CBSE 12th Standard Physics Subject Wave Optics HOT Questions 5 Mark Questions 2021
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CBSE 12th Standard Physics Subject Wave Optics HOT Questions 5 Mark Questions 2021
12th Standard CBSE
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Reg.No. :
Physics
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The critical angle between a given transparent medium and air is denoted by C. A ray of light in air enters this transparent medium at an angle of incidence equal to polarizing angle p. Deduce a relation for the angle of refraction in terms of C.
(a) -
Suppose while sitting in a parked car, you notice a jogger approaching towards you in the rear view mirror having R = 2m. If the Jogger is running at a speed of 5m/s, how fast is the image of Jogger moving. When the Jogger is (i) 39m (ii) 29m (iii) 19 m and (iv) 9 m away?
(a) -
A teacher has given three lenses of power 0.5D, 4D and 10D to a studet. He is not sure as to which lens should he use for constructing a good astronomical telescope. So, he consults his seniors and the teacher, and then constructs a telescope. Later, he shows this telescope to the junior classes and ex[plains about the choice of lenses.
Read the above passage and give the answer of the following questions:
(i) What values has he shown by doing these?
(ii) Which lenses are used as objective and which one as eyepiece?(a) -
(i) Obtain the conditions for the bright and dark fringes in diffraction pattern due to a single narrow slit illuminated by a monochromatic source. Explain clearly, why the secondary maxima go on becoming weaker with increasing of their order?
(ii) When the width of the slit is made double, how would this affect the size and intensity of the central diffraction band? Justify your answer.(a) -
(i) In a double slit experiment using light of wavelength 600 nm, the angular width of the fringe formed on a distant screen is \(0.1^o\) . Find the spacing between the two slits.
(ii) Light of wavelength 5000\(\dot { A } \) propagating in air gets partly reflected from the surface of water. How will the wavelengths and frequencies of the reflected and refracted light be affected?(a)
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CBSE 12th Standard Physics Subject Wave Optics HOT Questions 5 Mark Questions 2021 Answer Keys
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According to Brewster's law,
\(\mu =tanp....(i)\)
\(Also \ \mu =\frac { 1 }{ sin\quad C } .....(ii)\)
\( From \ (i) \ and \ (ii), \ tanp=\frac { 1 }{ sin \ C } \)
\(If \ r \ is \ angle \ of \ refraction, \ then \ p=90°-r\)
\(or \ tan \ r=sin \ C \ r={ tan }^{ -1 }(sin \ C)\) -
Here R = 2m,
\(f=\frac { R }{ 2 } =\frac { 2 }{ 2 } =1m\)
Using morror formula, we have
\(\frac { 1 }{ f } =\frac { 1 }{ v } +\frac { 1 }{ u } \Rightarrow \frac { 1 }{ v } =\frac { 1 }{ f } -\frac { 1 }{ u }\)
\( \Rightarrow \frac { 1 }{ v } =\frac { u-f }{ fu } \Rightarrow v=\frac { fu }{ u-f } \)
When Jogger is 39 m away, then u = -39m
\(\Rightarrow \) we get
\(v=\frac { fu }{ u-f } =\frac { 1\left( -39 \right) }{ -39-1 } or \ v=\frac { 39 }{ 40 } m\)
As, the Jogger is running at a constant speed of 5 m/s, after 1 s, the positionof the image(v) for
u = -39 + 5
u = -34m
Again using the Equation we get
\(\Rightarrow v=\frac { fu }{ u-f } =\frac { 1\left( -34 \right) }{ -34-1 } or\quad v=\frac { 34 }{ 35 } m\)
Difference in apparent position of Jogger in 1s
\(=\frac { 39 }{ 40 } -\frac { 34 }{ 35 } =\frac { 1365-1360 }{ 1400 } =\frac { 1 }{ 280 } m\)
Average speed of Joggers image \(=\frac { 1 }{ 280 } m/s\)
Similarly, for u = -29m, -19m and -9 m, average speed of Jogger image is \(\frac { 1 }{ 150 } m/s\),\(\frac { 1 }{ 60 } m/s\),\(=\frac { 1 }{ 10 } m/s\) respectively
The speed increases as the Jogger approaches the car>
This can be experienced by the person in the car. -
(i) The values shown by him are as follows:
(a) Consulting others in case of need
(b) Curosity
(c) Sharing knowledge.
(ii) From these three lenses, he will use a lens of power 0.5D for objective and lens of power 10D for eyepiece. -
(ii) As, the number of point sources increases, their contribution towards intensity also increases. Intensity varies as square of the slit width. Thus, when the width of the slit is made double the original width, intensity will get four times of its original value.
Width of central maximum is given by \(\beta =\frac { 2D\lambda }{ b } \)
Where, D = distance between screen and slit,
\(\lambda \) = wavelength of the light,b = size of slit.
So, with the increase in size of slit, the width of central maxima decreases. Hence, double the size of the slit would result as half the width of the central maxima. -
(i) Here,
\(\lambda =600 \ nm=600\times { 10 }^{ -9 }m=6\times { 10 }^{ -7 }m\)
\(\theta ={ 0.1 }^{ \circ }=\frac { 0.1\pi }{ 180 } rad,d=?\)
From angular width,\(\theta =\frac { \lambda }{ d } \)
\(\Rightarrow \ d=\frac { \lambda }{ \theta } =\frac { 6\times { 10 }^{ -7 } }{ \frac { \pi }{ 180 } \times 0.1 } =3.44\times { 10 }^{ -4 }m\)
(ii) The frequency and wavelength of reflected wave will not change. The refracted wave will have same frequency. The velocity of light in water is given by \(v=f\lambda \)
where, v = velocity of light
f = frequency of light
\( lambda =wavelength \ of \ light\)
If velocity will decrease, then wavelength (\(\lambda \)) will also decrease.