Ray Optics Class 12 important questions with Answers PDF
Dear students class 12th chapter 9 ray optics and optical instruments important short questions with answers pdf have been collected at one place to get you all prepared for your 12th Bihar board exam, as well as for UP board exams ,Jharkhand board exam ,MP board exams ,Rajsthan board exams ,cbse board exams and other board exams. Visit my nawendu classes youtube channel for more help.
1. What is mirage?
Answer-
μ2 > μ1
(Where μ2 = refractive index of lens
μ2 = refractive index of lens)
P = (μ2 – μ1) (1/R1 – 1/R2)
power of the lens will decreases as well as the focal length will increase. But nature of the lens will not change.
μ2 =μ1
P = 0
then power of lens is zero and focal length will be infinity.
μ2 < μ1
In this case power of lens will decrease as well as focal least will increase but nature of the lets will be changed. I.e., convex lens will act as concave lens and vice-versa.
9. What is difference between magnification and power of magnification.
Bihar Board 2010 ,2020
Answer- Magnification
(i)Ratio of height of image to height of object is called magnification.
(ii)Magnification increases with increase in distance of image.
(iii) Magnification lies between -infinity to +infinity.
Power of magnification
(i) Ratio of angle formed by image at eye and that by object at eye is called power of magnification.
M = β/α
(ii) Power of magnification decreases with increase in distance of image.
(iii) Power of magnification lies between D/f and (1+D/f)
10. What do you mean by power of magnification?
M = β/α
11. When an object is seen after watching towards the sun appears black. Why?
Answer- when we watch towards the sun then the pupil of eye shrinks due to large intensity of light. If we see other objects sufficient amount of light does not enter in eye that is why the object appears black.
Due to this reason nothing is visible except screen in cinema hall.
12. Danger sign is red. Why?
Bihar Board 2018
Answer- wavelength of red-light is maximum among all seven light rays of spectrum. According to Rayleigh Intensity of scattered light is indirectly proportional to fourth power of wavelength. So scattering of red light will be minimum so absorption of red light while propagating through a medium will also be minimum This is why danger-sign is red because it can travel for long distances.
13. Why does the sun appear red at the time of sunrise and sunset?
Answer- According to Rayleigh intensity of scattered light is indirectly proportional to fourth power of wavelength. So among seven colors of white-light red light is scattered in minimum amount that is why it is absorbed in minimum amount also among seven colors of white-light which travels larger distance while sunrise and sunset. Therefore the sun appears red while surmise and sunset.
14. Why does sky appear blue?
Answer- When we see towards sky scattered light is seen. According to Rayleigh we know that intensity of scattered light is indirectly proportional to fourth power of wavelength. So Violet is scattered in maximum amount and red is scattered in minimum amount. But intensity of violet light is already in less amount in sunlight so sky appears blue due to second and third most scattered light indigo-blue.
15. All the time of sunset (or sunrise) the sun appears some what higher than its actual position. Why?
Answer-
Answer- Emission spectrum – spectrum obtained from the radiation of
a hot body is called emission spectrum.
e. g - spectrum obtained from electric
bulb, carbon lamp and sun.
emission spectrum are of three types.
(i) Continuous spectrum
(ii) Line spectrum
Absorption
spectrum – When a radiation passes through a
refractive solid, liquid or gas medium then obtained spectrum is called
absorption spectrum.
Absorption spectrum are of
three types.
(ii) Line spectrum
21. Diamond appears very bright. Why?
22. What type of change is produced
by a glass slab in the path of ray?
23. Why does red rose appears black
in presence of blue light?
24. Distinguish between ultraviolet
and infrared radiation?
Ultraviolet
(iii) Its energy is greater than infrared.
Infrared
(ii) Its frequency is less than ultraviolet.
(iii) Its energy is less than ultraviolet.
(iv) It was discovered by by William Hershel in 1800 AD.
25. What do you mean by spectrometer?
(i) Collimator (ii) telescope (iii) prism table.
Answer-
Answer-
In summer, in desert air
consists of layers of different density. The layers close to the earth surface is rarer
and keeps on being denser with height from earth surface.
The light rays coming from any object (plant) towards earth frequently refracted at different layers of air. At each refraction, it bends away from normal so its angle of incidence keeps on increasing for next pair of layers.
When angle of incidence becomes more than critical angle total internal reflection takes place due to which imaginary inverted image is seen. The phenomena are known as mirage.
2. Explain the cause of dispersion of light?
Answer- Wavelengths of different colors of light of white light are different. Now from cuchy’s formula we know that μ=A+B/ λ²+ ...
