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Electromagnetic Waves - Electromagnetic Spectrum

Grade 12CBSEPhysics

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Electromagnetic (EM) Spectrum is the classification of electromagnetic waves according to their frequency (ν\nu) or wavelength (λ\lambda).

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All electromagnetic waves travel at the speed of light in vacuum, c≈3×108 m/sc \approx 3 \times 10^8 \text{ m/s}, regardless of their wavelength.

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Radio Waves: Produced by accelerated motion of charges in conducting wires. Wavelength range: >0.1 m> 0.1 \text{ m}. Used in AM/FM radio and television communication.

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Microwaves: Short-wavelength radio waves produced by special vacuum tubes (klystrons/magnetrons). Wavelength range: 0.1 m0.1 \text{ m} to 1 mm1 \text{ mm}. Used in Radar and microwave ovens.

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Infrared (IR) Waves: Produced by hot bodies and molecules. Often called 'heat waves'. Wavelength range: 1 mm1 \text{ mm} to 700 nm700 \text{ nm}. Used in remote controls, greenhouses, and night vision.

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Visible Light: The narrow band detected by the human eye. Wavelength range: 400 nm400 \text{ nm} (violet) to 700 nm700 \text{ nm} (red).

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Ultraviolet (UV) Rays: Produced by the sun and very hot bodies. Wavelength range: 400 nm400 \text{ nm} to 1 nm1 \text{ nm}. Used in water purifiers and LASIK eye surgery; blocked by the ozone layer.

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X-rays: Produced by bombarding a metal target with high-energy electrons. Wavelength range: 1 nm1 \text{ nm} to 10−3 nm10^{-3} \text{ nm}. Used as a diagnostic tool in medicine and for cancer treatment.

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Gamma Rays: Highest frequency waves produced in nuclear reactions and by radioactive nuclei. Wavelength range: <10−3 nm< 10^{-3} \text{ nm}. Used in medicine to destroy cancer cells.

📐Formulae

c=νλc = \nu \lambda

E=hν=hcλE = h\nu = \frac{hc}{\lambda}

c=E0B0c = \frac{E_0}{B_0}

v=1μϵv = \frac{1}{\sqrt{\mu \epsilon}}

p=Ucp = \frac{U}{c}

💡Examples

Problem 1:

A radio station broadcasts at a frequency of 90 MHz90 \text{ MHz}. Calculate the wavelength of the electromagnetic waves emitted by the station and identify the region of the EM spectrum.

Solution:

Given frequency ν=90 MHz=90×106 Hz\nu = 90 \text{ MHz} = 90 \times 10^6 \text{ Hz}. Using the formula c=νλc = \nu \lambda, we have λ=cν=3×10890×106≈3.33 m\lambda = \frac{c}{\nu} = \frac{3 \times 10^8}{90 \times 10^6} \approx 3.33 \text{ m}.

Explanation:

Since the wavelength is 3.33 m3.33 \text{ m}, which falls in the range >0.1 m> 0.1 \text{ m}, these waves belong to the Radio Waves region of the electromagnetic spectrum.

Problem 2:

Identify the electromagnetic waves used in: (i) Radar systems for aircraft navigation, (ii) Treating muscular strain, and (iii) Killing germs in water purifiers.

Solution:

(i) Microwaves, (ii) Infrared waves, (iii) Ultraviolet (UV) rays.

Explanation:

Microwaves are used in Radar due to their short wavelengths which allow for minimal diffraction. Infrared waves provide heat treatment for muscles. UV rays have enough energy to destroy the DNA of microorganisms, effectively sterilizing water.

Electromagnetic Spectrum - Revision Notes & Key Formulas | CBSE Class 12 Physics