Understanding Waves: Edexcel IGCSE Physics Explained

Waves are the mechanism by which energy and information travel through the universe. The Edexcel IGCSE Physics waves section demands both conceptual clarity and precise calculation.

The waves edexcel igcse section spans four topics: units, properties of waves, the electromagnetic spectrum, and light and sound. It is one of the most regularly examined areas of the specification, and the range of question types is broad. You may be asked to calculate wave speed, explain total internal reflection, identify regions of the electromagnetic spectrum, or describe how an oscilloscope displays a sound wave. Thorough preparation here pays dividends on both papers.

These edexcel igcse physics revision notes provide a complete treatment of the waves content. Each concept is explained, each equation is demonstrated with a worked example, and the common pitfalls are flagged. These revision notes serve as your comprehensive reference for edexcel igcse physics waves content.

Units for waves

Properties of waves

Transverse and longitudinal waves

In a transverse wave, the oscillations are perpendicular to the direction of energy transfer. Examples include light waves, water surface waves, and waves on a string. In a longitudinal wave, the oscillations are parallel to the direction of energy transfer. Sound waves are the principal example at this level. Both types transfer energy and information without transferring matter.

Key definitions

• Amplitude: The maximum displacement of a point on the wave from its rest position.
• Wavelength: The distance between two consecutive points in phase (e.g. crest to crest).
• Frequency: The number of complete waves passing a point per second, measured in hertz (Hz).
• Period: The time for one complete wave to pass a point. Period and frequency are inverses: f = 1 / T.
• Wavefront: A line joining points on a wave that are in phase.

The wave equation

Wave speed = frequency x wavelength, v = f x λ. This is the single most important equation in the waves section.

The Doppler effect

When a wave source moves towards an observer, the observed frequency increases and the observed wavelength decreases. When the source moves away, the frequency decreases and the wavelength increases. This is the Doppler effect. A common example is the change in pitch of a siren as an ambulance passes you.

The Doppler effect is not just a curiosity. It has profound applications in astrophysics, where it forms the basis of red-shift observations that tell us the universe is expanding. At the IGCSE level, you need to know the qualitative description: source moving towards you means shorter wavelength and higher frequency; source moving away means longer wavelength and lower frequency. The quantitative red-shift equation appears in the astrophysics section, not here.

Reflection and refraction

All waves can be reflected and refracted. Reflection occurs when a wave bounces off a surface. Refraction occurs when a wave changes speed as it passes from one medium to another, causing a change in direction (unless the wave enters at 90 degrees to the boundary).

The electromagnetic spectrum

The electromagnetic (EM) spectrum is a continuous range of waves that all travel at the same speed in free space (3 x 10⁸ m/s). The order, from longest wavelength to shortest, is:

1. Radio waves
2. Microwaves
3. Infrared
4. Visible light (red to violet)
5. Ultraviolet
6. X-rays
7. Gamma rays

As wavelength decreases, frequency increases. All EM waves are transverse.

Uses and dangers

Protective measures include sunscreen (UV), lead shielding (X-rays, gamma), limiting exposure time, and increasing distance from the source.

Light and sound

Reflection of light

Light waves are transverse. The law of reflection states that the angle of incidence equals the angle of reflection. Both angles are measured from the normal (the line perpendicular to the surface at the point of incidence).

Refraction of light

When light passes from a less dense medium (like air) into a more dense medium (like glass), it slows down and bends towards the normal. When it passes from a more dense medium into a less dense medium, it speeds up and bends away from the normal.

The quantitative relationship is Snell's law: n = sin i / sin r, where n is the refractive index of the material, i is the angle of incidence (in the less dense medium), and r is the angle of refraction (in the denser medium).

Total internal reflection and critical angle

When light travels from a denser medium to a less dense medium, increasing the angle of incidence eventually reaches a point where the refracted ray travels along the boundary. This is the critical angle, c. Beyond this angle, all light is reflected back into the denser medium. This is total internal reflection.

The relationship between critical angle and refractive index is: sin c = 1 / n.

Total internal reflection is used in optical fibres (for high-speed communication) and in prisms (for periscopes and binoculars).

Sound waves

Sound waves are longitudinal. They can be reflected (echoes) and refracted. The frequency range for human hearing is 20 Hz to 20,000 Hz. An oscilloscope connected to a microphone can display a sound wave as a trace on the screen. The frequency of the sound can be determined from the time period shown on the oscilloscope.

The pitch of a sound corresponds to its frequency: higher frequency means higher pitch. The loudness corresponds to its amplitude: larger amplitude means louder sound. You can investigate the speed of sound in air by measuring the time between seeing a distant event (like a starting pistol flash) and hearing it, then dividing the distance by the time delay.

Common mistakes in waves

1. Confusing frequency and wavelength. As frequency increases, wavelength decreases (for waves travelling at the same speed). Students sometimes state both increase together.
2. Measuring angles from the surface instead of the normal. The angle of incidence and the angle of refraction are always measured from the normal, not from the surface. This is a common error in ray diagram questions.
3. Forgetting the conditions for total internal reflection. Two conditions must be met: the light must be travelling from a denser medium to a less dense medium, and the angle of incidence must exceed the critical angle. Stating only one of these loses marks.
4. Saying sound waves are transverse. Sound waves are longitudinal. Light waves are transverse. This distinction is frequently tested.
5. Mixing up uses and dangers of EM waves. Microwaves are used for cooking and satellite transmission, not for medical imaging. X-rays are used for observing internal structures, not for communication. Learn the correct pairings.

Self-check: edexcel igcse physics practice questions

1. A sound wave has a frequency of 256 Hz and travels at 340 m/s. Calculate its wavelength.
2. The refractive index of water is 1.33. Calculate the critical angle for water.
3. A wave has a time period of 0.005 s. What is its frequency?
4. Explain why a prism can split white light into a spectrum.
5. Describe two uses and two dangers of ultraviolet radiation.

Answers: (1) λ = v / f = 340 / 256 = 1.33 m; (2) sin c = 1 / 1.33 = 0.7519, c = 48.8°; (3) f = 1 / T = 1 / 0.005 = 200 Hz; (4) Different colours of light have different wavelengths, and the refractive index varies slightly with wavelength, so each colour refracts by a different amount as it enters and exits the prism, spreading the colours out; (5) Uses: fluorescent lamps, detecting forged banknotes. Dangers: damage to surface cells, can cause blindness.

These edexcel igcse physics notes cover the entire waves section of the igcse 4ph1 waves specification. The edexcel igcse physics explained treatment above gives you every equation, every definition and every concept you need for the exam. Work through the examples, attempt the practice questions on your own, and this section of the edexcel exam will hold no surprises.