Tag: the nature of light

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Questions Related to the nature of light

Multiple choice physics option a: relativity the nature of light speed of light and optical density introduction to light

If $v _s$ , $v _x$ and $v _m$ are the velocities of soft gamma rays, X-rays and Microwaves respectively in vacuum, then

  1. $v _s < v _x < v _m$

  2. $v _s = v _x = v _m$

  3. $v _s > v _x > v _m$

  4. $v _x < v _s < v _m$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Soft gamma rays, $x-$rays and microwaves are namely eletromagnetic having different wavelengths. 

But, all of them propagate through space with the same speed,
$e=3\times 10^{8}\ m/s$
 so,
$v _{s}=v _{x}=v _{m}$

Multiple choice physics option a: relativity the nature of light speed of light and optical density introduction to light

If velocity of an electromagnetic wave in a medium is $3\times 10^8m/s$ then find refractive index of medium.

  1. $1$

  2. $2$

  3. $0.5$

  4. $0.25$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

We know that 
$\mu=\dfrac{V _{n}}{V _{m}}$

When $\mu=$ refractive index of wave
$V _{n}=$ velocity of wave in vacuum
$V _{m}=$ velocity of wave in medium
Now, velocity of wave in vacuum $=3\times 10^{8}m/s$
$\Rightarrow V _{n}=3\times 10^{8}m/s$
velocity of wave in medium $=3\times 10^{8}m/s$ (given)
$\Rightarrow V _{m}=3\times 10^{8}m/s$
Hence, $\mu=\dfrac{3\times 10^{8}}{3\times 10^{8}}=1$
$\Rightarrow \mu=1 =$ option $A$


Multiple choice physics option a: relativity the nature of light speed of light and optical density introduction to light

Three observers $A,B$ and $C$ measure the speed of light coming from a source as $v _A,v _B$ and $v _C$. Observer $A$ moves towards the source, $C$ moves away from source and $B$ stays stationary. The surrounding medium is water. If A and C are moving at the same speed, then

  1. $\displaystyle v _A>v _B>v _C$

  2. $\displaystyle v _A < v _B < v _C $

  3. $\displaystyle v _A=v _B=v _C$

  4. $\displaystyle v _B=\frac{1}{2}(v _C+v _A)$
Reveal answer Fill a bubble to check yourself
A,D Correct answer
Explanation

Speed of the light is constant with respect to any observer holds only for the vacuum. In different medium (say water) if observer is moving with respect to medium towards the source, the speed observed by the observer will be more compared to a stationary observer. 
Same thing holds for a observer moving away from the source. Hence Option A. 
Since they are moving at the same speed, change in speed will be same and hence Option D. 
So answer is A and D.

Multiple choice physics option a: relativity the nature of light speed of light and optical density introduction to light

Three observers $A,B$ and $C$ measure the speed of light coming from a source to be $v _A,v _B$ and $v _C$. The observer $A$ moves towards the source and $C$ moves away from the source at the same speed. The observer $B$ stays stationary. The surrounding space is vacuum everywhere.

  1. $\displaystyle v _A>v _B>v _C$

  2. $\displaystyle v _A < v _B < v _C$

  3. $\displaystyle v _A=v _B=v _C$

  4. $\displaystyle v _B=\frac{1}{2}(v _A+v _C)$
Reveal answer Fill a bubble to check yourself
C,D Correct answer
Explanation

Relativity states that the speed of light in vacuum will be same independent of the frame of reference. The frame of reference can be stationary or moving.
So all  three will find the speed of light to be $ c $
Also $ c=\dfrac{1}{2}(c+c) $
So, (C) and (D) are correct.

Multiple choice physics option a: relativity the nature of light speed of light and optical density introduction to light

Which of the following properties of light conclusively support wave theory of light?

