# Assertion Reason Physics Chapter 8 Electromagnetic Waves

Questions of Assertion Reason of Physics Chapter 8 Electromagnetic Waves CBSE Class 12 are very simple to understand as this chapter deals with Electromagnetic Waves. To solve assertion reason type questions of Physics, a deeper understanding of concept about Displacement Current, Electromagnetic Waves and Electromagnetic Spectrum 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 8 Electromagnetic Waves

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: Electromagnetic wave are transverse in nature.
Reason: The electric and magnetic fields in electromagnetic waves are perpendicular to each other and the direction of propagation.

Ans.1. (a)
Transverse waves are those waves in which the particles of the medium oscillate perpendicular to the direction of wave propagation.

Question.2.
Assertion: Electromagnetic waves interact with matter and set up oscillations.
Reason: Interaction is independent of the wavelength of the electromagnetic wave.

Ans.2. (c)
Electromagnetic waves interact with matter via their electric and magnetic field which in oscillation of charges present in all matter. The detailed interaction and so the mechanism of absorption, scattering, etc. depend of the wavelength of the electromagnetic wave, and the nature of the atoms and molecules in the medium.

Question.3.
Assertion: Electromagnetic waves carry energy and momentum.
Reason: Electromagnetic waves can be polarised.

Ans.3. (b)
Consider a plane perpendicular to the direction of propagation of the electromagnetic wave. If electric charges are present in this plane, they will be set and sustained in motion by the electric and magnetic fields of the electromagnetic wave. The charge thus acquired energy and momentum from the wave. This illustrate the fact that an electromagnetic wave like other waves carries energy and momentum.

Question.4.
Assertion: Electromagnetic waves exert radiation pressure.
Reason: Electromagnetic waves carry energy.

Ans.4. (a)
Electromagnetic waves have linear momentum as well as energy. This concludes that they can exert radiation pressure by falling beam of electromagnetic radiation on an object.

Question.5.
Assertion: The electromagnetic wave is transverse in nature.
Reason: Electromagnetic wave propagates parallel to the direction of electric and magnetic fields.

Ans.5. (c)
This electromagnetic wave contains sinusoidally time varying electric and magnetic field which act perpendicular to each other as well as at right angle to the direction of propagation of waves, so electromagnetic waves are transverse in nature. Electromagnetic wave propagate in the perpendicular direction to both fields.

Question.6.
Assertion: The velocity of electromagnetic waves depends on electric and magnetic properties of the medium.
Reason: Velocity of electromagnetic waves in free space is constant.

Ans.6. (b)
Both Assertion and Reason are correct but Reason is not a correct explanation
of the Assertion.

Question.7.
Assertion: The basic difference between various types of electromagnetic waves lies in their wavelength or frequencies.
Reason: Electromagnetic waves travel through vacuum with the same speed.

Ans.7. (a)
The basic difference between various types of electromagnetic waves lies in their wavelengths or frequencies since all of them travel through vacuum with the same speed. Consequently, the waves differ considerably in their mode of interaction with matter.

Question.8.
Assertion: Microwaves are better carrier of signals than optical waves.
Reason: Microwaves move faster than optical waves.

Ans.8. (d)
The optical waves used in optical fibre communication are better carrier of signals than microwaves. The speed of microwave and optical wave is the same in vacuum.

Question.9.
Assertion: Infrared radiation plays an important role in maintaining the average temperature of earth.
Reason: Infrared radiations are sometimes referred to as heat waves.

Ans.9. (b)
Infrared radiation help to maintain the earth warmth through the greenhouse effect. Incoming visible light which passes relatively easily through the atmosphere is absorbed by the earth’s surface and re-radiated as infrared radiation. The radiation is trapped by greenhouse gases such as carbon dioxide and water vapour and they heat up and heat their surroundings.