3. What do you mean by persistence of vision?
Answer- Image formed on retina is unstable yet after removal of an object its image persists on retina (brain) up to 1/10 sec.
This is known as persistence of vision.
Due to persistence of vision luminous ring appears when illuminated object is whirled and time – interval between two scenes in cinema hall is on the same – principle.
4. At night, it is difficult to see objects outside from the glass window of a lighted room easily. Explain.
Answer- The intensity of reflected light inside the room is large while the light coming from outside the room is negligible so it is difficult to see objects outside the room.
It the light is off then we see lights only coming from outside the room therefore we see outside objects easily.
5. Which is more suitable to collect solar energy?
(a) a polished concave mirror
(b)a convex lens
Answer-
The light rays coming from any object (plant) towards earth frequently refracted at different layers of air. At each refraction, it bends away from normal so its angle of incidence keeps on increasing for next pair of layers.
When angle of incidence becomes more than critical angle total internal reflection takes place due to which imaginary inverted image is seen. The phenomena are known as mirage.
2. Explain the cause of dispersion of light?
Answer- Wavelengths of different colors of light of white light are different. Now from cuchy’s formula we know that μ=A+B/ λ²+ ...
3. What do you mean by persistence of vision?
Answer- Image formed on retina is unstable yet after removal of an object its image persists on retina (brain) up to 1/10 sec.
This is known as persistence of vision.
Due to persistence of vision luminous ring appears when illuminated object is whirled and time – interval between two scenes in cinema hall is on the same – principle.
4. At night, it is difficult to see objects outside from the glass window of a lighted room easily. Explain.
Answer- The intensity of reflected light inside the room is large while the light coming from outside the room is negligible so it is difficult to see objects outside the room.
It the light is off then we see lights only coming from outside the room therefore we see outside objects easily.
5. Which is more suitable to collect solar energy?
(a) a polished concave mirror
(b)a convex lens
Answer-
When light passes through
a refractive medium then some part of its energy is absorbed by the medium so
to collect solar energy polished concave mirror is more suitable than a convex
lens.
6. What is effect of nature of medium on velocity of light?
Answer- μ=c/v
Hence refractive index of a medium is indirectly proportional to velocity of light. Refractive index of denser medium is greater than that of rarer medium. So velocity of light in rarer medium is greater than that in denser medium.
7. What do you mean by the minimum deviation of a prism?
Answer- Minimum deviation produced by a prism is called minimum deviation.
For minimum deviation
and \[{{r}_{1}}={{r}_{2}}\]
8. When a lens is dipped in water then its power changes. Why?
6. What is effect of nature of medium on velocity of light?
Answer- μ=c/v
Hence refractive index of a medium is indirectly proportional to velocity of light. Refractive index of denser medium is greater than that of rarer medium. So velocity of light in rarer medium is greater than that in denser medium.
7. What do you mean by the minimum deviation of a prism?
Answer- Minimum deviation produced by a prism is called minimum deviation.
For minimum deviation
and \[{{r}_{1}}={{r}_{2}}\]
8. When a lens is dipped in water then its power changes. Why?
OR
How does the focal length of a lens get changed when dipped in a liquid?
Answer- From lens makers formula we
know thatμ2 > μ1
(Where μ2 = refractive index of lens
μ2 = refractive index of lens)
P = (μ2 – μ1) (1/R1 – 1/R2)
power of the lens will decreases as well as the focal length will increase. But nature of the lens will not change.
μ2 =μ1
P = 0
then power of lens is zero and focal length will be infinity.
μ2 < μ1
In this case power of lens will decrease as well as focal least will increase but nature of the lets will be changed. I.e., convex lens will act as concave lens and vice-versa.
9. What is difference between magnification and power of magnification.
Bihar Board 2010 ,2020
Answer- Magnification
(i)Ratio of height of image to height of object is called magnification.
(ii)Magnification increases with increase in distance of image.
(iii) Magnification lies between -infinity to +infinity.
Power of magnification
(i) Ratio of angle formed by image at eye and that by object at eye is called power of magnification.
M = β/α
(ii) Power of magnification decreases with increase in distance of image.
(iii) Power of magnification lies between D/f and (1+D/f)
10. What do you mean by power of magnification?
M = β/α
11. When an object is seen after watching towards the sun appears black. Why?
Answer- when we watch towards the sun then the pupil of eye shrinks due to large intensity of light. If we see other objects sufficient amount of light does not enter in eye that is why the object appears black.
Due to this reason nothing is visible except screen in cinema hall.
