Tag: physics

Quizzes (598) Questions (13441)

Questions Related to physics

Multiple choice luminous intensity measurements physics

Light from a source is analysed by an analyser. When the analyser is rotated, the intensity of the emergent light.

  1. Does not vary

  2. Remains uniformly dark

  3. Varies between maximum and zero

  4. Varies between maximum and minimum
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Let the intensity of the unpolarised light be $I _o$.
An analyser is used to polarized the light in one direction.
Intensity of light after passing through the analyser  $I' = \dfrac{I _o}{2}$
Intensity of light after passing through analyser remains constant $I _o/2$ even if the analyser is rotated by some angle.

Multiple choice luminous intensity measurements physics

 If light falls on the surface at an angle of $60^{0}$, then illuminance will be____

  1. $12$ lux
  2. $6$ lux
  3. $3$ lux
  4. $1.5$ lux
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Illuminance ($E$)  is  measured  in  lux.  The  lux  is  an  SI  unit 

used  when  characterizing illumination conditions of a surface:
$E = \frac {I}{R^2}  cos \alpha     lux$
where $I$ is the luminous intensity, $R$ is the distance and $\alpha$ is the surface angle.
Given $I = 300 cd$, $R = 10 m$, $\alpha = 60^o$
$\implies E = \frac {300}{10^2}    cos  60^o= 1.5   lux$

Multiple choice luminous intensity measurements physics

A lamp is hanging along the axis of a circular table of radius r. At what height should the lamp be placed above the table, so that the illuminance at the edge of the table is $\displaystyle \frac{1}{8}$ of that at its centre?

  1. r/2
  2. r/$\sqrt{2}$

  3. r/3
  4. r/$\sqrt{3}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$E _2 = \displaystyle \frac{1}{8} E _1$ or $\displaystyle \frac{1}{(r^2 + h^2)} \times \frac{h}{\sqrt{r^2 + h^2}} = \frac{1}{8} \frac{1}{h^2}$
(by lambert's cosine law)
or, $(r^2 + h^2)^{3/2} = (2h)^3 $ or $(r^2 + h^2)^{1/2} = 2h$
or $r^2 + h^2 = 4h^2$
$h = r / \sqrt{3}$

Multiple choice physics sound sound as a wave of disturbance vibrations in a tuning fork vibrations in tuning fork

The frequency of a man's voice is $300\space Hz$. If the velocity of sound waves is $336\space ms^{-1}$, the wavelength of the sound is

  1. $1.12\space m$
  2. $300\times336\space m$
  3. $330/336\space m$

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

We are given, frequency $f=300 Hz$, and velocity $v=336 ms^{-1}$,

From the relation $v=f \times \lambda$,

$\lambda = v/f =336/300=1.12 m$

Option "A" is correct.

Multiple choice physics sound sound as a wave of disturbance vibrations in a tuning fork vibrations in tuning fork

 769Hz longitudinal wave in air has a speed of 344m/s. At a particular instant, what is the phase difference (in degrees) between two points 5.0 cm apart?

  1. 30

  2. 40

  3. 45

  4. 60
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Given that,

The frequency of wave is $\nu = 769 Hz$

The speed of wave is $v = 344 ms^{-1}$

The wavelength of wave is given by

$\lambda = \dfrac{v}{\nu}$

$\lambda = \dfrac{344}{769}$

$\lambda = 0.447 m = 44.7 cm$

Hence, the phase difference between two points 5.0 cm apart is

$\dfrac{5}{44.7} \times 360^\circ = 0.1118 \times 360^\circ = 40.24^\circ \approx40^\circ$

Multiple choice physics sound sound as a wave of disturbance vibrations in a tuning fork vibrations in tuning fork

A rod $70\space cm$ long is clamped from middle. The velocity of sound in the material of the rod is $3500\space ms^{-1}$. The frequency of fundamental note produced by it is :

  1. $3500\space Hz$
  2. $2500\space Hz$
  3. $1250\space Hz$
  4. $700\space Hz$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

SInce rod is clamped at middle, therefore only nodes can form at that point, and at free end only antinode can form, therefore for fundamental mode of frequency,

$\lambda/4=L/2$

$\lambda=2L=2\times 70 cm=1.4 m$

$v=3500\  ms^{-1}$ is given, 

We know, $v=f \times \lambda$ or $f = \dfrac{v}{\lambda}=3500/1.4=2500 Hz$

Option "B" is correct.

Multiple choice physics sound sound as a wave of disturbance vibrations in a tuning fork vibrations in tuning fork

A sonometer wire, $100\ \text{cm}$ in length has a fundamental frequency of $330\ \text{Hz}$. The velocity of propagation of transverse waves along this wire is :

  1. $330\ \text{ms}^{-1}$
  2. $660\ \text{ms}^{-1}$
  3. $115\ \text{ms}^{-1}$
  4. $990\ \text{ms}^{-1}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

For the fundamental frequency $(f _0=330 Hz),\ \lambda=2 L=200\ \text{cm} = 2\ \text{m}$


then from the formula, $v=f _0 \times \lambda= 330 \times 2.0= 660 \text{ms}^{-1}$

Multiple choice physics sound sound as a wave of disturbance vibrations in a tuning fork vibrations in tuning fork

If the frequency of a sound wave is increased by 25%, then the change in its wavelength will be

  1. 25% decrease

  2. 20% decrease

  3. 20% increase

  4. 25% increase
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
The frequency  of wave is given by

$\nu = \dfrac{v}{\lambda}$

$\lambda = \dfrac{v}{\nu}$

When the frequency of a sound wave is increased by 25%, then the new 
wavelength is

$\lambda' = \dfrac{v}{\nu+\dfrac{25}{100}\nu}$

$\lambda' = \dfrac{v}{\nu+\dfrac{1}{4}\nu}$

$\lambda' = \dfrac{v}{\dfrac{5}{4}\nu}$

$\lambda' = \dfrac{4v}{5\nu}$

Hence, the percent change in wavelength is

$\dfrac{\lambda - \lambda'}{\lambda} \times 100 = \dfrac{\dfrac{v}{\nu} - \dfrac{4v}{5\nu}}{\dfrac{v}{\nu}} \times 100$

$\Rightarrow -\dfrac{1}{4} \times 100 = -20$%.

Hence, wavelength decreases by 20%