Light Book Back Questions 9th Science Lesson 6

9th Science Lesson 6

6] Light

Book Back Questions with Answer and Do You Know Box Content

Do You Know?

The most common usage of mirror writing can be found on the front of ambulances, where the world “AMBULANCE” is often written in very large mirrored text.

Problem 1: Find the size, nature and position of the image formed when an object of size 1 cm is placed at a distance of 15 cm from a concave mirror of focal length 10 cm.

Solution: Object distance, u = -15 cm (to the left of mirror)

Image distance, v = ?

Focal length, f = -10 cm (concave mirror)

Using mirror formula, 1/v + 1/u = 1/f

1/v + 1/-15 = 1/-10

1/v – 1/15 = -1/10

1/v = -1/10 + 1/15 = -3+2/30 = -1/30

Thus, image distance, v = -30 cm (negative sign indicates that the image is on the left side of the mirror).

⸫ Position of image is 30 cm in front of the mirror. Since the image is in front of the mirror, it is real and inverted.

To find the size of the image, we have to calculate the magnification.

m = -v/u = – (-30) / (-15) = -2

We know that, m = h₂/h₁

Here, height of the object h₁ = 1 cm

-2 = h₂/1

h₂ = -2 x 1 = -2 cm

The height of images is 2 cm (negative sign shows that the image is formed below the principle axis).

Problem 2: An object 2 cm high is placed at a distance of 16 cm from a concave mirror which produces a real image 3 cm high. Find the position of the image.

Solution: Height of object h₁ = 2 cm

Height of real image h₂ = -3 cm

Magnification m = h₂/h₁ = -3/2 = -1.5

We know that, m = -v/u

Here, object distance u = -16 cm

Substituting the value, we get

-1.5 = -v / (-16)

-1.5 = v/16

v = 16 x (-1.5) = -24 cm

The position of image is 24 cm in front of the mirror (negative sign indicates that the image is on the left side of the mirror).

Stellar objects are at an infinite distance. Therefore, the image formed by a concave mirror would be diminished and inverted. Yet, astronomical telescopes use concave mirrors.

In the rear view mirror, the following sentence is written. “Objects in the mirror are closer than they appear”. Why?

Problem 3: A car is fitted with a convex mirror of focal length 20 cm. Another car is 6 m away from the first car. Find the position of the second car as seen in the mirror of the first. What is the size of the image if the second car is 2 m broad and 1.6 m high?

Solution: Focal length = 20 cm (convex mirror)

Object distance = -6 m = -600 cm

Image distance v =?

1/f = 1/u + 1/v

1/20 = 1/-600 + 1/v

1/v = 1/20 – 1/-600 = 1/20 + 1/600 1/v = 30+1/600 = 31/600

v = 600/31 = 19.35 cm

(b) Size of the image

m = -v/u = -v / (-u) = 600/31 x 1/-600 = 1/31

Breadth of image = 1/31 x 200 cm = 6.45 cm

Height of image = 1/31 x 160 cm = 5.16 cm

Some organisms can make their own light too? This ability is called bioluminescence. Worms, fish, squid, starfish and some other organisms that live in the dark sea habitat glow or flash light to scare off predators

Problem 4: The speed of light in air is 3 x 10⁸ ms^-1 and in glass it is 2 x 10⁸ ms^-1. What is the refractive index of glass?

Solution: _aµ_g = 3 x 10⁸/2 x 10⁸ = 3/2 = 1.5

Problem 5: Light travels from a rarer medium to a denser medium. The angles of incidence and refraction are respectively 45^o and 30^o. Calculate the refractive index of the second medium with respect to the first medium.

Solution: ₁µ₂ = sin i/sin r = sin 45^o/sin 30^o = 1√2 / ½ = √2 = 1.414

An Indian-born physicist Narinder Kapany is regarded as the Father of Fibre Optics.

Choose the best answers:

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

(a) 0^o

(b) 45^o

(c) 90^o

2. __________ is used as reflectors in torchlight.

(a) Concave mirror

(b) Plane mirror

(c) Convex mirror

3. We can create enlarged, virtual images with

(a) Concave mirror

(b) Plane mirror

(c) Convex mirror

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

(a) Concave mirror

(b) Convex mirror

(c) Plane mirror

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

(a) Reflected

(b) Only deviated

(c) Deviated and dispersed

6. The speed of light is maximum in

(a) Vacuum

(b) Glass

(c) Diamond

State true or false. If false, correct the statement:

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

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

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

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

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

Fill in the blanks:

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

2. The mirror used in search light is ___________.

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

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

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

Match the following:

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

Consider the statements given below and choose the correct option:

(a) If both assertion and reason are true and reason is the correct explanation of assertion

(b) If both assertion and reason are true but reason is not the correct explanation of assertion

(c) If assertion is true but reason is false

(d) If assertion is false but reason is true

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 or view than a plane mirror or a concave mirror.

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) = 0^o.

Answers:

Choose the best answers:

1. 45° 2. Concave mirror 3. Concave mirror 4. convex mirror 5. deviated and dispersed 6. vacuum

State true or false. If false, correct the statement:

1.True

2.False

Correct answer: When a light travel from one medium to another, it suffers dispersion

3.True

4.False

Correct answer: The image formed will be Real, inverted and same size of the object

5.True

Fill in the blanks:

1. Towards normal 2. Concave mirror 3. Prism and angle of incident 4. 10 Cm 5. Concave

Match the following:

1.Ratio of height of image to height of object – Magnification

2.Used in hairpin bends in mountains – Convex mirror

3.Coin inside water appearing slightly raised – Refraction

4.Mirage- Total internal reflection

5.Used as Dentist’s mirror- Concave mirror

Consider the statements given below and choose the correct option:

1.If assertion is false but reason is true

2.If both assertion and reason are true and reason is not the correct explanation.