2. Optics
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?
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 __________ scattering. Ans : Elastic
4. According to Rayleigh’s scattering law, the amount of scattering of light is inversely proportional to the fourth power of its __________. Ans : wave length
5. Amount of light entering into the eye is controlled by __________. Ans : iris
III. True or False. If false correct it.
1. Velocity of light is greater in denser medium than in rarer
medium.
Ans : False.
Velocity of light is greater in rare medium than in denser medium.
2. The power of lens depends on the focal length of the lens.
Ans : True.
3. Increase in the converging power of eye lens cause
‘hypermetropia’.
Ans : True.
4. The convex lens always gives small virtual image.
Ans : False. Only concave lens always gives small virtual image.
IV. Match the following:
|
|
Coloumn I |
Column
II |
|
1 |
Retina |
Screen of the eye |
|
2 |
Pupil |
Path way of light |
|
3 |
Ciliary muscles |
Power of accomodation |
|
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 the assertion and the reason are true and the reason is the correct explanation of assertion.
b) If both the assertion and the reason are true, but the reason is not the correct explanation of the assertion.
c) Assertion is true, but the reason is false.
d) Assertion is false, but the 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.
Ans : (a)
both the
assertion and the reason are true and the 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.
Ans : (b) both the assertion and the reason are true, but the reason is not the correct explanation of the assertion
VI. Answer Briefly
1. What is refractive index?
Refractive index as the ratio of sine of the angle incidence to the sine of angle of refraction. It can be alsobe defined as ratio of speed of light in air to the speed of light in medium. It has no unit.
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. This law is also known as Snell's law.
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 lightis 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.
Rayleigh's scattering law states that "the amount of scattering of light is inversaly proportional to the fourth power of its wavelength.
Let amount of scattering be 'A'
6.
Differentiate convex lens and concave lens.
|
S.No. |
Convex lens |
Concave lens |
|
1 |
A convex lens is thicker in the middle than
at edges |
A concave lens is thinner in the middle than
at edges |
|
2 |
It is having converging lens |
It is having diverging lens |
|
3 |
It produces mostly real images |
It produces virtual images |
|
4 |
It is used to treat hypermeteropia |
It is used to treat myopia |
7. What is power of accommodation of eye?
The ability of the eye lens to focus nearby as well as the distant objects on the refina of the eye is called power of accommodation of the eye.
8. What are the causes of ‘Myopia’?
Myopia is caused due to,
i) Lengthening of eye ball.
ii) Decrease in the focal length of the eyelens i.e., excessive curvature of the eye lens. The eyelens becomes more convergent.
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 bue colour.
10. Why are traffic signals red in colour?
Red colour has longest wavelength and scattered by a least amount and travels longer distance in atmosphere. So it used in traffic signals.
VII. Give the answer in detail
1. List any five properties of light.
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 m/s. Since, light is in the form of waves, it is characterized by a wavelength (l) and a frequency ( ), which are related by the following equation. C = l (C – velocity of light).
v) Different coloured light has
different wavelength and frequency.
vi) Among the visible light, Violet
light has the lowest wavelength and Red light has the highest wavelength.
vii) 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.
Rule 1 : When a ray of light strikes the convex lens obliquely at its optical centre, it continues to follow its path without any deviation. Rays passing through the optical centre
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 lens, the refracted ray will be parallel to the principal axis.
Rays passing through or directed towards the principal axis
Answer :
|
S.No. |
Myopia |
Hypermetropia |
|
|
1 |
It is also known as short sightedness occurs due to the
lengthening of eye ball |
It is
also known as long sightedness, occurs due to the shortening of eye ball |
|
|
2 |
With
this defect, nearyby objects can be seen clearly but distant objects
cannot be seen clearly |
With
this defect, distant objects can be seen clearly, but nearby objects
cannot be seen clearly |
|
|
3 |
The focal length of eye
lens is reduced or the distance between eye lens and retina increases |
The focal length of eye
lens is increased or the distance between eye lens and retina decreases |
|
|
4 |
The far point will not
be infinity for such eyes and the far point has come closer |
The near point will not
be at 25 cm for such eyes and the near point has moved farther |
|
|
5 |
Due to this the image
of distant objects are formed before the retina |
Due to this, the image
of nearby objects are formed behind the retina |
|
|
6 |
This defect can be
corrected by using a concave lens |
This defect can be
corrected by using a convex lens |
|
|
7 |
A suitable focal length
of the concave lens to be used to correct this defect |
A suitable focal length
of the convex lens to be used to correct this defect |
|
|
8 |
 |
 |
|
|
9 |
The
focal length of the required concave lens is
 |
The
focal length of the required convex lens is f  |
4. Explain the construction and working of a 'Compound Microscope'. Compound microscope is used to see the tiny objects. It has higher magnification power than simple microscope.
Construction :
ii) 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 the larger focal length and larger aperture placed near the observer's eye is called eye lens or eye piece. Both the lenses are fixed in a narrow tube with adjustable provision.
Working :
i) The object (AB) is placed at a distance slightly greater than the focal length of objective lens (u > fo). A real, inverted and magnified image (A'B') is formed at the other side of the object lens.
ii) This image behaves as the object for the eye lens. 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 errect image (A"B") on the same side of the object.
iii) Compound microscope has 50 to 200 times more magnification power than simple microscope.
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.
u = –20 cm, f = 10 cm
This indicates the size of the image is of the same size but inverted on the other side of the object.
b) Yes, focal length will get changed.