Evan's Space

Wonders of Physics


Light and sound wave diagram in different mediums with different density

Light and sound are both waves. So both carry energy from one place to another.

Light, which is part of the electromagnetic spectrum, is a transverse wave, It can travel through a vacuum at speed 3.0 x 108 m/s. As the light travels from an optically less dense medium (air) to an optically denser medium (liquid or glass), the light undergoes refraction and bends towards the normal due to a decrease in speed.

Light: Optically less dense medium to denser medium: 
– speed decreases
– wavelength shorter
– frequency remains constant

Sound is a longitudinal wave. It requires a medium to pass through and it cannot pass through a vacuum. Opposite to light, as the sound travels from a less dense medium (air) into a denser medium (water or solid), the speed increases.

Sound: Less dense medium to denser medium:
– speed increases
– wavelength longer
– frequency remains constant

Refers to the image below to understand how the waves behave in different mediums.
Click here to revise on the calculation of refractive index for light

light and sound

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Using Slinky Coil to demonstrate Transverse and Longitudinal Waves

Though slinky coil is commonly used to demonstrate transverse and longitudinal waves, you must not quote it as an example for either of the waves.

  • Transverse waves are waves in which the direction of the wave is perpendicular to the direction of the vibration of the particles. Examples are light wave, water wave or all the waves in the electromagnetic spectrum (which light is one of the waves.
  • Longitudinal waves are waves in which the direction of the wave is parallel to the direction of the vibration of the particles.  Example is sound wave.


Transverse Waves (slinky coil)

Longitudinal Waves (slinky coil)

Click here to see the simulations of transverse and longitudinal waves.



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Different kind of sounds

Students tend to think that ultrasound is a totally different kind of wave, or it is part of the electromagnetic spectrum, which is wrong!

The different kind of waves below are just different kind of sounds. So the properties of sound apply to all. Ultrasound is also sound wave, just that the frequency is greater than 20 kHz which is outside the range of audible frequencies for us.

different type of sounds

Radio Waves vs Sound – Similarities and Differences

Transverse Waves and Longitudinal Waves


Radio Waves vs Sound – Similarities and Differences

A common misconception by students is to associate sound waves produced by a sound system with radio waves. Signal is transmitted in the air via radio waves from the radio stations in the form of transverse waves. When sound which you hear that is produced from the speaker is sound waves which are longitudinal waves.

radio waves vs sound


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Sonic Boom – Faster than speed of sound

Sonic Boom


When you break the sound barrier (speed of sound is approximately 330 m/s), a sonic boom is created. Take a look at this great video clip on sonic boom. Enjoy =)

Recall speed of light is 3 x 108 m/s. Hence during stormy days, you see the lightning first before you hear the thunder 😉

SR-71 Blackbird is the fastest aircraft in the world, though it is no longer in service. It is the the first aircraft to break the sound barrier.


N2005P1Q24 Sound – Reflection of sound from 2 walls

A man who is standing at a point X between two parallel walls (as shown in the diagram) fires a starting pistol.


He hears the first echo after 0.6s and another one after 0.8s. How long after firing the pistol will he hear the next echo?


View video tutorial to visualise the distance travelled.

Time taken to hear next echo = 0.6 + 0.8 = 1.4s

The distance travelled by sound (left and right) from X is the same during 3rd and 4th echos.

Hence 3rd and 4th echos actually coincide. And time taken is 0.6 + 0.8 = 1.4s.