Waves

Common error: Do not assume sound behaves like light. Light can cross a vacuum but sound cannot, because sound must be passed on by particles.

Exam tip: Learn the pairs: loudness goes with amplitude, and pitch goes with frequency.

Common error: Check the prefixes: 15 kHz is 15 000 Hz, not 15 Hz. Convert to hertz before comparing with the 20 Hz – 20 000 Hz range.

Common error: In this method the sound travels only one way, so do not halve or double the distance. Halving is needed only for echoes.

Common error: Always measure the angles of incidence, reflection and refraction from the normal. If an angle is given to the surface, subtract it from 90° first.

Exam tip: The image distance is measured from the mirror. The distance between the object and its image is double this.

Common error: A ray along the normal is not deviated, but it still changes speed. Refraction is caused by the change in speed.

Common error: Red is deviated least and violet most. The order of the spectrum is red, orange, yellow, green, blue, indigo, violet.

Exam tip: Both conditions are needed for total internal reflection. Saying only 'a large angle' does not earn the mark; compare the angle of incidence with the critical angle.

Exam tip: Learn one order (radio, microwave, infrared, visible, ultraviolet, X-ray, gamma). Wavelength decreases and frequency increases along it. Read the question to see which direction it asks for.

Common error: High-frequency waves such as gamma rays do not travel faster than radio waves. They carry more energy, but their speed in a vacuum is identical.

Common error: Do not assume sterilising means heating. Gamma sterilisation works by killing microorganisms with ionising radiation, not by making the instruments hot.

Exam tip: Thermal imaging, remote controls, electric grills and some optical fibres all use infrared.

Exam tip: Write v = fλ before substituting. Check that m/s comes out as (1/s) × m.

Common error: Do not confuse the direction of vibration with the direction the energy travels. For a longitudinal wave the two directions are parallel.

Common error: Read whether the question asks for the distance to the mirror or the distance to the image.

Exam tip: Frequency is the number of complete waves per second. Divide the number of oscillations by the time taken.

Exam tip: When explaining a safety precaution, state both the hazard and how the precaution reduces it. Each idea is worth a separate mark.

Common error: Rearrange v = fλ correctly. The wavelength is v divided by f, not v multiplied by f.

Exam tip: The distance from the person to their image is twice the distance from the person to the mirror.

Common error: The prism does not add colour to the light. The colours are already present in white light; the prism only separates them because each colour travels at a different speed in glass.

Exam tip: When describing how sound is produced, always start with something vibrating. Then say how the vibration is passed through the medium, for example as compressions and rarefactions in the air.

Common error: An echo time covers the journey there and back. Halve the total distance to find how far away the reflecting surface is.

Exam tip: Rearrange v = fλ before putting numbers in. A wavelength of under a metre is sensible for a musical note in air.

Exam tip: Compare the object distance with f and 2f first. Beyond 2f gives a diminished image, between f and 2f gives a magnified image, and inside f gives a virtual image.

Exam tip: A magnifying glass always gives a virtual image. You can only see a virtual image by looking through the lens; it cannot be caught on a screen.

Exam tip: Match each hazard to its region: microwaves give internal heating, infrared gives skin burns, ultraviolet damages surface cells and eyes, and X-rays and gamma rays cause cell mutation.

Exam tip: When a wave enters a new medium, its frequency never changes. Speed and wavelength change together.

Common error: A crest-to-trough distance is half a wavelength. One full wavelength runs from crest to crest.

Common error: Turning the volume down does not change the frequency or the wave speed. Only the amplitude changes.

Exam tip: Frequency and wavelength run in opposite directions along the spectrum. Check which one the question asks for before writing the order.

Common error: Dividing the distance by 10 instead of 9 gives the wrong wavelength. Count the gaps between crests, not the crests themselves. Convert cm to m before giving the speed in m/s.

Common error: Changing the source does not change the wave speed. The medium sets the speed, so f and λ change in opposite directions.

Common error: In a vacuum the speed is fixed, so frequency and wavelength are inversely proportional. If one doubles, the other halves.

Exam tip: For any ultrasound use, the same idea earns the marks: a pulse is sent, it reflects at a boundary, and the echo time gives a distance. Remember to halve the total time, because the pulse travels there and back.

Common error: Do not think that denser materials 'block' sound and slow it down. Sound is faster in solids than in liquids, and faster in liquids than in gases.

Exam tip: Compression means particles are close together (higher pressure). Rarefaction means particles are spread out (lower pressure). The particles only vibrate about fixed positions.

Common error: Ultrasound is defined by its frequency (above 20 kHz), not by its loudness. It is still a sound wave and still needs a medium.

Exam tip: Check that your calculator is in degree mode. Then check that your refractive index is greater than 1; if it is not, you have probably inverted the ratio.

Exam tip: Use n = 1/sin c, so c = sin⁻¹(1/n). A denser material has a larger n and therefore a smaller critical angle.

Common error: Short sight is corrected with a diverging lens and long sight with a converging lens. Learn where each eye defect focuses the light, not just the lens type.

Common error: Mega means 10⁶. Write 95 MHz as 95 × 10⁶ Hz before substituting into λ = v/f.

Exam tip: For mobile phones and wireless internet, give two reasons: microwaves pass through walls, and only a short aerial is needed.

Common error: Light follows an optical fibre by total internal reflection at the glass surface. Diffraction plays no part in guiding it.

Common error: A wider gap does not give more diffraction. What matters is how the gap width compares with the wavelength.

Exam tip: Say that the wavelength is unchanged by diffraction. Link the amount of spreading to how the gap size compares with the wavelength.

Exam tip: Convert milliseconds to seconds (×10⁻³) before calculating. Remember that an echo travels to the reflector and back, so halve the total distance.

Exam tip: Keep the unrounded value of n (1.326) in your calculator for part (b). Using a rounded value can change the final digit of an answer that is given to more significant figures.

Exam tip: Keep your calculator in degree mode. Carry the unrounded value of n into the next part.

Exam tip: Use the speed of sound for the medium the wave actually travels through. Here that is sea water, which is much faster than air.

Common error: Do not divide the distance by the difference in speeds. Work out each travel time separately, then subtract the two times.

Exam tip: For light leaving a dense medium, find the critical angle first. If the angle of incidence is larger than the critical angle, the ray is totally internally reflected.

Exam tip: When two faces meet at 90°, their normals are also at 90°. The angle of incidence at the second face is therefore 90° minus the angle of refraction at the first face.

Exam tip: Check that an answer makes sense. A frequency around 10¹⁸ Hz must give a tiny wavelength, roughly the size of an atom.

Exam tip: Diffraction is greatest when the wavelength is comparable to the size of the obstacle or gap. Long-wavelength radio waves bend around hills; short-wavelength waves do not.

Exam tip: Check that your answer is greater than 1. A refractive index below 1 cannot be right.

Exam tip: n = speed in air ÷ speed in the medium. Because the frequency is constant, the wavelength changes by the same factor as the speed.

Exam tip: In part (c), the ray inside the glass is the hypotenuse of a right-angled triangle. The block thickness is the side next to angle r.

Common error: Total internal reflection needs the angle of incidence to be greater than the critical angle. At exactly the critical angle, the refracted ray travels along the boundary. Below it, most of the light refracts out of the glass.