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Multiple choice physics wave optics huygens wave theory and wavefront wave propagation (huygens' construction) theories on light wave behaviour

Who proposed wave nature of light ?

  1. Huygen

  2. Young

  3. Fresnel

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

Huygens believed that light was made up of waves vibrating up and down perpendicular to the direction of  the light travels. This is known as 'Huygens' Principle'.  According to this theory, light waves are spherical, and these wave surfaces advance or spread out as they travel at the speed of light. This theory explains why light shining through a pin hole or slit will spread out rather than going in a straight line.

Multiple choice physics wave optics huygens wave theory and wavefront wave propagation (huygens' construction) theories on light wave behaviour

Mark the CORRECT statements(s)

  1. Direction of wave propagation is along the normal to wavefront.

  2. For a point source of light, the shape of wavefront can be considered to be plane at very large

    distance from the source.

  3. A point source of light is placed at the focus of a thin spherical lens, then the shape of the wavefront for emerged light may be plane.

  4. The shape of the wavefront for the light incident on a thin spherical lens (kept in vacuum) is plane, the shape of the wavefront corresponding to emergent light would be always spherical.
Reveal answer Fill a bubble to check yourself
A,B,C,D Correct answer
Explanation

According to convention, direction of wave propagation is along the normal to wavefront.

The wavefront of a point source of light is spherical. A sphere with large radius is like a plane at its surface. Thus for a point source of light, the shape of wavefront can be considered to be plane at very large distance from the source.
Beam of light from a point source emerges parallel to principle axis when kept at focus of a thin spherical lens. Thus the wavefront emergent from the lens is planar.

Multiple choice physics wave optics huygens wave theory and wavefront wave propagation (huygens' construction) theories on light wave behaviour

A: The corpuscular theory fails in explaining the velocities of light in air and water.
B: According to corpuscular theory, the light should travel faster in a 'denser medium than in a rarer medium.

  1. If both A and B are true but the B is the correct explanation of A

  2. If both A and B are true but the B is not the correct explanation of A

  3. If A is true but B is false

  4. If both the A and B are false

  5. If B is true but A is false
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Corpuscular theory fails to explain the velocity of light in air and water because it predicted light to have more velocity in denser medium where as the fact is just the opposite. So option (a) is correct.

Multiple choice physics electric current, potential difference and resistance drift velocity and mobility drift speed drift velocity & mobility

A charged particle having drift velocity of $7.5\times 10^{-4}m$ $s^{-1}$ in electric field of $3\times 10^{-10}$V $m^{-1}$, mobility is?

  1. $6.5\times 10^6m^2V^{-1}s^{-1}$
  2. $2.5\times 10^6m^2V^{-1}s^{-1}$
  3. $2.5\times 10^4m^2V^{-1}s^{-1}$
  4. $6.5\times 10^4m^2V^{-1}s^{-1}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Given:
$V _d=7.5 \times 10^{-4}$
$E=3 \times 10^{-10}$
Mobility of charged particle
$\mu =\dfrac{|v _d|}{E}=\dfrac{7.5\times 10^{-4}}{3\times 10^{-10}}=2.5\times 10^6m^2V^{-1}s^{-1}$.

Multiple choice physics electric current, potential difference and resistance drift velocity and mobility drift speed drift velocity & mobility

Which of the following characteristics of electrons determines the current in a conductor?

  1. Drift velocity alone

  2. Thermal velocity alone

  3. Both drift velocity and thermal velocity

  4. Neither drift nor thermal velocity
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A. Drift velocity only.


The current in a conductor,

$I= neA V _{d}$
    where, $n=$ no. of free charge density
                $A=$ cross-sectional area of conductor
                 $V _{d}=$ Drift velocity
From the above, Drift velocity is only responsible for the current in a conductor.

Multiple choice physics electric current, potential difference and resistance drift velocity and mobility drift speed drift velocity & mobility

When current flows through a conductor, then the order of drift velocity of electrons will be:-

  1. $10^{10} cms^{-1}$
  2. $10^{-2} cms^{-1}$
  3. $10^{4} cms^{-1}$
  4. $10^{-1} cms^{-1}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The drift velocity of electrons in a conductor is of the order of $10^{−4} m/s. It is very small compared to the thermal speed which is of the order of 105 m/s.

The answer is $10^{-2}$

Multiple choice physics electric current, potential difference and resistance drift velocity and mobility drift speed drift velocity & mobility

Potential difference of $100 V$ is applied to the ends of a copper wire one metre long. Find the ratio of average drift velocity and thermal velocity of electrons at $27^\circ C$. (Consider there is one conduction electron per atom. The density of copper is $9.0 \times 10^3$; Atomic mass of copper is $63.5 g$.
$N _A = 6.0 \times 10^{23}$ per gram-mole, conductivity of copper is $5.81 \times 10^{7} \Omega^{-1}$.$K =1.38 \times 10^{-23} JK^{-1}$).

  1. $3.67 \times 10^{-6}$
  2. $4.3 \times 10^{-6}$
  3. $6 \times 10^{-5}$
  4. $5.6 \times 10^{-6}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Drift velocity is given by v_d = (eE tau) / m, and thermal velocity is v_th = sqrt(3kT / m). Using the provided conductivity and density, the ratio is calculated as approximately 3.67 x 10^-6.

Multiple choice physics electric current, potential difference and resistance drift velocity and mobility drift speed drift velocity & mobility

Assertion: When a straight wire is bent to form L-shape, its resistance increases.
Reason: Electrons take longer time to travel along a bent wire, as compared to travel along a straight wire.

  1. A

  2. B

  3. C

  4. D
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Resistance is a property of the material and its geometry (length and area). Bending a wire does not change its length or cross-sectional area, so the resistance remains constant. The reason provided is also physically incorrect as drift velocity is determined by the electric field and scattering, not the path geometry.

Multiple choice physics electric current, potential difference and resistance drift velocity and mobility drift speed drift velocity & mobility

When the current in a wire is 1A, the drift velocity is $1.2\times 10^{-4}ms^{-1}$. The drift velocity when current becomes 5 A is

  1. $1.2\times 10^{-4}ms^{-1}$
  2. $3.6\times 10^{-4}ms^{-1}$
  3. $6\times 10^{-4}ms^{-1}$
  4. $4.8\times 10^{-4}ms^{-1}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Given
Initial current through the wire is $I = 1A$
Initial drift velocity is, $v _d = 1.2 \times 10^{-4} ms^{-1}$
Increased current is, $I' = 5A$
The current, I through the wire is given by
$I = \mu _e.e.A.v _d$
where, $\mu _e$ is the free electron density, e is the charge on electron, A is the area of cross section of the wire and v_d is the drift velocity of electrons. 
Since,  the free electron density, the charge on electron and the area of cross section of the wire are constant, hence
$I \propto v _d$.................(1)
Now, current through the wire is increased to 5 A, if the new drift velocity of electrons is $v' _d$ then
$I' \propto v' _d$................(2)


From (1) and (2), we can write

$\dfrac{v' _d}{v _d} = \dfrac{I'}{I}$

$v' _d = \dfrac{I'}{I} v _d$

$v' _d = \dfrac{5}{1} 1.2 \times 10^{-4}$ 

$v' _d = 6 \times 10^{-4} ms^{-1}$