# Assertion Reason Physics Chapter 10 Wave Optics

Questions of Assertion Reason of Physics Chapter 10 Wave Optics CBSE Class 12 are very simple to understand as this chapter deals with Wave Optics. To solve assertion reason type questions of Physics, a deeper understanding of concept about Huygens Principle, Refraction and Reflection of Plane Waves using Huygens Principle, Coherent and Incoherent Addition of Waves, Interference of Light Waves and Young’s Experiment, Diffraction and Polarisation is required. In order to help the student in solving Assertion Reason type of questions, detailed process is explained.

How to solve Assertion Reason Type Questions?
In Assertion-Reason type of question, two statements are given, first is Assertion and second is called Reason. Student must have to think critically about both the statements in Assertion Reason Questions, since it combines multiple choice questions and true/false type of questions which requires a higher level of understanding.

How many types are there of Assertion Reason Type Questions?
Assertion-Reason type of questions can be asked either with four MCQ options or with five MCQ options. First four options are same in both the cases only one more options is being provided i.e. last one. One extra option increases the difficulty level of the questions.

In assertion reason type of questions, all 4 or 5 options are same for each question, which are as:

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
(c) Assertion (A) is true but reason (R) is false.
(d) Assertion (A) is false but reason (R) is true.
(e) Both Assertion and Reason are false.

Then a question haunts in every student’s mind, which option is correct and when?
This can be understood clearly with the following table:

Assertion (A)Reason (R)MCQ Options (Fixed for all questions)
If TrueTrue(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
If TrueTrue(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
If TrueFalse(c) Assertion (A) is true but reason (R) is false.
If FalseTrue(d) Assertion (A) is false but reason (R) is true.
If FalseFalse(e) Both Assertion and Reason are false.

Now, lets practice some Assertion Reason Questions of Physics: Chapter 10 Wave Optics

For question given below, two statements are given- one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below:

(a) Both A and R are true and R is correct explanation of the assertion.
(b) Both A and R are true but R is not the correct explanation of the assertion.
(c) A is true but R is false.
(d) A is false but R is true.

Question.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.

Ans.1. (b)
Both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.

Question.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.

Ans.2. (a)
Both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.

Question.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.

Ans.3. (a)
Both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.

Question.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.

Ans.4. (d)
For interference, the waves may be of unequal intensities.

Question.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.

Ans.5. (c)
Interference will take place in green light only.

Question.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.

Ans.6. (d)
Both the Assertion and Reason are incorrect.

Question.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.

Ans.7. (a)
Both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.

Question.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.

Ans.8. (b)
Both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.

Question.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.

Ans.9. (a)
Both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.

Question.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.

Ans.10. (a)
Both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.

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

Ans.11. (b)
Both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.