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Chapter: 10th Science : Chapter 2 : Optics

Book Back Questions with Answers

I. Choose the correct answer II. Fill in the blanks: III. True or False. If false correct it. IV. Match the following: V. Assertion and reasoning type VI. Answer Briefly VII. Give the answer in detail VIII. Numerical Problems: IX. Higher order thinking (HOT) questions:

Optics - Science


I. Choose the correct answer

 

1. The refractive index of four substances A, B, C and D are 1.31, 1.43, 1.33, 2.4 respectively. The speed of light is maximum in

a) A

b) B

c) C

d) D

 

2. Where should an object be placed so that a real and inverted image of same size is obtained by a convex lens

a) f

b) 2f

c) infinity

d) between f and 2f

 

3. A small bulb is placed at the principal focus of a convex lens. When the bulb is switched on, the lens will produce

a) a convergent beam of light

b) a divergent beam of light

c) a parallel beam of light

d) a coloured beam of light

 

4. Magnification of a convex lens is

a) Positive

b) negative

c) either positive or negative

d) zero

 

5. A convex lens forms a real, diminished point sized image at focus. Then the position of the object is at

a) focus

b) infinity

c) at 2f

d) between f and 2f

 

6. Power of a lens is –4D, then its focal length is

a) 4m

b) –40m

c) –0.25 m

d) –2.5 m

 

7. In a myopic eye, the image of the object is formed

a) behind the retina  

b) on the retina

c) in front of the retina   

d) on the blind spot

 

8. The eye defect ‘presbyopia’ can be corrected by

a) convex lens

b) concave lens

c) convex mirror

d) Bi focal lenses

 

9. Which of the following lens would you prefer to use while reading small letters found in a dictionary?

a) A convex lens of focal length 5 cm

b) A concave lens of focal length 5 cm

c) A convex lens of focal length 10 cm

d) A concave lens of focal length 10 cm

 

10. If VB, VG, VR be the velocity of blue, green and red light respectively in a glass prism, then which of the following statement gives the correct relation?

a) VB = VG = VR

b) VB > VG >VR

c) VB < VG < VR

d) VB < VG > VR

 

II. Fill in the blanks:


1. The path of the light is called as ray

2. The refractive index of a transparent medium is always greater than one

3. If the energy of incident beam and the scattered beam are same, then the scattering of light is called as elastic scattering.

4. According to Rayleigh’s scattering law, the amount of scattering of light is inversely proportional to the fourth power of its wavelength

5. Amount of light entering into the eye is controlled by Iris

 

III. True or False. If false correct it.


1. Velocity of light is greater in denser medium than in rarer medium - False

Velocity of light is lesser in denser medium than in rarer medium.

2. The power of lens depends on the focal length of the lens - True

3. Increase in the converging power of eye lens cause ‘hypermetropia’ - True

4. The convex lens always gives small virtual image. - False

Concave lens always gives small virtual image.

 

IV. Match the following:

Column - I Column - II

1 Retina a Path way of light

2 Pupil b Far point comes closer

3 Ciliary muscles c near point moves away

4 Myopia d Screen of the eye

5 Hypermetropia f Power of accom- modation

Answer:

(1) Retina - Screen of the eye

(2) Pupil - Path way of light

(3) Ciliary muscles - Power of accommodation

(4) Myopia - Far point comes closer

(5) Hypermetropia - near point moves away


 

V Assertion and reasoning type

Mark the correct choice as

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) Assertion is true but reason is false.

d) Assertion is false but reason is true.

1. Assertion: If the refractive index of the medium is high (denser medium) the velocity of the light in that medium will be small

Reason: Refractive index of the medium is inversely proportional to the velocity of the light

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

2. Assertion: Myopia is due to the increase in the converging power of eye lens.

Reason: Myopia can be corrected with the help of concave lens.

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

 

VI. Answer Briefly


1. What is refractive index?

The ratio of speed of light in vacuum to the speed of light in a medium is defined as refractive index ‘μ’ of that medium.


2. State Snell’s law.

The ratio of the sine of the angle of incidence and sine of the angle of refraction is equal to the ratio of refractive indices of the two media.

sin i / sin r  = μ2/ μ1


3. Draw a ray diagram to show the image formed by a convex lens when the object is placed between F and 2F.


4.              Define dispersion of light

When a beam of white light or composite light is refracted through any transparent media such as glass or water, it is split into its component colours. This phenomenon is called as dispersion of light.


5. State Rayleigh’s law of scattering

The amount of Rayleigh scattering is inversely proportional to the fourth power of the wavelength. This is called as Rayleigh scattering law.

Amount of scattering a  1/λ4


6. Differentiate convex lens and concave lens.

Convex Lens

1. A convex lens is thicker in the middle than at edges.

2. It is converging

3. It produces mostly real images

4. It is used to treat hypermeteropia

Concave Lens

1. A concave lens is thinner in the middle than at edges

2. It is diverging

3. It produces a virtual image

4. It is used to treat myopia


7. What is power of accommodation of eye?

The ability of the eye to focus nearby as well as the distant objects on the retina of our eye is called power of accommodation of the eye.


