# Assertion and Reason Questions on class 12 Physics Chapter 10 Wave Optics

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Directions: These questions consist of two statements, each printed as Assertion and Reason. While answering these questions, you are required to choose any one of the following four responses.
(a) If both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.
(b) If both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.
(c) If the Assertion is correct but Reason is incorrect.
(d) If both the Assertion and Reason are incorrect.

Q.1. Assertion : According to Huygen’s principle, no backward wave-front is possible.
Reason : Amplitude of secondary wavelet is proportional to (1 + cos θ) where θ is the angle between the ray at the point of consideration and the direction of secondary wavelet.

Q.2. Assertion : Thin film such as soap bubble or a thin layer of oil on water show beautiful colours when illuminated by white light.
Reason : It happens due to the interference of light reflected from upper and lower face of the thin film.

Q.3. Assertion : No interference pattern is detected when two coherent sources are infinitely close to each other.
Reason : The fringe width is inversely proportional to the distance between the two sources.

Q.4. Assertion : It is necessary to have two waves of equal intensity to study interference pattern.
Reason : There will be an effect on clarity if the waves are of unequal intensity.

Answer (d) For interference, the waves may be of unequal intensities.

Q.5. Assertion : White light falls on a double slit with one slit is covered by a green filter. The bright fringes observed are of green colour.
Reason : The fringes observed are coloured.

Answer (c) Interference will take place in green light only

Q.6. Assertion : In YDSE, if a thin film is introduced in front of the upper slit, then the fringe pattern shifts in the downward direction.
Reason : In YDSE if the slit widths are unequal, the minima will be completely dark.

Q.7. Assertion : In Young’s double slit experiment if wavelength of incident monochromatic light is just doubled, number of bright fringe on the screen will increase.
Reason : Maximum number of bright fringe on the screen is inversely proportional to the wavelength of light used

Q.8. Assertion : In YDSE number of bright fringe or dark fringe can not be unlimited
Reason : In YDSE path difference between the superposing waves can not be more than the distance between the slits.

Q.9. Assertion : Interference pattern is made by using yellow light instead of red light, the fringes becomes narrower.
Reason : In YDSE, fringe width is given by β=λD/d

Q.10. Assertion : Coloured spectrum is seen when we look through a muslin cloth.
Reason : It is due the diffraction of white light on passing through fine slits.

Q.11. Assertion : Diffraction takes place for all types of waves mechanical or non-mechanical, transverse or longitudinal.
Reason : Diffraction’s effect are perceptible only if wavelength of wave is comparable to dimensions of
diffracting device.

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