Refraction of a Wave of Light

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The diagram shows an incident wave of light with a wavelength corresponding with red approaching the boundary between air and glass.


  • As the wave crosses the boundary into the glass it bends towards the normal (the dotted green line).
  • Refraction is towards the normal because the ray travels from air, the faster, less optically dense medium with a smaller refractive index into the glass, a slower, more optically dense medium with a higher refractive index.

Description

Refraction of a Wave of Light

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
Yes! Refraction occurs as light crosses the boundary between transparent media with different refractive indices.
Blue and violet are two of the colours in the visible spectrum with the shortest wavelengths.
The normal is an imaginary line drawn on a ray-tracing diagram perpendicular to, so at a right angle (90 degrees), to the boundary between two media.

About the diagram

Overview of this page

  • This page takes a close look at the refraction of a ray of red light at the boundary between air and glass.
  • Related topics including reflection and dispersion are covered on other pages of this series.
  • Introductions to the terms refractive index and the law of refraction (sometimes called Snell’s law) also appear on later pages in the series.

An overview of refraction

  • Refraction refers to the way that light (electromagnetic radiation) changes direction and speed as it travels from one transparent medium into another.
  • Refraction takes place as light travels across the boundary between different transparent media and is a result of their different optical properties.
  • When light is refracted its path bends and so changes direction.
  • The effect of refraction on the path of a ray of light is measured by the difference between the angle of incidence and the angle of reflection.
  • As light travels across the interface between different media (such as between air and glass) it changes speed.
  • Depending on the media through which light is refracted, its speed can increase or decrease.

The diagram

The diagram shows an incident wave of light with a wavelength corresponding with red approaching the boundary between air and glass.

  • As the wave crosses the boundary into the glass it bends towards the normal (the dotted green line).
  • Refraction is towards the normal because the ray travels from air, the faster, less optically dense medium with a smaller refractive index into the glass, a slower, more optically dense medium with a higher refractive index.
  • The diagram also shows that the wavelength of the light gets shorter and reduces speed as it crosses the boundary into the glass.
  • Notice that the frequency and so the colour of the wave remain the same.

Remember:

  • When light passes from one medium to another, the wavelength changes but the frequency and so the colour remains the same.
  • In terms of the physics of light, it is the frequency of a wave that determines the colour seen by an observer.
  • The colour of the ray of light doesn’t change as it crosses the boundary between different transparent media because although the wavelength gets shorter and the wave reduces speed the frequency is unchanged.

About wavelength, frequency and colour

  • To understand the relationship between wavelength and frequency at the boundary between two different media, imagine standing in front of the block of glass (as shown in the diagram) watching the waves go past.
  • Now imagine that as the wave enters the glass from the left that its speed is halved. The result is that its wavelength will be halved. that means the length of each wave will be half as long.
  • But if the wavelength is halved then each one will pass by at double the pace. This means that their frequency will double as twice as many waves rush pass by.
  • The outcome is that because the frequency at which the waves pass by remain the same the colour is unchanged.

A useful rule for frequency and colour

When light crosses the boundary into a medium with higher optical density the result is: shorter wavelength, slower speed, same frequency.

When light crosses the boundary into a medium with lower optical density the result is: longer wavelength, faster speed, same frequency.

Some key terms

Wavelength is a measurement from any point on the path of a wave to the same point on its next oscillation. The measurement is made parallel to the centre-line of the wave.

  • The wavelength of an electromagnetic wave is measured in metres.
  • Each type of electromagnetic radiation, such as radio waves, visible light and gamma waves,  forms a band of wavelengths on the electromagnetic spectrum.
  • The visible part of the electromagnetic spectrum is composed of the range of wavelengths that correspond with all the different colours we see in the world.
  • Human beings don’t see wavelengths of visible light, but they do see the spectral colours that correspond with each wavelength and the other colours produced when different wavelengths are combined.
  • The wavelength of visible light is measured in nanometres. There are 1,000,000,000 nanometres to a metre.

The angle of refraction measures the angle to which light bends as it passes across the boundary between different media.

  • The angle of refraction is measured between a ray of light and an imaginary line called the normal.
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
  • See this diagram for an explanation: Refraction of a ray of light
  • If the boundary between the media is curved, the normal is drawn perpendicular to the boundary.

The frequency of electromagnetic radiation (light) refers to the number of wave-cycles of an electromagnetic wave that pass a given point in a given amount of time.

  • Frequency is measured in Hertz (Hz) and signifies the number of wave-cycles per second. Sub-units of Hertz enable measurements involving a higher count of wave-cycles within a single second.
  • The frequency of electromagnetic radiation spans a broad range, from radio waves with low frequencies to gamma rays with high frequencies.
  • The wavelength and frequency of light are closely linked. Specifically, as the wavelength becomes shorter, the frequency increases correspondingly.
  • It is important not to confuse the frequency of a wave with the speed at which the wave travels or the distance it covers.
  • The energy carried by a light wave intensifies as its oscillations increase in number and its wavelength shortens.

In physics and optics, a wave diagram uses a set of drawing conventions and labels to describe the attributes of light waves including wavelength, frequency, amplitude and direction of travel.

  • A wave diagram illustrates what happens to a wave as it encounters different media or objects.
  • The aim of a wave diagram is to demonstrate optical phenomena such as reflection and refraction.

The angle of incidence measures the angle at which incoming light strikes a surface.

  • The angle of incidence is measured between a ray of incoming light and an imaginary line called the normal.
  • See this diagram for an explanation: Reflection of a ray of light
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
  • If the boundary between the media is curved, then the normal is drawn at a tangent to the boundary.

Incident light refers to light that is travelling towards an object or medium.

  • Incident light refers to light that is travelling towards an object or medium.
  • Incident light may come from the Sun, an artificial source or may have already been reflected off another surface, such as a mirror.
  • When incident light strikes a surface or object, it may be absorbed, reflected, refracted, transmitted or undergo any combination of these optical effects.
  • Incident light is typically represented on a ray diagram as a straight line with an arrow to indicate its direction of propagation.

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