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Electromagnetic Waves (HL IB Physics)

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The Electromagnetic Spectrum

  • An electromagnetic wave is generated by the combined oscillation of an electric and a magnetic field
  • These fields oscillate perpendicularly to each other and to the direction of motion of the wave (i.e. the direction in which energy is transferred)

4-2-3-oscillating-electric-and-magnetic-fields_sl-physics-rn

An electromagnetic wave is generated by the combined oscillation of an electric and a magnetic field

 

  • Electromagnetic waves are transverse waves and, as such, they can travel through vacuum
  • Regardless of their frequency, all electromagnetic waves travel at the speed of light c = 3 × 108 m s–1 in vacuum
  • Electromagnetic waves form a continuous spectrum based on their frequency (or wavelength)
  • The shorter the wavelength, or higher the frequency, the greater the energy of the wave

 3-2-4-ib-electromagnetic-spectrum

The electromagnetic spectrum

  • Humans can only sense electromagnetic waves with wavelengths in the range 700 nm - 400 nm, which are the limits of the so-called visible spectrum 
    • Electromagnetic waves with longer and shorter wavelengths are invisible to the human eye

  • Knowing the wavelengths of electromagnetic waves, their frequencies can be calculated using
    • The wave equation
    • The fact that the speed of light (c = 3 × 108 m s–1) in a vacuum is constant

Worked example

:The wavelength of blue light falls within the range 450 nm – 490 nm.

Determine the range of frequencies of blue light.

Answer:

Step 1: Write down the known quantities 

  • Note that you must convert the values of the wavelength from nanometres (nm) into metres (m)
    • λlower = 450 nm = 4.5 × 10–7 m
    • λhigher = 490 nm = 4.9 × 10–7 m

Step 2: Remember that all electromagnetic waves travel at the speed of light in vacuum

  • From the data booklet, c = 3.00 × 108 m s–1

Step 3: Write down the wave equation 

v space equals space f lambda

Step 4: Rearrange the above equation to calculate the frequency f 

f space equals space v over lambda

Step 5: Substitute the lower and higher values of the wavelength to calculate the limiting values of the frequency of blue light 

  • The lower frequency flower corresponds to the higher value of the wavelength λhigher

f subscript l o w e r end subscript space equals space fraction numerator 3.00 space cross times space 10 to the power of 8 space over denominator 4.9 space cross times space 10 to the power of negative 7 end exponent end fraction space equals space 6.1 space cross times space 10 to the power of 14 space Hz

 

  • The higher frequency fhigher corresponds to the lower value of the wavelength λlower

f space equals space fraction numerator space 3.00 space cross times space 10 to the power of 8 over denominator 4.5 space cross times space 10 to the power of negative 7 end exponent end fraction space equals space 6.7 space cross times space 10 to the power of 14 space Hz

Step 6: Write down the range of frequencies of blue light 

f space equals space 6.1 space cross times space 10 to the power of 14 space – space 6.7 space cross times space 10 to the power of 14 space Hz

Examiner Tip

You don't need to memorise the order or the wavelengths of waves in the electromagnetic spectrum, as this is given in your data booklet. However, you must remember all electromagnetic waves travel at the speed of light, c.

Comparing Mechanical & Electromagnetic Waves

  • Travelling waves can be of two types, mechanical and electromagnetic
Mechanical Waves Electromagnetic Waves
Require a medium, such as a fluid or solid to propagate through Do not require a medium
Can be transverse or longitudinal Are only transverse
Cannot travel through a vacuum Can travel through a vacuum
Are produced by the oscillation of particles in a medium Are produced by oscillating charged particles
Examples: Sound waves, waves on the surface of the ocean  Examples: Radio waves, UV rays, X-rays
Travel a lot slower than the speed of light Travel at the speed of light

3-2-4-em-vs--mechanical-waves

Electromagnetic waves can travel in a vacuum (such as space) whilst mechanical waves require a medium (such as water) 

Examiner Tip

You must be able to identify the features of either mechanical or electromagnetic waves, depending on the question.

A common misconception is that mechanical waves are just longitudinal, like sound waves. However, they can also be transverse waves that travel through a medium such as seismic waves (created during an earthquake) and water waves.

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    Ashika

    Author: Ashika

    Expertise: Physics Project Lead

    Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.