12. Danger sign is red. Why?
Bihar Board 2018
Answer- wavelength of red-light is maximum among all seven light rays of spectrum. According to Rayleigh Intensity of scattered light is indirectly proportional to fourth power of wavelength. So scattering of red light will be minimum so absorption of red light while propagating through a medium will also be minimum This is why danger-sign is red because it can travel for long distances.
13. Why does the sun appear red at the time of sunrise and sunset?
Answer- According to Rayleigh intensity of scattered light is indirectly proportional to fourth power of wavelength. So among seven colors of white-light red light is scattered in minimum amount that is why it is absorbed in minimum amount also among seven colors of white-light which travels larger distance while sunrise and sunset. Therefore the sun appears red while surmise and sunset.
14. Why does sky appear blue?
Answer- When we see towards sky scattered light is seen. According to Rayleigh we know that intensity of scattered light is indirectly proportional to fourth power of wavelength. So Violet is scattered in maximum amount and red is scattered in minimum amount. But intensity of violet light is already in less amount in sunlight so sky appears blue due to second and third most scattered light indigo-blue.
15. All the time of sunset (or sunrise) the sun appears some what higher than its actual position. Why?
Answer-
Air is denser near the
earth surface and its density decrease on going to words the sun. So light rays
coming from the sun frequently refracted to-wards normal so imaginary image of
the sun is seen higher than its actual position.
16. After the operation of eye green curtain is used before eyes. Why?
L – type cells are mainly to
identify green color. So our eyes are more sensitive for green color so when
we see green light our eyes feel relaxed so green curtain is used before eyes
after the operation.
Answer- When we see an object of
higher intensely for long time then conical and cylindrical cell of our retina
fatigued. Due to which black strips are seen while watching lower intensity
object. This is known as retinal fatigue.
Answer- There are following
advantages of pair of eyes instead of a single eye.
(i) There is a correct estimation
of distance by both eyes instead of one eye.
This is why it is difficult to fill ink in a pen or string in the hole of needle by one eye.
(ii) Instead of one eye, both
eyes increase range of vision.
(iii) Instead of one eye, both eyes help us to see 3 – D shape of an object.
19. What is difference between
primary and secondary rainbow?
16. After the operation of eye green curtain is used before eyes. Why?
Answer- Our eyes are more
sensitive for green color of light. There are three types of conical cell in
our retina. (i) L – type (Large) (ii) M – type (Medium) (iii) S – type (Small)
17. What is retinal fatigue?
18. What are advantage of pair of eye
instead of a single eye?
This is why it is difficult to fill ink in a pen or string in the hole of needle by one eye.
(iii) Instead of one eye, both eyes help us to see 3 – D shape of an object.
Bihar Board 2019
Answer- see in note of
(Nawendu physics classes–Muzaffarpur.)20. What is difference between
emission and absorption spectrum.
emission spectrum are of three types.
(ii) Line spectrum
(iii) Band spectrum
Spectrum of sun light
formed by a prism is a absorption spectrum.
(i) Continuous spectrum
(iii) Band spectrum
Answer- 
Critical angle of diamond
is 24.4 and that of ordinary glass is 41.8 Therefore total internal reflections inside diamond are greater than those in ordinary glass. Therefore diamond appears very bright.
Answer-
Deviation produced by a glass
slab is zero. When a light ray incident perpendicular to it passes with no
deviation but if a light ray incident at any other angle it is laterally
displaced with no deviation.
Answer- Red rose absorbs all light
ray except red light. Hence when a red rose is seen in blue light then it
absorbs blue light and no any light enters in observers eyes so it appears
black.
Answer-
Ultraviolet
(i) Its wavelength is less than 3900A.
(ii) Its frequency is greater than infrared.
(iv) It was discovered by Ritter in 1801 AD.
(i) Its wavelength is greater than 7800A.
(iii) Its energy is less than ultraviolet.
(iv) It was discovered by by William Hershel in 1800 AD.
25. What do you mean by spectrometer?
Answer- Spectrometer is a machine
which is used to obtain pure spectrum. It consists of following three main
parts.
26. What are the necessary conditions
to obtain pure spectrum?
(i) Source slit should be
narrow.
(iii) Prism should be in case of minimum
deviation.
27. If image of a candle is formed on
a screen by a convex lens. If half of lens is blackened then what will be the
size of image.
(ii) Incident rays on prism should be parallel.
(iv) Emergent rays should be converged on a
curtain by a convex lens.
Answer-
The ray coming from any
part of candle meets at the same point after refraction through half portion
of lens where it meets in case of total lens. Therefore size of image will be
same size but intensity of image will decrease because half of light rays will
not refract to form image.
