State the direction of the incident ray which after reflection from a spherical mirror retraces its path. Give a reason for your answer.

Incident ray is directed toward the center of curvature

Reason: The ray is normal to the spherical mirror, so∠i = 0, ஃ ∠r = 0

4. What is meant by magnification? Write its expression. What is its sign for real images and virtual images? [HY – 2019]

Magnification is the increase in the size of an image compared to the true size.

 Magnification m = height of the image h2 / height of the object h1 = − image distance v / object distance u so m = h2 / h1 = −v/u

(a) Negative sign – real image
(b) Positive sign – virtual image

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The provided 9th science Light Book Back Answers can be beneficial for your preparation. All the best!

Last Updated: 30th June 2023