# CBSE Class 12 Physics Chapter 10 - Wave Optics Important Questions

## Very Short Answer Type Questions

Question 1 Define the term wavefront?

Question 2 Give the relation between path difference and wavelength for constructive interference between two waves.

Question 3 State two conditions to obtain sustained interference of light

Question 4 If the separation between the two slits is decreased in Young's double-slit experiment keeping the screen position fixed. What will happen to the fringe width?

Question 5 Why are coherent sources necessary to produce a sustained interference pattern?

Question 6 Sunglasses are made of Polaroid and not colored glasses. Why?

Question 7 Why does the intensity of a secondary maximum become less as compared to the central maximum?

Question 8 Will ultrasonic big show any polarization? Give a reason for your answer.

Question 9 Why longitudinal waves cannot be polarized?

Question 10 Is there any difference between the colors emerging from a prism and the colors of a soap film seen in sunlight?

Question 1 How is a wavefront different from a ray? Draw the geometrical shape of the wavefront when
(a) light diverges from a point source and
(b) light emerges out of convex lens when a point source placed at its focus

Question 2 Two narrow slits are illuminated by a single monochromatic source. Name the pattern obtained on the screen. One of the slits is now completely covered what is the name of the pattern now obtained on the screen. Draw intensity pattern obtained in the two cases. Also, write two differences between the pattern obtained in the above two cases.

Question 3 What two main changes in the diffraction pattern of the single slit will you observe when the monochromatic source of light is replaced by a source of white light.

Question 4 When a sheet of transparent plastic is placed between two crossed polarizers, no light is transmitted. When the sheet is stretched in one direction, some light passes through the crossed polarizers. What is happening?

Question 5 Show that when a Ray of light is incident on the surface of a transparent medium at the polarizing angle, the reflected and transmitted rays are perpendicular to each other.

Question 6 Define critical angle and polarizing angle. What is the relation between the two angles.

## Multiple Choice Questions

Question 1
To demonstrate the phenomenon of interference, we require two sources, which emit radiation of
(a) nearly the same frequency
(b) the same frequency
(c) different wavelength
(d) the same frequency and have a definite phase relationship

Question 2
The maximum number of possible interference maxima for slit separation equal to twice the wavelength in Young’s double-slit experiment is
(a) infinite
(b) five
(c) three
(d) zero

Question 3
When an unpolarized light intensity $I_0$ is incident on a polarizing sheet, the intensity of the light which does not get transmitted is
(a) $\frac{I_0}{2}$
(b) $\frac{I_0}{4}$
(c) zero
(d) $I_0$

Question 4
Select the right option in the following
(a) Christian Huygens, a contemporary of Newton's established the wave theory of light by assuming that light waves are transverse.
(b) Maxwell provided the theoretical evidence that light is a transverse wave.
(c) Thomas Young experimentally proved the wave behavior of light and Huygens assumption.
(d) All the statements given above correctly answer the question whet is light.

Question 5
In a double-slit experiment, the distance between slits is increased ten times whereas their distance from the screen is halved. What is the fringe width?
(a) It remains the same
(b) it becomes 1/10
(c) it becomes 1/20
(d) it becomes 1/90

Question 6
The colors seen in the reflected white light from a thin oil film are due to
(a) diffraction
(b) interference
(c) polarization
(d) dispersion

Question 7 In diffraction from a single slit, the angular width of the central maxima does not depend on
(a) $\lambda$ of light used
(b) width of slit
(c) the distance of slit from screen
(d) ratio of $\lambda$ and slit width.

It does not depend on the distance of the slit from the screen.
Question 8 Which of the following can measure the position of particle most accurately?
(a) polarized light
(b) light with high wavelength
(c) light with low wavelength
(d) none of the above.

Question 9
We can obtain polarized light with the help of which of the following instrument?
(a) Nicol Prism
(b) bi prism
(c) Polarimeter
(d) none of the above

Question 10 If in Young's double-slit experiment of light interference is performed in water, which of the following is correct?
(a) fringe width will decrease
(b) fringe width will increase
(c) there will be no fringe
(d) fringe width will remain unchanged

Question 11 Interference occurs in which of the following waves
(a) longitudinal
(b) transverse
(c) electromagnetic
(d) all of these

Question 12 A monochromatic beam of light is used for the formation of fringes on the screen by illuminating the two sits in Young's double-slit experiment. When a thin film of mica is interposed in the path of one of the interfering beams, then
(a) the fringe width increases
(b) the fringe width decreases
(c) the fringe width remains same but the pattern shifts
(d) the fringe pattern disappears

Question 13 When exposed to sunlight, thin films of oil on water often exhibit brilliant colors due to the phenomenon of
(a) interference
(b) diffraction
(c) dispersion
(d) polarization

