Electromagnetic Waves (HSC SSCE Physics): Revision Notes

Electromagnetic Waves

What are electromagnetic waves?

Electromagnetic waves are a unique type of wave that can travel through empty space. Unlike mechanical waves such as sound, electromagnetic waves do not need a medium to propagate. We can picture electromagnetic radiation as two fields—one electric and one magnetic—that vibrate together at right angles to each other while moving through space.

The two fields are perpendicular to both each other and to the direction of travel. As the electric field oscillates, it creates a changing magnetic field. This changing magnetic field, in turn, generates an electric field. This continuous interaction allows the fields to sustain themselves and propagate forward without requiring any material medium.

Historical development

Early discoveries

The connection between electricity and magnetism was first identified by Hans Christian Ørsted in 1820. His discovery was subsequently investigated further by Michael Faraday and Humphry Davy, who explored how these two phenomena were related.

Maxwell's breakthrough

In the 1860s, James Clerk Maxwell developed mathematical equations that described electromagnetic radiation. His work showed that:

• A changing magnetic field induces an electric field perpendicular to it
• A changing electric field induces a magnetic field perpendicular to it
• These oscillations can self-propagate through space

This prediction proved correct with the later discovery of X-rays, radio waves, infrared radiation, and ultraviolet radiation.

The speed of light

Maxwell's equations revealed that the speed of light in a vacuum is $c = 2.9979 \times 10^8 \ m \cdot s^{-1}$, which is approximately $300\,000 \ \mathrm{km} \cdot s^{-1}$. For most calculations, this is rounded to $c = 3.0 \times 10^8 \ m \cdot s^{-1}$.

This speed turned out to be of fundamental importance. Albert Einstein's special theory of relativity identified it as the theoretical universal speed limit—nothing can travel faster than the speed of light in a vacuum.

The aether debate and Einstein's resolution

Maxwell's result showed that the speed of light was invariant (constant), suggesting that electromagnetic waves could travel through space without requiring a medium. This idea sparked intense debate among physicists. Many scientists believed there must be some medium—called the 'aether'—through which these waves propagated, and numerous experiments were conducted to try to detect it.

However, the existence of such a medium would have significant implications. It would mean the aether itself became the universal frame of reference, which contradicted fundamental principles. Einstein realised that the constant speed of electromagnetic wave propagation meant there was no aether. To reconcile Maxwell's theories with observations, Einstein developed his 'thought experiments', which showed that both time and space as experienced by observers would vary depending on their location and motion. This revolutionary thinking led to the special theory of relativity in 1905, which ushered in the modern physics we understand today.

The electromagnetic spectrum

The electromagnetic spectrum encompasses all types of electromagnetic radiation, distinguished by their wavelengths and frequencies. The spectrum includes an enormous range of wavelengths, from kilometres for long radio waves down to $10$ picometres ($10^{-12} \ \mathrm{m}$) for gamma rays.

The electromagnetic spectrum includes, in order of increasing wavelength:

• Gamma rays: Shortest wavelength (around $0.1$ Ångströms or $10^{-11} \ \mathrm{m}$), highest frequency
• X-rays: Wavelengths from about $0.1$ to $10 \ \mathrm{nm}$
• Ultraviolet: Wavelengths from about $10$ to $400 \ \mathrm{nm}$
• Visible light: Wavelengths from approximately $400 \ \mathrm{nm}$ (violet) to $700 \ \mathrm{nm}$ (red)
• Infrared: Divided into near, thermal, and far infrared, with wavelengths from about $700 \ \mathrm{nm}$ to $1 \ \mathrm{mm}$
• Microwaves: Including radar frequencies, wavelengths from about $1 \ \mathrm{mm}$ to $1 \ \mathrm{m}$
• Radio waves: Including TV, FM, VHF, UHF, and AM radio, wavelengths from about $1 \ \mathrm{m}$ to kilometres
• Long waves: Longest wavelengths in the spectrum

All these different types of electromagnetic radiation are fundamentally the same phenomenon—oscillating electric and magnetic fields—but with different wavelengths and frequencies. Crucially, they all travel at the speed of light when in a vacuum.

Properties of electromagnetic waves

No medium required

Unlike mechanical waves, electromagnetic waves can travel through a vacuum. The radiation reaching us from the Sun, distant stars, and pulsars travels through the emptiness of space at the speed of light. This is possible because the waves do not require matter to propagate—they consist of oscillating electric and magnetic fields, not vibrating particles.

Travelling through materials

While electromagnetic waves can travel through a vacuum, they can also pass through certain materials:

• Light can travel through the atmosphere, water, glass, and diamond
• Light cannot pass through rocks, wood, or metals
• Earth's atmosphere effectively absorbs or blocks most electromagnetic radiation, with the main exceptions being visible light, radio waves, and microwaves

Invisible oscillations

With electromagnetic waves, there is no physical vibration of matter. Instead, it is invisible electric and magnetic fields (which do not involve matter) that oscillate in space, transferring energy as the oscillating fields propagate forward.

Investigation 7.4: Differences between mechanical and electromagnetic waves

Aim

To observe differences between mechanical and electromagnetic waves.

Hypothesis

If the medium is removed, a mechanical wave will not pass but electromagnetic waves will.

Materials

• Well-greased air-tight bell jar with air outlet
• Strong vacuum pump
• Battery-powered bell

Risk assessment

What are the risks in doing this investigation?	How can you manage these risks to stay safe?
The vacuum pump is very heavy and may cause injury if dropped.	Place the vacuum pump well away from the edge of the bench.
A strong vacuum can cause a glass container to implode and shatter, spreading shards of glass.	Only use a purpose-built vacuum jar and wear safety glasses.

You should also consider any other risks associated with your investigation and how you can manage them safely.

Method

1. Place the battery-powered bell inside the bell jar, turn it on, and seal the jar
2. Connect the vacuum pump
3. Observe the sound of the bell
4. Start the vacuum pump and continue to observe the sound of the bell
5. Once the vacuum has been attained, observe the sound from the bell and observe the bell itself

Results

Record your observations in a suitable format. You should note both what you hear and what you see throughout the experiment.

Discussion

The experiment demonstrates a fundamental difference between mechanical and electromagnetic waves. When the vacuum is created inside the bell jar:

• The sound from the bell becomes much quieter or disappears completely (mechanical wave requiring a medium)
• The light from the bell (if it has a light) remains unchanged (electromagnetic wave not requiring a medium)

Conclusion

The investigation supports the hypothesis. When the medium (air) is removed from the bell jar, the mechanical wave (sound) cannot propagate effectively, but the electromagnetic wave (light) continues to travel through the vacuum without change. This demonstrates the fundamental difference between these two types of waves.

Key differences from mechanical waves

Calculating with electromagnetic waves

When working with electromagnetic waves, you can use the relationship between speed, distance, and time:

$\text{speed} = \frac{\text{distance}}{\text{time}}$

Remember!