EM Waves Properties & Propagation

# EM Waves Properties & Propagation | Physics Class 12 - NEET PDF Download

 Table of contents Electromagnetic Waves Properties of EM Waves Electromagnetic Spectrum Solved Examples on EM Waves

## Electromagnetic Waves

Electromagnetic waves are those waves in which electric and magnetic field vectors changes sinusoidally and are perpendicular to each other as well as at right angles to the direction of propagation of wave.

The equation of plane progressive electromagnetic wave can be written as

E = Eo sin Ω (t – x / c) and B = Bo sin Ω (t – x / c). Where, Ω =2πv

Electromagnetic waves are produced by accelerated charge particles.

## Properties of EM Waves

(i) These waves are transverse in nature.

(ii) These waves propagate through space with speed of light, i.e., 3 * 108 m / s.

(iii) The speed of electromagnetic wave,

c = 1 / √μo εo

where, μo is permittivity of free space,

∴ c = Eo / Bo

where Eo and Bo are maximum values of electric and magnetic field vectors.

According to Maxwell, when a charged particle is accelerated, it produces electromagnetic wave. The total radiant flux at any instant is given by,

p = q2a2 / 6 πεoc2

(iv) The rate of flow of energy in an electromagnetic wave is described by the vector S called the poynting vector, which is defined by the expression,

S = 1 / μo E * B

SI unit of S is watt/m2.

(v) Its magnitude S is related to the rate at which energy is transported by a wave across a unit area at any instant.

(vi) The energy in electromagnetic waves is divided equally between electric field and magnetic field vectors.

(vii) The average electric energy density.

UE = 1 / 2 εo   E2 = 1 / 4 εo E2o

(viii) The average magnetic energy density,

UB = 1 / 2 B2 / μo = 1 / B2o / μo

(ix) The electric vector is responsible for the optical effects of an electromagnetic wave.

(x) Intensity of electromagnetic wave is defined as energy crossing per unit area per unit time perpendicular to the directions of propagation of electromagnetic wave.

(xi) The intensity I is given by the relation,

I = < μ > c = 1 / 2 εo E2oc

(xii) The existence of electromagnetic waves was confirmed by Hertz experimentally in 1888.

### Propagation of Electromagnetic Waves

In radio wave communication between two places. the electromagnetic waves are radiated out by the transmitter antenna at one place which travel through the space and reach the receiving antenna at the other place.

## Electromagnetic Spectrum

The arranged array of electromagnetic radiations in the sequence of their wavelength or frequency is called electromagnetic spectrum

Radio and microwaves are used in radio and TV communication,

Infrared rays are used to

(i) Treat muscular straw.
(ii) For taking photographs’ in fog or smoke.
(iii) In green house to keep plants warm.
(iv) In weather forecasting through infrared photography.

Ultraviolet rays are used

(i) In the study of molecular structure.
(ii) In sterilizing the surgical instruments.
(iii) In the detection of forged documents, Eringer prints.

X-rays are used

(i) In detecting faults, cracks, flaws and holes in metal products.
(ii) In the study of crystal structure.
(iii) For the detection of pearls in oysters.

γ – rays are used for the study of nuclear structure.

## Solved Examples on EM Waves

Question 1: Calculate the frequency and wavelength of an electromagnetic wave with an energy of 6.626×10−19 J.

Answer:

• Frequency (f):E/h=6.626×10−19/6.626×10−34=1015 Hz.
• Wavelength (λ): $c/f = \frac{3 \times 10^8}{10^{15}} = 3 \times 10^{-7}$c/f=3×108/1015=3×10−7 meters.

Question 2: What are the applications of X-rays?

Answer: X-rays are used in medical diagnostics to detect bone fractures and other ailments. They are also useful for ionization purposes.

Question 3: Are X-rays and gamma rays suitable for broadcasting radio, TV, or mobile signals?

Answer: No, X-rays and gamma rays are unsuitable for broadcasting because they have short ranges and are harmful. Their high penetrating power can damage living tissue.

Question 4: Identify which of the following is not a property of electromagnetic waves:

1. Momentum
2. Energy
3. Pressure
4. Heat energy

Answer:

• 1. Electromagnetic (EM) waves can transfer momentum to materials they interact with.
• 2. EM waves carry energy and can transmit it through a vacuum.
• 3. EM waves exert pressure, as demonstrated by a radiometer, which spins due to light-induced pressure differences.
• 4. EM waves do not carry heat energy, but they can heat objects upon absorption.

Question 5: A sunlight ray passing through your kitchen window hits a prism, creating a rainbow on the windowsill. If you place a radiometer on a specific color of the rainbow and measure the energy as 4.0×10−19 joules, what color might you have measured? Use Planck’s constant $6.6256 \times 10^{-34}$6.6256×10−34 J·s to determine this.

Answer:

• Equation to use: E=h⋅f

• Solving for frequency (f): $f = E / h$f=E/h

• Given:

• Energy (E) = 4.0×10−19 joules
• Planck’s constant (h) = $6.6256 \times 10^{-34}$6.6256×10−34 J·s
• Calculation: f=4.0×1019 / 6.6256×10−34≈6.03×1014 Hz

This frequency falls within the visible spectrum, near the green color range. Therefore, the measured color is likely green, though it could also be cyan or blue.

The document EM Waves Properties & Propagation | Physics Class 12 - NEET is a part of the NEET Course Physics Class 12.
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## Physics Class 12

105 videos|425 docs|114 tests

## FAQs on EM Waves Properties & Propagation - Physics Class 12 - NEET

 1. What are the properties of electromagnetic waves?
Ans. Electromagnetic waves are transverse waves that can travel through a vacuum at the speed of light. They have electric and magnetic fields perpendicular to each other and the direction of propagation.
 2. How do electromagnetic waves propagate through space?
Ans. Electromagnetic waves propagate through space by oscillating electric and magnetic fields. They do not require a medium to travel through, as they can travel through a vacuum.
 3. What are the different types of electromagnetic waves?
Ans. The electromagnetic spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. Each type of wave has a different wavelength and frequency.
 4. How do electromagnetic waves interact with matter?
Ans. Electromagnetic waves can be absorbed, reflected, refracted, diffracted, or scattered when they interact with matter. The interaction depends on the properties of the material and the wavelength of the wave.
 5. How are electromagnetic waves used in everyday life?
Ans. Electromagnetic waves are used in various applications, such as communication (radio waves), cooking (microwaves), medical imaging (X-rays), and remote sensing (infrared). They play a crucial role in modern technology and everyday activities.

## Physics Class 12

105 videos|425 docs|114 tests

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