Cambridge IGCSE Combined Science 0653 Physics P3 connects wave measurements to light, the electromagnetic spectrum and sound. Waves transfer energy without carrying matter from source to receiver. Diffraction is not listed in this P3 syllabus boundary.
P3.1 Wave quantities and motion
Wave motion can be demonstrated by oscillations in a rope, spring or water surface. A point in the medium oscillates about an equilibrium position while the disturbance and energy propagate onward.
Amplitude is the maximum displacement from equilibrium. Wavelength is the distance between adjacent points in phase, such as crest to crest. Frequency is the number of complete oscillations per second, measured in hertz. Wave speed is the distance travelled by a wavefront per unit time.
v=fλ
Use consistent units. If frequency is in hertz and wavelength in metres, speed is in metres per second. A larger amplitude normally represents a larger energy transfer, but amplitude is not the same quantity as frequency.
Check this topic from memory
Attempt the matching topic bank before reopening the notes. Use each missed idea to decide what to review next.
In a transverse wave, vibration is perpendicular to propagation. Electromagnetic waves, water-wave models and seismic S-waves are examples. In a longitudinal wave, vibration is parallel to propagation. Sound in air and seismic P-waves are longitudinal.
Reflection and refraction
Reflection sends a wave back from a boundary. Refraction is a change in direction caused by a change in wave speed when a wave crosses a boundary. A ray entering along the normal does not change direction even though its speed may change.
The normal is an imaginary line perpendicular to the surface at the point of incidence. Angles of incidence, reflection and refraction are always measured from the normal, not the surface.
P3.2 Reflection of light
For a plane mirror, the angle of incidence equals the angle of reflection. A ray diagram should show a ruler-drawn mirror, normal and arrowed rays. The image is virtual, upright, laterally inverted, the same size as the object and the same perpendicular distance behind the mirror as the object is in front.
A virtual image cannot be projected onto a screen because the rays do not actually meet behind the mirror. The eye traces reflected rays backward to the apparent image position.
Refraction through transparent materials
Refraction occurs at the boundary between two media because light changes speed. Draw the incident ray, normal and refracted ray at the same point. The required treatment is limited to boundaries between two media and does not require refractive-index calculations.
Avoid memorising "towards the normal" without context. The direction depends on the speed change; use the information or diagram provided.
Thin converging lenses
A thin converging lens bends a parallel beam toward the principal focus. The principal axis passes through the optical centre and focus. Focal length is the distance from optical centre to principal focus.
For the required real-image diagrams, draw one ray from the top of the object parallel to the principal axis, then through the far focus after the lens. Draw a second ray through the optical centre undeviated. Their intersection locates the image. Describe it as enlarged, same size or diminished, and upright or inverted. The syllabus limits these diagrams to real images.
Dispersion and visible light
A glass prism disperses white light because different colours refract by different amounts. In increasing frequency, the order is red, orange, yellow, green, blue, indigo, violet. Wavelength decreases as frequency increases when wave speed is fixed.
Dispersion separates colours already present in white light. The prism does not manufacture the colours.
P3.3 Electromagnetic spectrum
The order from lowest frequency and longest wavelength is radio, microwave, infrared, visible, ultraviolet, X-ray and gamma. All travel at the same speed in a vacuum:
c=3.0×108m⋅s−1
Applications include radio and television transmission and radar; satellite links, mobile phones and cooking with microwaves; infrared remote controls and thermal imaging; vision and photography; ultraviolet bank-note checks; X-ray medical and security imaging; and gamma detection or treatment of cancer.
Excess ultraviolet can damage surface cells and eyes, increasing risks of skin cancer and eye conditions. X-rays and gamma rays are ionising and can mutate or damage body cells. A complete application answer balances the useful property with controlled exposure.
P3.4 Sound
Vibrating sources produce sound. Sound needs a medium and cannot travel through a vacuum. In air it consists of compressions, where particles are closer and pressure is higher, alternating with rarefactions, where particles are farther apart and pressure is lower.
Human hearing is approximately 20Hz to 20kHz. Ultrasound has frequency above 20kHz. Greater frequency gives higher pitch; greater amplitude generally gives louder sound. An echo is reflected sound.
To measure sound speed, measure a known distance and the travel time, then use speed equals distance divided by time. Sound generally travels faster in solids than liquids and faster in liquids than gases because of their particle interactions.
Worked application: identify an unknown wave
A wave has frequency 2.5×103Hz and wavelength 0.14m. Its speed is v=fλ=2.5×103×0.14=350m⋅s−1, consistent with sound in air under ordinary conditions. It cannot be an electromagnetic wave in a vacuum because that would travel at 3.0×108m⋅s−1. Since 2.5kHz lies within the approximate human hearing range, a person with typical hearing may hear it. Increasing amplitude would make it louder, while increasing frequency would raise its pitch.
Common misconceptions and corrections
Matter travels with the wave. Medium particles oscillate locally while energy propagates.
Amplitude and wavelength are measured the same way. Amplitude is from equilibrium; wavelength is between adjacent in-phase points.
Angles are measured from the mirror. Measure them from the normal.
A plane-mirror image is on the mirror surface. It appears behind the mirror at equal distance.
A prism creates colours. It separates wavelengths already in white light.
All electromagnetic waves have different vacuum speeds. They share the same vacuum speed.
Sound can cross empty space. Sound requires a medium.
Diffraction is required in 0653 P3. It is outside this syllabus boundary.
Assessment guidance
Label every quantity and unit before using v=fλ. In ray diagrams use straight lines, arrowheads, a normal and measured angles. For electromagnetic questions, connect each use or hazard to the relevant wave property rather than reciting a list. For sound, distinguish particle vibration direction from wave propagation direction. When comparing pitch and loudness, refer to frequency and amplitude respectively. Do not import refractive index, total internal reflection or diffraction unless the question supplies an extension context.
When a question asks for an explanation, finish the chain from the named wave property to the observed result. A correct definition without that application may not answer the command word.
Retrieval practice
Distinguish transverse and longitudinal waves using vibration direction.
Calculate wave speed for f=50Hz and λ=1.2m.
State four characteristics of a plane-mirror image.
Describe two principal rays used for a converging-lens image.
Explain one use and one hazard of a named electromagnetic region.
Explain compressions and rarefactions in particle terms.
