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
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About the diagram
Have you already checked out An Introduction to Reflection, Refraction and Dispersion?
It is the opening page of our Reflection, Refraction and Dispersion Series and contains masses of useful information. This is the table of contents:
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
Wave diagram
Medium
Angle of incidence
Angle of refraction
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