Sound is a longitudinal wave that needs a medium. Electromagnetic waves are transverse and travel through a vacuum at 3.0 × 10⁸ m s⁻¹.
This lesson is part of SPM Physics waves. It follows reading interference patterns.
How do sound and electromagnetic waves compare?
| Property | Sound | Electromagnetic waves |
|---|---|---|
| Type of wave | Longitudinal | Transverse |
| Needs a medium | Yes | No |
| Speed in air | About 340 m s⁻¹ | About 3.0 × 10⁸ m s⁻¹ |
| Example | A guitar note | Light, radio, X-rays |
| Reflect, refract, diffract, interfere | Yes | Yes |
The first three rows show where the two differ. The last row is what they share, which is why both follow v = fλ.
Worked example: distance to a lightning strike
An original storm shows a flash, and the thunder arrives 6.0 s later. The light arrives almost instantly, so the time is the travel time of the sound.
Distance = speed × time = 340 × 6.0 = 2040 m, which is about 2.0 km.
Worked example: a ship’s depth sounder
A sounder sends a pulse down to the seabed and receives the echo after 0.80 s. The speed of sound in seawater is 1500 m s⁻¹.
The pulse travels down and back, so the total distance is 1500 × 0.80 = 1200 m. The depth is half of that, 600 m.
The mistake that costs marks
The slip is to use the whole echo time as the one-way time. The echo covers the journey twice.
| Step | Wrong | Right |
|---|---|---|
| Distance | 1500 × 0.80 = 1200 m | 1500 × 0.80 ÷ 2 = 600 m |
| Meaning | Depth | Down-and-back distance |
A second slip is to write that sound needs no medium, or that light is longitudinal. Match each wave type to its row in the table.
Why are the speeds so different?
Sound passes from particle to particle, so its speed depends on the medium. Electromagnetic waves are disturbances in electric and magnetic fields, so they do not need particles.
Over 3.0 km, light takes 3000 ÷ 3.0 × 10⁸ = 1.0 × 10⁻⁵ s. Sound takes 3000 ÷ 340 ≈ 8.8 s. The difference explains why you see fireworks before you hear them.
Check yourself
A student stands 85 m from a wall and claps once. Find the time before the echo returns, using 340 m s⁻¹ for the speed of sound.
Answer
The sound travels to the wall and back: 2 × 85 = 170 m.
t = distance ÷ speed = 170 ÷ 340 = 0.50 s.
What to study next
Put all the quantities onto graphs in interpreting wave graphs without confusing axes. Then try the waves practice set.
To build comparison answers with a teacher, see online one-to-one Physics tuition.