  1. Light obeys laws of reflection

  2. Speed of light in water is smaller than the speed in vacuum

  3. Light doesn't show interferrence

  4. Light shows photoelectric effect
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
Laws of reflection was successfully explained by both wave theory and corpuscular theory of light.
Only Speed of light in water is smaller than the speed in vacuum was was proven by wave theory of light.
Multiple choice physics option a: relativity the nature of light speed of light and optical density introduction to light

An electromagnetic wave of frequency $v=3.0:MHz$ passes from vacuum into a dielectric medium with permittivity $\epsilon=4.0$. Then

  1. wavelength is halved and frequency remains unchanged.

  2. wavelength is doubled and frequency becomes half.

  3. wavelength is doubled and frequency remains unchanged.

  4. wavelength and frequency both remain unchanged.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Frequency remains constant during refraction.

$\displaystyle v _{med}=\dfrac{1}{\sqrt{\mu _0\epsilon _0\times4}}=\dfrac{c}{2}$

$\displaystyle\dfrac{\lambda _{med}}{\lambda _{air}}=\dfrac{v _{med}}{v _{air}}=\dfrac{\displaystyle\dfrac{c}{2}}{c}=\dfrac{1}{2}$

$\therefore$ wavelength is halved and frequency remains unchanged.

Multiple choice physics option a: relativity the nature of light speed of light and optical density introduction to light

The electromagnetic waves

  1. travel with the the speed of sound.

  2. travel with the the same speed in all media.

  3. travel in free space with the speed of light.

  4. do not travel through a medium.
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The electromagnetic waves of all wavelengths travel with the same speed in space which is equal to velocity of light.

Multiple choice physics option a: relativity the nature of light speed of light and optical density introduction to light

The apparent wavelength of light from a star moving away from the earth is 0.02% more than actual wavelength. What is the velocity of the star.

  1. $\displaystyle 30{ kms }^{ -1 }$

  2. $\displaystyle 60{ kms }^{ -1 }$

  3. $\displaystyle 90{ kms }^{ -1 }$

  4. None of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Answer is B.

The apparent change in the wavelength is given as follows.
$\displaystyle \frac { \Delta \lambda  }{ \lambda  } =\frac { v }{ C } ,Hence\quad V=\frac { \Delta \lambda  }{ \lambda  } C$
$\displaystyle =\frac { 0.02 }{ 100 } \times 3\times { 10 }^{ 8 }{ ms }^{ -1 }=60{ kms }^{ -1 }$.
Hence, the velocity of the star is 60 km/s.

Multiple choice physics option a: relativity the nature of light speed of light and optical density introduction to light

All colours travel with the same speed in :

  1. Vaccum

  2. Glass 

  3. Water

  4. All of the above
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

In a vacuum, the refractive index is 1 for all frequencies, so all colors (wavelengths) travel at the same speed c. In media like glass or water, dispersion occurs, meaning speed depends on the refractive index, which varies with wavelength.

Multiple choice physics option a: relativity the nature of light speed of light and optical density introduction to light

An electromagnetic wave of frequency $\mathrm{v}=3.0$ MHz passes from vacuum into a dielectric medium with permittivity $\epsilon =4.0$. Then : 

  1. wavelength is doubled and frequency remains unchanged

  2. wavelength is doubled and frequency becomes half

  3. wavelength is halved and frequency remains unchanged

  4. wavelength and frequency both remain unchanged
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

$f=3.0mHz$

$E=4.0$

$C=fZ$

C is the speed of light into the medium

${ C } _{ 0 }=f _{ i }{ Z } _{ i }  C=\dfrac { { C } _{ 0 } }{ n } \\ C={ f } _{ f }{ Z } _{ f }  n=\sqrt { \dfrac { \varepsilon }{ { \varepsilon } _{ 0 } } } \\ C=\dfrac { { C } _{ 0 } }{ 2 } $

Now since frequency depends on source

Hence ${ f } _{ f }={ f } _{ i }\\ { Z } _{ f }=\dfrac { { Z } _{ i } }{ 2 } $

Speed and wavelength will be halved