28. Why is aperture of objective kept small in compound microscope?
Answer-
28. Why is aperture of objective kept small in compound microscope?
Answer- Aperture of objective is
kept small in compound microscope so that light rays coming from small object
expand in small aperture. As a result image of the object is more bright and
clear.
29. Why is aperture of objective kept large in telescope?
Answer- Aperture of objective is kept large in telescope so that more light (coming from long distant objects) is collected in large aperture objective. As a result image of the object is more bright and clear.
30. What is optical fiber ? Describe
its some important applications.
It is a fiber (tube) of
glass or quartz which works on the principle of total internal reflection.
Applications- (ii) In communication system with the help of optical fibres light signals are sent from one place to the other place..
31. What is refractive index. How it
related to critical angle.
Bihar Board 2020
(i) In the field of computer
lines.
(iii) In endoscopy (in medical
field).
Answer- see in (Nawendu physics
classes) Note .
32. What do you mean by dispersive
power?.
Ratio of angle of dispersion and
angle of deviation of yellow ray is called dispersive power.
$\omega =\frac{\theta }{{{\delta }_{Y}}}=\frac{{{\delta }_{V}}-{{\delta }_{R}}}{{{\delta }_{Y}}}=\frac{{{\mu }_{V}}-{{\mu }_{R}}}{{{\mu }_{Y}}-1}$
33. Difference between reflecting and refracting telescope.
Answer-
Reflecting telescope
(i) Objective is large concave mirror .
Refracting telescope
(ii) Image is less bright and clear than reflecting telescope.
34. What do you mean by distortion in a lens?
Answer- The defect in a lens due to which magnification of different point of image is different.
Distortion is of two types.
(i) Pin-cushion distortion- when straight lines are curved outward then distortion is called Pin-cushion distortion.
(ii) Barrel distortion – when straight lines are curved inward then distortion is called barred distortion.
Bihar Board 2010
Answer-
Answer- It is denoted by ω.
33. Difference between reflecting and refracting telescope.
Answer-
Reflecting telescope
(i) Objective is large concave mirror .
(ii) Image is more bright and clear than refracting telescope.
(i) Objective is a lens.
34. What do you mean by distortion in a lens?
Answer- The defect in a lens due to which magnification of different point of image is different.
(i) Pin-cushion distortion- when straight lines are curved outward then distortion is called Pin-cushion distortion.
(ii) Barrel distortion – when straight lines are curved inward then distortion is called barred distortion.
35. What is chromatic aberration? How
is it reduced?
Answer-
The image of an object in
white light formed by a lens is usually colored and blurred. This defect of
image is called chromatic aberration.
Focus of a lens for violet ray will be nearest and that for red ray will be farthest.
Difference between fv and fr is measure of aberration.
Chromatic aberration = fr-fv = -df ...(i)
From lens-maker formula
\[\frac{1}{f}=(\mu -1)(\frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}})\] ...(ii)
Differentiating both sider.
$\frac{-df}{{{f}^{2}}}=d\mu \left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)$ ....(iii)
Apply (iii)/
-df/f = dμ/μ-1
-df/f = ω
Aberration = -df = ωf _______(iv)
To ignore aberration two lenses will be combined
Differentiation both sides
$\frac{-df}{{{f}^{2}}}=\frac{-d{{f}_{1}}}{{{f}_{1}}^{2}}+\frac{-d{{f}_{2}}}{{{f}_{2}}^{2}}$
The combination will be free from chromatic aberration if df = o
-$\frac{-d{{f}_{1}}}{{{f}_{1}}^{2}}+\frac{-d{{f}_{2}}}{{{f}_{2}}^{2}}=0$
$\frac{{{\omega }_{1}}{{f}_{1}}}{{{f}_{1}}^{2}}+\frac{{{\omega }_{2}}{{f}_{2}}}{{{f}_{2}}^{2}}=0$
$\frac{{{\omega }_{1}}}{{{f}_{1}}}+\frac{{{\omega }_{2}}}{{{f}_{2}}}=0$
This is condition to reduce chromatic aberration.
Answer- Filament bulb is a thermal
source of light from which ultraviolet and infrared radiation are emitted
besides light.
So fluorescent tube provides
more light than filament bulb of same wattage.
Answer- powdered color glass reflects all seven color light rays of white light similarly so it appears white.
Answer- Oil has refractive index very close to paper so when oil is applied on paper they both act as a same medium . Therefore amount of light propagating across paper increases so it becomes translucent.