8. What are the causes of ‘Myopia’?

(i) Myopia, also known as short sightedness, occurs due to the lengthening of eye ball.

(ii) Nearby objects can be seen clearly but distant objects cannot be seen clearly.

(iii) The focal length of eye lens is reduced or the distance between eye lens and retina increases.

(iv) Due to this, the image of distant objects are formed before the retina.


9. Why does the sky appear in blue colour?

When sunlight passes through the atmosphere, the blue colour (shorter wavelength) is scattered to a greater extent than the red colour (longer wavelength). This scattering causes the sky to appear in blue colour.


10. Why are traffic signals red in colour?

Red has the longest wavelength so it is mostly scattered by atmospheric particles. As a result whether it is fog or smoke, red light passes comparatively easily through them.

 

VII. Give the answer in detail


1. List any five properties of light

Answer:

(i) Light is a form of energy.

(ii) Light always travels along a straight line.

(iii) Light does not need any medium for its propagation. It can even travel through vacuum.

(iv) The speed of light in vacuum or air is,  c = 3 × 108 ms-1.

(v) Different coloured light has different wavelength and frequency.

(vi) When light is incident on the interface between two media, it is partly reflected and partly refracted.


2. Explain the rules for obtaining images formed by a convex lens with the help of ray diagram.

Answer:

When an object is placed in front of a lens, the light rays from the object fall on the lens.

Rule-1 : When a ray strikes convex lens obliquely at its pole or optical centre, it continues to follow its path without any deviation.


Rule-2 : When rays parallel to the principal axis strikes a convex lens, the refracted rays are converged to (convex lens) the principal focus.


Rule-3 : When a ray passing through (convex lens) the principal focus strikes a convex, the refracted ray will be parallel to the principal axis.



3. Differentiate the eye defects: Myopia and Hypermetropia

Answer:


 Myopia

(i) Myopia, also known as Short sightedness, occurs due to the lengthening of eye ball.

(ii) Nearby objects can be seen clearly but distant objects cannot be seen clearly.

(iii) The focal length of eye lens is reduced or the distance between eye lens and retain increases.

(iv) Due to this, the image of distant objects are formed before the retina.

(v) This defect can be corrected using a concave lens.

Hypermeteropia

(i) Hypermeteropia, also known as long sightedness, occurs due to the shortening of eye ball.

(ii) Distant objects can be seen clearly but nearby objects cannot be seen clearly.

(iii) The focal length of eye lens is increased or the distance between eye lens and retina decreases.

(iv) Due to this, the image of nearby objects are formed behind the retina

(v) This defect can be corrected using a convex lens.


4. Explain the construction and working of a ‘Compound Microscope’.

Answer:

(i) Compound microscope is used to see the tiny objects.

(ii) A compound microscope consists of two convex lenses. The lens with the shorter focal length is placed near the object, and is called as ‘objective lens’ or ‘objective piece’.

(iii) The lens with larger focal length and larger aperture placed near the observer’s eye is called as ‘eye lens’ or ‘eye piece’. Both the lenses are fixed in a narrow tube with adjustable provision.

Working :


(iv) The object (AB) is placed at a distance slightly greater than the focal length of objective lens (u > F0). A real, inverted and magnified image (A'B') is formed at the other side of the objective lens. This image behave as the object for the eye lens.

(v) The position of the eye lens is adjusted in such a way, that the image (A'B') falls within the principal focus of the eye piece. This eye piece forms a virtual, enlarged and erect image (A"B") on the same side of object.

 

VIII. Numerical Problems:


1. An object is placed at a distance 20cm from a convex lens of focal length 10cm. Find the image distance and nature of the image.

Object distance, u = - 20 cm

Focal length of convex lens f = 10 cm .

To find: Image distance v = ? and Nature of the the image = ?

Solution

If the object is placed on the left side of the lens, then f =  10 cm ; u = - 20 cm


Distance of the image v = 20 cm.

Enlarged and inverted image at a distance of 20 cm on the right side of the lens.

m = v/u = 20/-20 = -1

(-ve sign indicates the inverted image).

[Nature of the image]


2. An object of height 3cm is placed at 10cm from a concave lens of focal length 15cm. Find the size of the image.

Given

Object distance u =  - 10 cm

[Object is placed on the left side]

Focal length f = - 15 cm [concave lens]

To find : Image distance v = ?

Height of the object h = 3 cm.

Solution


Magnification m = Distance of the image / Distance of the object

m = v/u

m = 6/10 = 0.6

m =  height of the, image / height of the object

= h1/h

h1 = m x h = 0.6 x 3

Size of the image, h1 = 1.8 cm

 

IX. Higher order thinking (HOT) questions:


1. While doing an experiment for the determination of focal length of a convex lens, Raja Suddenly dropped the lens. It got broken into two halves along the axis. If he continues his experiment with the same lens, (a) can he get the image? (b) Is there any change in the focal length?

(a) Yes, he can get the image of same size.

(b)  No, there is no change in the focal length of the convex lens even if it is broken into two halves. Only the intensity of the image obtained will be less.


2. The eyes of the nocturnal birds like owl are having a large cornea and a large pupil. How does it help them?

Increase in their field of vision and an increase retinal surface help them to collect more ambient light during night.



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