CBSE Class 12 Physics Chapter-10 Important Questions – Free PDF Download
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CBSE Class 12 Physics Chapter-10 – Wave Optics
CBSE Class 12 Physics Important Questions Chapter 10 – Wave Optics
2 Marks Questions
1.Obtain an expression for the ratio of intensities at maxima and minima in an interference pattern.
Ans. Suppose 
be the amplitudes and 
the intensities of light waves which interfere each other
After interference (applying superposition principle)
Amplitude at maxima = 
Amplitude at minima = 
= amplitude ratio of two waves.
2.A slit S is illuminated by a monochromatic source of light to give two coherent sources
. These given bright and dark bands on a screen. At a point R, on the screen, there is a dark fringe. What relation must exist between the lengths
?
Ans. There will be a dark fringe at point R When path difference
.
Where 
is the wavelength of the light and n = 0, 1, 2, 3 ———–
3.In young’s double slit experiment how is the fringe width change when
(a) Light of smaller frequency is used
(b) Distance between the slits is decreased?
Ans.
If light of smaller frequency is of higher wavelength is used the fringe width will increase.
(b) If distance between the slits is decreased
i.e 
. Fringe width will increase.
4.Write two points of difference between interference and diffraction?
Ans.
| S. | Interference | Diffraction |
| 1 | Interference occurs due to superposition of light coming from two coherent sources. | It is due to the superposition of the waves coming from different parts of the same wave front. |
| 2 | All bright fringes are of equal intensity | The intensity of bright fringes decreases with increasing distance from the central bright fringes. |
5.Consider interference between two sources of intensities I and 4I. What will be the intensity at points where phase differences is 
Ans. 
Where a and b are amplitudes of two coherent waves having phase difference of
.
Here 
, 
I = I + 4I + 2 
I = 5I + 4I cos
(i) 
I = 5I
(ii) Why 
I = 5I + 4I cos
I = 5I – 4I
I = I
6.Can white light produce interference? What is the nature?
Ans. White light produces interference but due to different colour present in white light interference pattern overlaps the central bright fringe for all the colours is at the position, so its colour is white. The white central bright fringe is surrounded by few coloured rings.
7.(a) The refractive index of glass is 1.5. What is the speed of light in glass? Speed of light in vacuum is 
(b) Is the speed of light in glass independent of the colour of light? If not, which of the two colours red and violet travels slower in a glass prism?
Ans.(a) Refractive index of glass, 
= 1.5
Speed of light, c = 
m/s
Speed of light in glass is given by the relation,
Hence, the speed of light in glass is 
m/s.
(b) The speed of light in glass is not independent of the colour of light.
The refractive index of a violet component of white light is greater than the refractive index of a red component. Hence, the speed of violet light is less than the speed of red light in glass. Hence, violet light travels slower than red light in a glass prism.
8.What is the Brewster angle for air to glass transition? (Refractive index of glass = 1.5.)
Ans.Refractive index of glass, 
Brewster angle = 
Brewster angle is related to refractive index as:
Therefore, the Brewster angle for air to glass transition is 
9.Estimate the distance for which ray optics is good approximation for an aperture of 4 mm and wavelength 400 nm.
Ans.Fresnel’s distance (ZF) is the distance for which the ray optics is a good approximation. It is given by the relation,
Where,
Aperture width, a = 4 mm = 
Wavelength of light, 
= 400 nm = 
Therefore, the distance for which the ray optics is a good approximation is 40 m.
10.Let us list some of the factors, which could possibly influence the speed of wave propagation:
(i) Nature of the source.
(ii) Direction of propagation.
(iii) Motion of the source and/or observer.
(iv) Wave length.
(v) Intensity of the wave. On which of these factors, if any, does
(a) The speed of light in vacuum,
(b) The speed of light in a medium (say, glass or water), depend?
Ans.(a) The speed of light in a vacuum i.e., 
(approximately) is a universal constant. It is not affected by the motion of the source, the observer, or both. Hence, the given factor does not affect the speed of light in a vacuum.
(b) Out of the listed factors, the speed of light in a medium depends on the wavelength of light in that medium.
11. For sound waves, the Doppler formula for frequency shift differs slightly between the two situations: (i) source at rest; observer moving, and (ii) source moving; observer at rest. The exact Doppler formulas for the case of light waves in vacuum are, however, strictly identical for these situations. Explain why this should be so. Would you expect the formulas to be strictly identical for the two situations in case of light travelling in a medium?
Ans.No Sound waves can propagate only through a medium. The two given situations are not scientifically identical because the motion of an observer relative to a medium is different in the two situations. Hence, the Doppler formulas for the two situations cannot be the same.
In case of light waves, sound can travel in a vacuum. In a vacuum, the above two cases are identical because the speed of light is independent of the motion of the observer and the motion of the source. When light travels in a medium, the above two cases are not identical because the speed of light depends on the wavelength of the medium.
12. In double-slit experiment using light of wavelength 600 nm, the angular width of a fringe formed on a distant screen is 
. What is the spacing between the two slits?
Ans.Wavelength of light used, = 6000 nm =
Angular width of fringe,
Angular width of a fringe is related to slit spacing (d) as:
Therefore, the spacing between the slits is
.
13.In deriving the single slit diffraction pattern, it was stated that the intensity is zero at angles of
. Justify this by suitably dividing the slit to bring out the cancellation.
Ans.Consider that a single slit of width d is divided into n smaller slits.
Width of each slit, 
Angle of diffraction is given by the relation,
Now, each of these infinitesimally small slit sends zero intensity in direction θ. Hence, the combination of these slits will give zero intensity.
14.Answer the following questions:
(a) When a low flying aircraft passes overhead, we sometimes notice
a slight shaking of the picture on our TV screen. Suggest a possible explanation.
(b) As you have learnt in the text, the principle of linear superposition of wave displacement is basic to understanding intensity distributions in diffraction and interference patterns. What is the justification of this principle?
Ans.(a) Weak radar signals sent by a low flying aircraft can interfere with the TV signals received by the antenna. As a result, the TV signals may get distorted. Hence, when a low flying aircraft passes overhead, we sometimes notice a slight shaking of the picture on our TV screen.
(b) The principle of linear superposition of wave displacement is essential to our understanding of intensity distributions and interference patterns. This is because superposition follows from the linear character of a differential equation that governs wave motion. If 
and 
are the solutions of the second order wave equation, then any linear combination of and will also be the solution of the wave equation.