Waves
Unit 3 of Cambridge IGCSE Physics 0625. Transverse and longitudinal waves, reflection, refraction, total internal reflection, lenses, the electromagnetic spectrum and sound.
What you need to know
- A transverse wave vibrates at right angles to the direction of travel. A longitudinal wave vibrates along it. Sound in air is longitudinal.
- When a wave refracts, the frequency never changes. The speed and the wavelength change.
- Light bends towards the normal going into a denser medium, and away from the normal coming out.
- Total internal reflection happens only when light travels from denser to less dense and the angle of incidence is greater than the critical angle.
- The electromagnetic spectrum in order: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. All travel at 3 x 10^8 m/s in a vacuum.
- Sound needs a medium. It cannot travel through a vacuum.
Equations
- v = f x wavelength
- n = sin i / sin r (refractive index)
- n = 1 / sin c (critical angle)
- speed of sound in air is about 330 to 340 m/s
Waves questions students ask
What is the difference between transverse and longitudinal waves?
In a transverse wave the vibration is at right angles to the direction of travel, like light or water waves. In a longitudinal wave the vibration is along the direction of travel, like sound.
Does the frequency change when a wave refracts?
No. The frequency is set by the source and never changes at a boundary. The speed changes and the wavelength changes with it.
Why does light bend when it enters glass?
It slows down in the denser medium. If it arrives at an angle, one side of the wavefront slows first, which turns the wave towards the normal.
Where do you measure the angle of incidence from?
From the normal, the line at right angles to the surface. Measuring from the surface itself is one of the most common ways students lose marks.
What is the critical angle?
The angle of incidence in the denser medium that produces an angle of refraction of exactly 90 degrees. Above it, no light escapes.
When does total internal reflection happen?
Only when light travels from a denser medium into a less dense one, and the angle of incidence is greater than the critical angle.
How does an optical fibre work?
Light enters at a shallow angle and hits the wall above the critical angle every time, so it totally internally reflects along the fibre instead of escaping.
What is the order of the electromagnetic spectrum?
Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. Wavelength decreases and frequency increases along that list. All travel at 3 times 10 to the 8 metres per second in a vacuum.
Why can sound not travel through a vacuum?
Sound is a longitudinal wave that needs particles to compress and expand. A vacuum has no particles, so there is nothing to carry it.
How do you calculate the speed of a wave?
Speed equals frequency times wavelength. Keep the units consistent, hertz and metres give metres per second.
Mistakes that cost marks
- Saying the frequency changes during refraction. It does not.
- Drawing the ray bending the wrong way at the boundary.
- Measuring angles from the surface instead of from the normal.
- Forgetting that total internal reflection needs the ray to start in the denser medium.
- Writing that light is a longitudinal wave.
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