Bihar Board 2016
Answer - When the angle of incidence (in a denser medium) is greater than the critical angle, then light rays return to the same medium, this phenomenon is called total internal induction.
Focus of a lens for violet ray will be nearest and that for red ray will be farthest.
Difference between fv and fr is measure of aberration.
Chromatic aberration = fr-fv = -df ...(i)
\[\frac{1}{f}=(\mu -1)(\frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}})\] ...(ii)
$\frac{-df}{{{f}^{2}}}=d\mu \left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)$ ....(iii)
-df/f = ω
To ignore aberration two lenses will be combined
$\frac{1}{f}=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}$
Differentiation both sides
$\frac{-df}{{{f}^{2}}}=\frac{-d{{f}_{1}}}{{{f}_{1}}^{2}}+\frac{-d{{f}_{2}}}{{{f}_{2}}^{2}}$
The combination will be free from chromatic aberration if df = o
$\frac{{{\omega }_{1}}{{f}_{1}}}{{{f}_{1}}^{2}}+\frac{{{\omega }_{2}}{{f}_{2}}}{{{f}_{2}}^{2}}=0$
This is condition to reduce chromatic aberration.
36. A fluorescent tube of 40 watt provides more light than that of filament bulb of 40 watt. Why?
While inside the fluorescent tube
ultraviolet radiation is emitted with light due to gas discharge. Ultraviolet
radiation is modified into visible light by photo luminescent.
37. When color glass is powdered it
appears white. Why?
38. When some oil is introduced on
paper. It becomes translucent. Why?
39. What is total internal reflection? Write the necessary conditions for this event ?
Answer - When the angle of incidence (in a denser medium) is greater than the critical angle, then light rays return to the same medium, this phenomenon is called total internal induction.
Conditions - (i) Light rays must be incident in a denser medium for total internal reflection.
(ii) The angle of incidence must be greater than the critical angle.
40. Define critical angle and write its terms.
Bihar Board 2012
Answer- The angle of incidence of the denser medium for which the angle of refraction is 90 degrees is called the critical angle.
(ii) The angle of incidence must be greater than the critical angle.
40. Define critical angle and write its terms.
Bihar Board 2012
Answer- The angle of incidence of the denser medium for which the angle of refraction is 90 degrees is called the critical angle.
Conditions - For critical angle light rays must be refracted from denser medium to rarer medium.
41. Explain secondary rainbow.
Bihar Board 2021
Answer- see in note of (Nawendu physics classes–Muzaffarpur.)
42. Write necessary conditions for total internal reflection of light.
Bihar Board 2021
Answer- Followings are the conditions for total internal reflection of light
(i) Light rays must be incident in a denser medium for total internal reflection.
(ii) The angle of incidence must be greater than the critical angle.43. What do you mean by power of accommodation of eye ?
Bihar Board 2021
Answer- The ability of eye to adjust focal length of eye lens ( with the help of ciliary muscles ) to form images clearly on retina of near and distant objects is called power of accommodation of eye.
Bihar Board 2021
Answer- The ability of eye to adjust focal length of eye lens ( with the help of ciliary muscles ) to form images clearly on retina of near and distant objects is called power of accommodation of eye.
44. What do you mean by refractive index of medium ?
Bihar Board 2022
Bihar Board 2022
Answer- see in (Nawendu physics classes) Note
45. Explain dispersion of light
Bihar Board 2022
Bihar Board 2022
Answer- see in (Nawendu physics classes) Note
chapter 1 ELECTRIC CHARGES AND FIELDS short Q-A
chapter 2 ELECTROSTATIC POTENTIAL AND CAPACITANCE short Q-A
chapter 3 CURRENT ELECTRICITY short Q-A
chapter 4 MOVING CHARGES AND MAGNETISM short Q-A
chapter 5 MAGNETISM AND MATTER short Q-A
chapter 6 ELECTROMAGNETIC INDUCTION short Q-A
chapter 7 ALTERNATING CURRENT short Q-A
chapter 8 ELECTROMAGNETIC WAVES short Q-A
chapter 2 ELECTROSTATIC POTENTIAL AND CAPACITANCE short Q-A
chapter 3 CURRENT ELECTRICITY short Q-A
chapter 4 MOVING CHARGES AND MAGNETISM short Q-A
chapter 5 MAGNETISM AND MATTER short Q-A
chapter 6 ELECTROMAGNETIC INDUCTION short Q-A
chapter 7 ALTERNATING CURRENT short Q-A
chapter 8 ELECTROMAGNETIC WAVES short Q-A
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