Physics

Wave Optics and Diffraction

148 Questions

Wave optics explores light phenomena like interference and diffraction. This hub features problems on Young's double slit experiment, Fresnel biprism, and single slit diffraction patterns. These topics regularly appear in physics sections of competitive exams.

Young's double slitFresnel biprismSingle slit diffractionConstructive interferenceFringe width calculations

Wave Optics and Diffraction Questions

Multiple choice physics superposition of waves coherence young's double slit experiment interference

In Young's double slit experiment, one slit is covered with red filter and another slit is covered by green filter, then interference pattern will be

  1. red

  2. green

  3. yellow

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

When the red and green filters are used, the rays from slits will have wavelengths of red and green light.  The condition of mono chromatic sources will not be met. Hence, the fringe pattern will disappear.

Multiple choice physics superposition of waves coherence young's double slit experiment interference

Two coherent sources of intensity ratio $\beta$ interfere. Then the value of $\displaystyle \left( {\frac{{{I _{\max }} - {I _{\min }}}}{{{I _{\max }} + {I _{\min }}}}} \right)$ is:

  1. $\dfrac{{1 + \beta }}{{\sqrt \beta }}$
  2. $\sqrt {\left( {\dfrac{{1 + \beta }}{\beta }} \right)} $
  3. $\dfrac{{1 + \beta }}{{2\sqrt \beta }}$
  4. $\dfrac{{2\sqrt \beta }}{{1 + \beta }}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$\displaystyle \frac{I _2}{I _1}=\beta$                        $\displaystyle \frac{I _{min}}{I _{max}}=\frac{\beta - 2 \sqrt{\beta}+1}{\beta + 2 \sqrt{\beta +1}}$
$\frac{A _2}{A _1} =\sqrt{\beta}$
$\displaystyle \frac{A _2 -A _1}{A _2 + A _1}=\frac{\sqrt{\beta}-1}{\sqrt{\beta}+1}$     $\displaystyle \frac{I _{max}- I _{min}}{I _{max}+ I _{min}}=\frac{(2)}{(2)} \left [ \frac{2 \sqrt{\beta}}{\beta +1}\right ]$

Multiple choice physics superposition of waves coherence young's double slit experiment interference

Interference fringes were produced in Young's double slit experiment using light of wavelength $5000\overset{o}{A}$. When a film of thickness $2.5\times 10^{-3}$cm was placed in front of one of the slits, the fringe pattern shifted by a distance equal to $20$ fringe-widths. The refractive index of the material of the film is?

  1. $1.25$
  2. $1.35$
  3. $1.4$
  4. $1.5$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

$n\lambda =\left( \mu -1 \right) t$\ $20\times 5\times  { 10 }^{ -7 }=\left( \mu -1 \right) 25\times { 10 }^{ -6 }$\ $\mu =1.4$

Multiple choice image formation by lens image formation by lenses ray optics and optical instruments optics physics

The distance between a point source of light and a screen which is $60$ $cm$ is increased to 180 cm. The intensity on the screen as compared with the original intensity will be : 

  1. $\cfrac { 1 } { 9 }$ times
  2. $\cfrac { 1 } {3 }$ time
  3. $3$ times
  4. $9$ times
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Intensity is inversely proportional to the square of the distance from a point source (I proportional to 1/r^2). The distance increases from 60 cm to 180 cm, a factor of 3. Thus, the intensity changes by a factor of 1/(3^2) = 1/9.

Multiple choice evs light, shadows and images what makes things visible nature and sources of light light travels in straight line

In a YDSE with two identical slits, when the upper slit is covered with a thin, perfectly transparent sheet of mica, the intensity at the centre of screen reduces to $75\%$ of the initial value. Second minima is observed to be above this point and third maxima below it. Which of the following can be a possible value of phase difference caused by the mica sheet.

  1. $\dfrac{\pi}{3}$
  2. $\dfrac{13\pi}{3}$
  3. $\dfrac{17\pi}{3}$
  4. $\dfrac{11\pi}{3}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Here when second order minica replaces bright spot change in path difference$3\pi $


Second order minica phase difference$\dfrac { \pi  }{ 2 } $

Phase difference lies between $3\pi\ to\ 4\pi $ i.e. $\dfrac{11\pi}{3}$

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

At the first minimum adjacent to the central maximum of a single slit diffraction plate the phase difference between the huygens wavelet from the edge of the slit and the wavelet from the midpoint of the slate

  1. $\pi/8$
  2. $\pi/4$
  3. $\pi/2$
  4. $\pi$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

At the first minimum, the path difference between the edges of the slit is lambda. The path difference between the edge and the midpoint is lambda/2, which corresponds to a phase difference of pi.