Question 14 The ratio of intensity of two waves is given by 4:1. Then ratio of amplitudes of two waves is
(a) 2:1
(b) 1:2
(c) 4:1
(d) 1:4

Question 15 In a Fresnel bi-prism experiment, the two positions of lense gives separation between the slits as 16 cm and 9 cm respectively. What is the actual distance of separation.
(a) 12.5 cm
(b) 12 cm
(c) 13 cm
(d) 14 cm

Question 16 Which of the following phenomenon is not common to sound and light waves?
(a) interference
(b) diffraction
(c) coherence
(d) polarization

Question 17 Which of the following statements is true?
(a) both light and sound waves can travel in vacuum
(b) both light and sound waves in air are transverse
(c) the sound waves in air are longitudinal, while light waves are transverse
(d) both light and sound waves in air are longitudinal

Question 18 Laser beam is coherent because it contains
(a) waves of several wavelength
(b) incoherent waves of single wavelength
(c) coherent waves of several wavelengths
(d) coherent waves of single wavelength

Question 19 Resolving power of a telescope can be increased by increasing
(a) the wavelength
(b) the diameter of objective
(c) the diameter of eyepiece
(d) the focal length of eyepiece

Question 20 A diffraction pattern is obtained by using a beam of red light. What will happen if red light is replaced by blue light?
(a) bands disappear
(b) bands become broader and farther apart
(c) no change will take place
(d) diffraction bands become narrower and crowded together.

Question 1
(a) Use Huygens's geometrical construction to show how a plane wavefront at $t=0$ propagates and produces a wavefront at a later time.
(b) Verify using Huygens's principal Snell's law of refraction of a plane wave propagating from a denser to a rarer medium.
(c) Illustrate with the help of diagrams the action of a convex lens and concave mirror on a plane wavefront incident on it
Question 2
(i) In a Young's double-slit experiment, deduce the conditions for constructive and destructive interference. Write the expression for the distance between two consecutive bright or dark fringes.
(ii) What changes in the interference pattern do you observe if the two slits $S_1$ and $S_2$ are taken as point sources?
(iii) Plot a graph of the intensity distribution vs. path difference in this experiment. Compare this with the intensity distribution of fringes due to diffraction at a single slit. What important difference do you observe?
Question 3
(i) State Huygens's principle. Using this principle explain, how a diffraction pattern is obtained on a screen due to a narrow slit on which and narrow beam coming from a monochromatic source of light is incident normally.
(ii) Show that the angular width of the first diffraction fringe is half of that of the central fringe.
(iii) If a monochromatic source of light is replaced by white light what changes would you observe in the differential pattern.
Question 4
(i) How does one demonstrate using a suitable diagram that unpolarized light, when passed through a Polaroid, get polarized.
(ii) A beam of unpolarized light is incident on a glass-air interface. Show using a suitable ray diagram that light reflected from the interface is totally polarized when $\mu =\tan i_B$ where $\mu$ is the refractive index of glass with respect to air and $i_B$ is the Brewster's angle.

## Numerical Type Questions

Question 1 Two identical coherent waves each of intensity $I$, are producing an interference pattern. Write the value of resultant intensity at a point of (i) constructive interference and (ii) destructive interference.

Question 2 If the angle between the planes of the polarizer and analyzer is $60^{\circ}$, by what factor does the intensity of transmitted light change when passing through the analyzer?

Question 3 The ratio of intensity of maxima and minima in an interference pattern is 100:64. Calculate the ratio of intensities of the coherent sources producing the pattern.

Question 4 Draw a diffraction pattern due to single slit illuminated by monochromatic source of light. Light of wavelength 500 nm, falls from a distant source of slit 0.50 mm wide. Find the distance between the two dark bands, on either side of the central bright band of the diffraction pattern observed on a screen placed 2m from the slit.

Question 5 Laser light of wavelength 630 nm incident on a pair of slits produces an interference pattern in which the bright fringes are separated by 8.1 mm. A second light produces an interference pattern in which the fringes are separated by 7.2 mm. Calculate the wavelength of the second light.

Question 6 Find the ratio of intensities of two points $P$ and $Q$ on a screen in Young’s double slit experiment when waves from sources $S_1$ and $S_2$ have phase difference of (i) $0^0$ and (ii) $\frac{\pi}{2}$ respectively.

Question 6 In Young’s Double Slit experiment using monochromatic light of wavelength $\lambda$, the intensity of light at a point on screen where path difference is $\lambda$; is $K$ units. What is the intensity of light at a point where path difference is $\frac{\lambda}{3}$?

Question 7 Interference pattern is obtained with two coherent light sources of intensity ratio n. Show that in the interference pattern
$\frac {(I_{max}-I_{min})}{(I_{max}+I_{min})}=2 \sqrt {\frac {n}{n+1}}$