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1. What is mirage?
Ans. - 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?
Ans. – Wavelengths of different colors of light of white light are different. Now from cuchy’s formula we know that μ=A+B/ λ²+ ...
refractive-index of a medium will be also different for different colors of light of white light. From formula 𝛿 = A (μ - 1) deviation of different light rays will be different so they disperse.
3. What do you mean by persistence of vision?
Ans. – 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.
Ans. – 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
Ans. – 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?
Ans. – 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?
Ans. – Minimum deviation produced by a prism is called minimum deviation.
For minimum deviation
\[{{i}_{1}}={{i}_{2}}\]
and \[{{r}_{1}}={{r}_{2}}\]
8. When a lens is dipped in water then its power changes. Why?
Ans. – From lens makers formula we know that
Case (i) if lens is dipped in a medium of lower refractive index.
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.
Case (ii) when lens is dipped in a medium of same refractive index.
μ2 =μ1
P = 0
then power of lens is zero and focal length will be infinity.
Case (iii) when lens is dipped in a medium of lower refractive index.
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
Ans. – 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?
Ans.- Ratio of angle formed at eye by an image to angle formed at eye by an object of same place is called power of magnification (or angular magnification).
M = β/α
11. When an object is seen after watching towards the sun appears black. Why?
Ans. – 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
Ans. – 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?
Ans.- 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?
Ans. – 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?
Ans. – 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?
Ans. – 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)
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.
17. What is retinal fatigue?
Ans. – 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.
18. What are advantage of pair of eye instead of a single eye?
Ans. – 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?
Ans. – see in note book of (Nawendu physics classes–Muz.)
Mo.- 9504928380, 8862880454
20. What is difference between emission and absorption spectrum.
Ans. – 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
(iii) Band spectrum
Absorption spectrum – When a radiation passes through a refractive solid, liquid or gas medium then obtained spectrum is called absorption spectrum.
Spectrum of sun light formed by a prism is a absorption spectrum.
Absorption spectrum are of three types.
(i) Continuous spectrum
(ii) Line spectrum
(iii) Band spectrum
21. Diamond appears very bright. Why?
Ans. – 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.
22. What type of change is produced by a glass slab in the path of ray?
Ans. –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.
23. Why does red rose appears black in presence of blue light?
Ans. – 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.
24. Distinguish between ultraviolet and infrared radiation?
Ans. -
Ultraviolet
(i) Its wavelength is less than 3900A.
(ii) Its frequency is greater than infrared.
(iii) Its energy is greater than infrared.
(iv) It was discovered by Ritter in 1801 AD.
Infrared
(i) Its wavelength is greater than 7800A.
(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?
Ans. –Spectrometer is a machine which is used to obtain pure spectrum. It consists of following three main parts.
(i) Collimator (ii) telescope (iii) prism table.
26. What are the necessary conditions to obtain pure spectrum?
Ans. – (i) Source slit should be narrow.
(ii) Incident rays on prism should be parallel.
(iii) Prism should be in case of minimum deviation.
(iv) Emergent rays should be converged on a curtain by a convex lens.
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.
Ans. – 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?
Ans. – 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?
Ans. – 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 fibre ? Describe its some important applications.
Ans. – It is a fiber (tube) of glass or quartz which works on the principle of total internal reflection.
Applications-
(i) In the field of computer lines.
(ii) In communication system with the help of optical fibres light signals are sent from one place to the other place..
(iii) In endoscopy (in medical field).
31. What is refractive index. How it related to critical angle.
Bihar Board 2020
Ans. – see in (Nawendu physics classes)
Note - book.
32. What do you mean by dispersive power?.
Ans. – It is denoted by ω.
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.
Ans. - -
Reflecting telescope
(i) Objective is large concave mirror .
(ii) Image is more bright and clear than refracting telescope.
Refracting telescope
(i) Objective is a lens.
(ii) Image is less bright and clear than reflecting telescope.
34. What do you mean by distortion in a lens?
Ans. – The defect in a lens due to which magnification of different point of image is different.
Distortion is of two typer.
(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?
Bihar Board 2010
Ans. – 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
$\frac{1}{f}=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}$
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
-$\frac{-d{{f}_{1}}}{{{f}_{1}}^{2}}+\frac{-d{{f}_{2}}}{{{f}_{2}}^{2}}=0$
$\frac{{{\omega }_{1}}{{f}_{1}}}{{{f}_{1}}^{2}}+\frac{{{\omega }_{2}}{{f}_{2}}}{{{f}_{2}}^{2}}=0$
$\frac{{{\omega }_{1}}}{{{f}_{1}}}+\frac{{{\omega }_{2}}}{{{f}_{2}}}=0$
This is condition to reduce chromatic aberration.
36. A fluorescent tube of 40 watt provides more light than that of filament bulb of 40 watt. Why?
Ans. – Filament bulb is a thermal source of light from which ultraviolet and infrared radiation are emitted besides light.
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.
So fluorescent tube provides more light than filament bulb of same wattage.
37. When color glass is powdered it appears white. Why?
Ans. –powdered colour glass reflects all seven color light rays of white light similarly so it appears white.
38. When some oil is introduced on paper. It becomes translucent. Why?
Ans.-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.
39. What is total internal reflection? Write the necessary conditions for this event
? Bihar Board 2016Answer - 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.
Conditions - For critical angle light rays must be refracted from denser medium to rarer medium.
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