📘 GCSE Physics · Notes & worksheet
Waves and the Electromagnetic Spectrum.
Notes
Wave basics
- Transverse: oscillation is perpendicular to the direction of energy transfer (light, water ripples).
- Longitudinal: oscillation is parallel (sound), with compressions and rarefactions.
- Waves transfer energy, not matter.
- Amplitude (maximum displacement), wavelength λ (distance between matching points), frequency f (waves per second, Hz), period T = 1/f.
Wave equations
| Quantity | Equation |
|---|---|
| Wave speed | v = fλ |
| Wave speed | v = distance ÷ time |
| Period | T = 1 ÷ f |
Reflection and refraction
- Reflection: angle of incidence = angle of reflection (measured from the normal).
- Refraction: a change in direction when a wave changes speed at a boundary. Entering a denser medium slows light and bends it towards the normal.
- Sound travels faster in solids than liquids and faster in liquids than gases.
Electromagnetic spectrum
In order of increasing frequency (decreasing wavelength): radio, microwave, infrared, visible light, ultraviolet, X-ray, gamma ray. All are transverse and travel at the same speed (about 3 × 10⁸ m/s) in a vacuum.
| Wave | Uses | Hazards |
|---|---|---|
| Radio | broadcasting, communications | — |
| Microwave | cooking, satellites | internal heating of tissue |
| Infrared | heaters, remote controls, thermal imaging | skin burns |
| Visible | vision, fibre optics | — |
| Ultraviolet | sterilising, security marking | skin cancer, eye damage |
| X-ray | medical imaging | ionising, damages cells |
| Gamma | cancer treatment, sterilising | ionising, damages cells |
Ultrasound and echo
Echo distance = (speed × time) ÷ 2, because the pulse travels there and back.
Lenses
A convex lens converges light. Magnification = image height ÷ object height.
Black-body radiation (higher)
All objects emit infrared radiation. Hotter objects emit more, and at higher frequencies. A body at constant temperature absorbs and emits radiation at the same rate. The Earth warms if it absorbs more than it emits, for example because of increased greenhouse gases.
Common mistakes
- Quoting frequency in kHz or MHz without converting to Hz in v = fλ.
- Saying "light waves travel through a vacuum so they are not waves".
- Forgetting to halve the time for echo problems.
Exam tips
- Convert units first: 1 MHz = 10⁶ Hz, 1 cm = 0.01 m.
- For refraction diagrams, draw the normal and label the angles from it.
Worksheet
Prints the questions only, without answers. Choose “Save as PDF” in the print window to download.
Total: 27 marks. Speed of electromagnetic waves in a vacuum = 3.0 × 10⁸ m/s.
- A sound wave has frequency 50 Hz and wavelength 6.8 m. Calculate its speed. (2)
- A radio station broadcasts at 100 MHz. Calculate the wavelength of the waves. (3)
- A ship sends an ultrasound pulse to the sea bed. The echo returns after 0.40 s. The speed of sound in sea water is 1500 m/s. Calculate the depth of the water. (3)
- In a ripple tank a student counts 12 waves passing a point in 4.0 s. The wavelength is 2.5 cm. Calculate the frequency and the wave speed. (4)
- List the electromagnetic spectrum from lowest to highest frequency. (2)
- Explain why ultraviolet radiation is more hazardous to humans than infrared radiation. (3)
- Light travels from air into glass at an angle.
(a) Describe what happens to its speed and direction. (2)
(b) Explain why the direction changes. (2) - An object 3.0 cm tall is placed in front of a convex lens. The magnification is 2.5. Calculate the image height. (2)
- Explain why the average temperature of the Earth remains roughly constant, and how an increase in greenhouse gases affects this. (4)
Show mark scheme and answers (tutor copy)
Try the worksheet first, then check your work here.
- v = fλ = 50 × 6.8 (M1) = 340 m/s (A1).
- f = 100 × 10⁶ Hz (B1). λ = v ÷ f = 3.0 × 10⁸ ÷ 1.0 × 10⁸ (M1) = 3.0 m (A1).
- Distance there and back = 1500 × 0.40 = 600 m (M1). Depth = 600 ÷ 2 (M1) = 300 m (A1).
- f = 12 ÷ 4.0 = 3.0 Hz (M1, A1). v = fλ = 3.0 × 2.5 (M1) = 7.5 cm/s (A1), which is 0.075 m/s.
- radio, microwave, infrared, visible light, ultraviolet, X-ray, gamma. (B2 for fully correct order; B1 if one adjacent pair is swapped.)
- Ultraviolet has a higher frequency and more energy (B1). It is ionising, so it can damage DNA and lead to cell mutations (B1) and skin cancer, and can also damage the eyes. Infrared mainly causes heating and burns at high intensity. (B1)
- (a) The light slows down (B1) and bends towards the normal. (B1)
(b) The wave fronts change speed at the boundary (B1); because one side of the wave front slows before the other, the wave front pivots and the direction changes. (B1) - Image height = magnification × object height = 2.5 × 3.0 (M1) = 7.5 cm (A1).
- The Earth absorbs radiation from the Sun and emits infrared radiation to space (B1). When absorption equals emission, the temperature remains constant (B1). Greenhouse gases absorb some of the outgoing infrared and re-emit it back towards the surface (B1), so less radiation escapes, the Earth emits less than it absorbs, and its temperature rises until a new balance is reached. (B1)