Multiple choice luminous intensity measurements physics

A lamp placed 60 cm from a screen produces the same illumination as a standard 100 W lamp placed 90 cm away on the other side of the screen. The luminous intensity of the first lamp is 

  1. $49.44W$
  2. $44.44W$
  3. $54.44W$
  4. $34.44W$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

For same illumination $\dfrac{Power} { r^{2}}$ will be same


=> $\dfrac {100} { 0.9^{2}} = \dfrac {P} {0.6^{2}}$
=> P = $44.44 W$

Multiple choice luminous intensity measurements physics

Two light sources of $8 Cd$ and $12 Cd$ are placed on the same side of the photometer screen at a distance of $40 cm$ from it. Where should a $80 Cd$ source be placed to balance the illuminance?

  1. $40 cm$
  2. $60 cm$
  3. $20 cm$
  4. $80 cm$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Intensity of illumination = $\dfrac {L} {d^{2}}$


Therefore
$I _1+I _2=I$

$\dfrac {8} {0.4^{2}} + \dfrac{12} {0.4^{2}} = \dfrac{80}{d^{2}}$

=>$\dfrac {20} {0.16} = \dfrac{80}{d^{2}}$

=> d = 0.8 m =80 cm

Answer. D) 80 cm

Multiple choice luminous intensity measurements physics

The luminous intensity of a light source is $300 Cd$. The illuminance of a surface lying at a distance of $10$ $m$ from it will be if light falls normally on it 

  1. $30$ lux
  2. $3$ lux
  3. $0.3$ lux
  4. $0.03$ lux
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Illuminance ($E$)  is  measured  in  lux.  The  lux  is  an  SI  unit  used  when  characterizing illumination conditions of a surface:
$E = \dfrac {I}{R^2}    lux$
where $I$ is the luminous intensity and $R$ is the distance.
Given $I = 300 cd$ and $R = 10 m$
$\implies E = \dfrac {300}{10^2} = 3   lux$

Multiple choice luminous intensity measurements physics

Two lamps of luminous intensity of $8$ $Cd$ and $32$ $Cd$ respectively are lying at a distance of $1.2$ $m$ from each other. Where should a screen be placed between two lamps such that its two faces are equally illuminated due to the two sources?

  1. $10$ $cm$ from $8$ $Cd$ lamp
  2. $10$ $cm$ from $32$ $Cd$ lamp
  3. $40$ $cm$ from $8$ $Cd$ lamp
  4. $40$ $cm$ from $32$ $Cd$ lamp
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let distance between 8 Cd lamp and screen be $ x  m$

Distance between 32 Cd lamp and screen $=$ $ (120-x) m$

Under equal illumination,
$ \dfrac{8}{x^{2}} = \dfrac{32}{(120-x)^{2}}$
=> x $=$ 40 cm

So the distance from 8 Cd lamp is 40 centimeters.

Multiple choice luminous intensity measurements physics

At what distance should a book be placed from a $50 Cd$ bulb so that the illuminance on the book becomes $2lm $ $m^{-2}$

  1. $1$m
  2. $5$m
  3. $10$m
  4. $50$m
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

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

used  when  characterizing illumination conditions of a surface:
$E = \dfrac {I}{R^2}    lux$
$\implies R = \sqrt {\dfrac {I}{E}}$
where $I$ is the luminous intensity and $R$ is the distance.
Given $I = 50 cd$ and $E = 2 lm  m^{-2}$
$\implies R = \sqrt {\dfrac {50}{2}} = 5   m$

Multiple choice luminous intensity measurements physics

The luminous intensity of a $100$ W unidirectional bulb is $100$ candela. The total luminous flux emitted from the bulb will be

  1. $100\pi$
  2. $200\pi$
  3. $300\pi$
  4. $400\pi$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

We have total luminous flux,
$L = 4 \pi I = 4 \pi \times 100 = 400 \pi$

Multiple choice luminous intensity measurements physics

The illuminance on screen distance $3$ m from a $100$ $W$ lamp is $25$ lm/$m^{2}$. Presuming normal incidence, the luminous intensity of the bulb will be 

  1. $100$ $Cd$
  2. $25$ $Cd$
  3. $225$ $Cd$

  4. none of these
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

We have illuminance
$I = \dfrac {\phi}{R^2}$
$\implies \phi = I  R^2 = 25 \times 3^2 = 225 \ Cd$