Summary
To fully understand the propagation of electromagnetic waves, you will need to study the next section on electromagnetism. However, it is enough to use what we know about other waves to understand the wave nature of light.
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Electromagnetic spectrum
Light is part of the electromagnetic spectrum. Electromagnetic waves are disturbances in electric and magnetic fields. They are not mechanical waves so require no medium.
Reflection
When light strikes the boundary between two media, part of it reflects.
Refraction
While part of the light reflects, the remainder will refract into the medium as it changes speed and direction. The angle of refraction can be calculated by Snell's law, where n is the refractive index (ratio of the speed of light to its speed in the material).
\({\sin i \over \sin r}={n_2 \over n_1}\)
NB: n1 is often equal to 1, as light usually enters a medium from air (where its speed is equal to that in a vacuum).
Total internal reflection
Total internal reflection occurs when light travels into a less optically dense medium at an angle of incidence that exceeds the critical angle - all of it reflects.
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Dispersion
The refractive index of glass depends on the wavelength (colour) of light. It is largest for violet light, but a useful reminder can also be "blue bends best" (rather than red!).
White light disperses into all the colours of the visible light spectrum when it passes through a prism.
Interference
Two coherent sources of light (equal frequencies) will interfere where they overlap. This causes the formation of bright and dark patches called fringes.
Diffraction
Light spreads out when it passes through a slit. The angle of spreading depends on the width of the slit:
\(d\sin \theta=\lambda\)
Test yourself
Use quizzes to practise application of theory.
START QUIZ!
Which of the following lists includes the em waves in order of increasing wavelength
that´s just the way it is
The velocity of em waves in a vacuum:
The velocity of em radiation is 3 x 108 ms-1
The electric and magnetic fields in an em wave are:
A ray of light is reflected off an inclined mirror as shown.
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The angle of reflection is
A ray of light is reflected by two mirrors as shown
What is the angle of reflection at the second mirror?
The diagram shows a ray of light incident on 4 blocks.
Which block is glass?
Refractive index is 1.5. C and D refract too much A not enough
The diagram shows 4 possible/impossible paths for light travelling through a glass block
The red line shows the path of red light. Which is the path of blue?
Blue light is refracted more but the emerging ray is still parallel to the incident ray.
A ray of light passes across a boundary as shown.
What is the refractive index of the boundary?
sini/sinr
A ray of light is incident on a glass - air boundary at 45° as shown
Which ray represents the light after hitting the boundary?
The critical angle for glass is 43.8° any angle above this will cause total internal reflection
Water has refractive index 1.3. What is the critical angle for water?
sin-1(1/n)
Visible diffraction effects can be observed when light passes through an opening of width
There would be diffraction for 1 nm but the amount of light passing would be too little to see
The diagram represents the interference of light passing through two slits.
The path difference between the waves at point A is:
The second region of destructive interference
The diagram represents the interference of light passing through two slits.
The phase difference between the waves at point A is:
Out of phase so phase difference is π
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