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Published on: 01/06/2021
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Questions + Answers key
Take MCQ Physics Test1.
The wavelength of light from sodium source in vacuum is 5893Å. What are its
(a) wavelength,
(b) speed and
(c) frequency when this light travels in water which has a refractive index of 1.33.
2.
Find the dispersive power of a prism if the refractive indices of flint glass for red, green and violet colours are 1.613, 1.620 and 1.632 respectively.
3.
The angle of minimum deviation for an equilateral prism is 37o . Find the refractive index of the material of the prism.
4.
A monochromatic light is incident on an equilateral prism at an angle 30o and is emergent at an angle of 75o . What is the angle of deviation produced by the prism?
5.
Light ray falls at normal incidence on the first face and emerges gracing the second face for an equilateral prism.
(a) What is the angle of deviation produced?
(b) What is the refractive index of the material of the prism?
1.
The refractive index of vacuum, n1 = 1
The wavelength in vacuum, λ1 = 5893 Å.
The speed in vacuum, c = v1 = 3 x 108 m s–1
The refractive index of water, n2 = 1.33
The wavelength of light in water, λ2
The speed of light in water, v2
(a) The equation relating the wavelength and refractive index is,
\(\cfrac { { \lambda }_{ 1 } }{ \lambda _{ 2 } } =\cfrac { { n }_{ 2 } }{ { n }_{ 1 } } \)
Rewriting, \({ \lambda }_{ 2 }=\cfrac { { n }_{ 1 } }{ { n }_{ 2 } } \times { \lambda }_{ 1 }\)
Substituting the values,
\({ \lambda }_{ 2 }=\cfrac { 1 }{ 1.33 } \times 5893\overset { o }{ A } =4431\overset { o }{ A } \)
\({ \lambda }_{ 2 }=4431\overset { o }{ A } \)
(b) The equation relating the speed and refractive index is,
\(\cfrac { { v }_{ 1 } }{ { v }_{ 2 } } =\cfrac { { n }_{ 2 } }{ { n }_{ 1 } } \)
Rewriting, \({ v }_{ 2 }=\cfrac { { n }_{ 1 } }{ { n }_{ 2 } } \times { v }_{ 1 }\)
Substituting the values,
\({ v }_{ 2 }=\cfrac { 1 }{ 1.33 } \times 3\times { 10 }^{ 8 }=2.256\times { 10 }^{ 8 }\)
v2 = 2.256 x 108 ms-1
(c) Frequency of light in vacuum is,
\({ v }_{ 1 }=\cfrac { c }{ { \lambda }_{ 1 } } \)
Substituting the values,
\({ v }_{ 1 }=\cfrac { 3\times { 10 }^{ 8 } }{ 5893\times { 10 }^{ -10 } } =5.091\times { 10 }^{ 14 }Hz\)
Frequency of light in water is, \({ v }_{ 2 }=\cfrac { v }{ { \lambda }_{ 2 } } \)
Substituting the values,
\(\\ { v }_{ 2 }=\cfrac { 2.256\times { 10 }^{ 8 }{ ms }^{ -1 } }{ 4431\times { 10 }^{ -10 } } =5.091\times { 10 }^{ 14 }Hz\)
The results show that the frequency remains same in all media.
2.
Given, nv = 1.632; nR = 1.613; nG = 1.620
Equation for dispersive power is,
\(\omega =\cfrac { \left( { n }_{ v }-{ n }_{ g } \right) }{ \left( { n }_{ G }-1 \right) } \)
Substituting the values,
\(\omega =\cfrac { 1.632-1.613 }{ 1.620-1 } =\cfrac { 0.019 }{ 0.620 } =0.0306\)
The dispersive power of the prism is,\(\\ \omega =0.0306\)
3.
Given, A = 60°; D = 37°
Equation for refractive index is,
\(n=\cfrac { \sin\left( \frac { A+D }{ 2 } \right) }{ \sin\left( \frac { A }{ 2 } \right) } \)
Substituting the values,
\(n=\cfrac { \sin\left( \frac { 60^{ o }+37^{ o } }{ 2 } \right) }{ \sin\left( \cfrac { { 60 }^{ o } }{ 2 } \right) } =\cfrac { \sin\left( 48.5^{ o } \right) }{ \sin\left( { 30 }^{ o } \right) } =1.5\)
The refractive index of the material of the prism is, n = 1.5
4.
Since, the prism is equilateral, A = 60o;
Given, i1 = 30o;i2 = 75o
Equation for angle of deviation, d = i1 + i2 – A
Substituting the values, d = 30°+ 75°– 60°= 45°
The angle of deviation produced d = 45o
5.
The given situation is shown in the figure
Given, A = 60o ; i1 = 0o; i2 = 90o
(a) Equation for angle of deviation,
d = i1 + i2 - A
Substituting the values
d = 0o + 90o- 60o = 30o
The angle of deviation produced is, d = 30°
(b) The light inside the prism must be falling on the second face at critical angle as it graces the boundary. ic = 90° – 30° = 60°
Equation for critical angle is, \(\sin i_{c}=\frac{1}{n}\)
\(n=\frac{1}{\sin i_{c}} ; \quad n=\frac{1}{\sin 60^{\circ}}=\frac{1}{\sqrt{3} / 2}=\frac{2}{\sqrt{3}}=1.15\)
The refractive index of the material of the prism is, n = 1.15
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