Light and Sound
You should know by now that light and sound travel in waves but in different forms - light is transverse while sound is longitudinal. Remember that both types can be reflected and refracted. Sometimes light waves can be refracted so much that no light passes out and its reflected within the material - it’s what makes diamonds so shiny.
Light Waves
Light waves are transverse waves, like all electromagnetic waves. This means they transmit energy through vibrations which move up and down (at right angles) to the direction of wave travel, just like a wave at sea. All light waves can be reflected and refracted. Reflection is when the light wave bounces off an object and travels in other direction. Refraction is when the light wave passes through another material of a different density and changes speed as it passes through it.
Reflection
Waves can ‘bounce off’ a surface in a process which we refer to as reflection. The reflection of light waves is the process that allows us to see the objects around us. Sound waves can also be reflected and is what happens when you hear an echo. If a wave hits a surface at a certain angle, it will be reflected at exactly the same angle. This principle is referred to as the law of reflection which can be summarised as:
the angle of incidence = the angle of reflection
The angle of incidence is measured between the incoming light ray (the incident ray) and the normal. The normal is an imaginary line at 90° to the reflecting surface and is drawn as a dotted line. The angle of reflection is measured between the reflected light ray and the normal.
The refraction of light rays make straws look bent in water
Refraction
Waves travel at different speeds depending on the material they are travelling through. The change of speed when a wave enters a new substance is known as refraction. This happens because denser material (where the particles are closer together) are generally more difficult for waves to travel through, therefore it takes longer to pass between the particles. For example, when light passes from air into glass, its wave speed decreases. The principles of refraction explain why it looks like a straw is bending when placed in a glass of water - light travels faster through air than water so the path of light bends as it enters the water.
When a wave slows down, it’s wavelength also decreases while the frequency stays the same. This makes sense when we refer back to the equation: wave speed = frequency x wavelength - if speed decreases then the wavelength must too.
On a ray diagram, the refracted ray will be emitted at a different angle to the incident ray. If the second material is more dense than the first, the refracted ray bends towards the normal, making the angle of refraction smaller than the angle of incidence. On the other hand, if the second material is less dense, the refracted ray is drawn away from the normal, with the angle of refraction larger than the angle of incidence.
Refractive index
Light travels at different speeds depending on the medium it is travelling through. It travels quickest through space (or a vacuum) where there are no particles around to slow it down. It travels a little slower in air and even more slowly in denser material such as glass or water. The refractive index is a measure of the change in speed of light as it passes from a vacuum into a certain material. It is calculated using the equation:
The higher the refractive index, the more the speed changes as it enters the material. For example, the refractive index of glass is higher than that of water (1.5 in glass compared to 1.3 in water), which means light travels slower in glass than water.
We can also calculate refractive index by measuring the angles of incidence and refraction. This is known as Snell’s law:
Critical angle
If light passes into a denser medium (such as air to water), the light waves slow down. The angle of refraction is smaller than the angle of incidence (the refracted ray moves closer to the normal). On the other hand, if light passes into a less dense medium (such as water to air), the light wave speeds up and the angle of refraction will be greater than the angle of incidence as the refracted ray moves away from the normal. When the angle of refraction is equal to 90o, the angle of incidence is called the critical angle.
If the angle of incidence is greater than the critical angle (i.e. greater than 90o), no light passes through the medium - it is totally internally reflected.
You can find the critical angle, C, of a material using the following equation:
The higher the refractive index, the lower the critical angle. In other words, the harder it is for light to pass through the material, the more likely the light ray is to be totally internally reflected. Diamond has a high refractive index and a low critical angle, which means that when light hits it it is reflected inside the gemstone rather than passing out of it, which is why diamonds are so sparkly.
Uses of total internal reflection
Total internal reflection is used in optical fibres and prisms. Optical fibres are made to be so narrow that the incident light ray always hits the fibre at an angle greater than the critical angle. If the optical fibre is made of glass the critical angle will be around 42o.
This also happens in prisms. When the incident light ray hits the back of the prism, it will be internally reflected at a 90o angle. The light ray emerges from the prism along a normal and continues out of the surface of the prism. Two prisms can be set up like this to function as a periscope.
Sound waves
Sound waves are longitudinal waves, which means they travel by particles vibrating side to side in the same direction as the direction of wave travel. Sound waves are made up of areas of compression (where the particles are bunched up together) and areas of rarefaction (where the particles are spread out). Like all other waves, they can be reflected and refracted.
The frequency range of human hearing is between 20 - 20,000 Hz. Our range of hearing decreases as we get older, so there are frequencies that only younger people can hear that older individuals will be completely deaf to. Frequencies higher than 20,000 Hz are called ultrasound and are outside the range of human hearing.
Oscilloscopes
An oscilloscope is a machine which shows the shape of sound waves. It is connected to a microphone, which detects the sound and converts it to an electrical signal. The oscilloscope then displays the sound wave on a screen. We can use the oscilloscope trace to determine information about the sound, such as its volume and frequency.
The amplitude of a wave corresponds to its volume - the higher the amplitude (i.e. the larger the distance between the rest position and the peak of the wave), the louder the sound. An iron maiden concert will be releasing higher amplitude sound waves than chanting monks.
The frequency of a wave corresponds to its pitch - the higher the frequency (i.e. the larger the number of complete cycles), the higher the pitch. Men generally have deeper voices than women so if two people of different genders spoke into a microphone, the oscilloscope should give traces with different frequencies. Men’s voices have a lower pitch so the sound wave will have a lower frequency, with fewer cycles displayed on the screen.
