VISIBLE LIGHT - ray box experiments - prisms and spectrum
Refraction and diffraction of light, diagrams and explanations
See also REFLECTION experiments
Doc Brown's Physics Revision Notes
Suitable for GCSE/IGCSE Physics/Science courses or their equivalent
This page will answer many questions e.g.
How do draw ray diagrams for visible light ray refraction experiments?
How do we explain refraction by light waves?
How do you explain diffraction of light waves?
Why and how does a triangular prism produce a 'rainbow of colours'?
Know and understand that waves can undergo a change of direction when they are refracted at an interface.
Refraction: The bending of the light ray at an interface between two media of different density.
Some sketches of light rays passing through transparent blocks or prisms (glass or Perspex).
Know that light waves are not refracted if travelling along the normal (diagram 1 below).
Looking in detail at two refractions situations involving visible light
I refer to them as refraction A and refraction B
The speed of light varies with the medium it is travelling through and this has important consequences for the behaviour of light when passing through a boundary between two transparent media of different densities.
Examples of the speed of light in different materials:
Waves travel at different speeds in different materials and this can result in a change of direction as the waves pass through a boundary from one material to another.
This change in direction at the boundary between two media is called refraction.
Refraction A: When light waves passing through a less dense medium, hit a boundary interface (not at 90o to it), and on entering a more dense medium, the light waves 'bend towards the normal' ie refraction occurs.
Refraction of light rays A from a less dense medium to a more dense medium
This happens because on entering the more dense medium, the light waves are slowed down causing the change in wave direction at the boundary interface - ray bent towards the normal.
Comparing refractions A and B
The above diagram illustrates the scientific model of the wave theory of refraction of light.
You can also see that in refraction A the wavelength is decreased as well as the velocity.
The bigger the change in speed the bigger the change in direction - the greater the angle of refraction.
The obvious examples in your school/college laboratory are the optics experiments you do in passing light rays passing from air into more dense transparent triangular or rectangular plastic/glass blocks or triangular prisms.
You see this effect in ripple tank experiments when you abruptly go from deeper water to shallower water the waves will change direction towards the normal.
Extra notes on refraction A - with refraction experiments, and real life too, you often get reflection too e.g.
Light rays passing from a less dense medium to a more dense transparent medium.
You ay 2 refracted. You do get some reflection too, ray 1. You see reflections on water and in shop windows.
Refraction B: When light waves from a more dense medium, hit a boundary interface (not at 90o to it), and on entering a less dense medium, the light waves 'bend away from the normal' i.e. refraction occurs.
Refraction of light rays B from a more dense medium to a less dense medium
Comparing refractions A and B
The above diagram illustrates the scientific model of the wave theory of refraction.
Wave theory of refraction B (light rays-waves passing from a more dense to a less dense medium):
You can also see that in refraction B the wavelength has increased as well as the velocity.
The bigger the change in speed the bigger the change in direction - the greater the angle of refraction of the light rays.
You see this effect in ripple tank experiments when you abruptly go from shallower water to deeper water the waves will change direction away from the normal.
Extra notes on refraction B - with refraction experiments, and real life too, you often get reflection too e.g.
Light rays passing from a more dense medium to a less dense transparent medium.
Note: Ray 2 refracted. You do get some reflection too, ray 1. For glass, if the internal angle of incidence is over 43o you get total internal reflection. You can investigate this with simple ray box experiments with glass blocks. This sort of reflection is part of the explanation of the formation of a rainbow.
The origin of the optical illusion when observing an object at an angle in water.
When you observe an object half in water and half in air e.g. poking a stick into still water, you see a 'bent' distorted image, because, the light rays from the object are bent at the air-water interface because of refraction. If you think of the actual object at the start of the incident ray, you think the object is higher up to the right compared to where it actually is - just follow the line back from the emerging refracted ray. You are dealing with a 'real' (deeper) and 'apparent' (shallower) depth - can be a bit disconcerting! and take care when diving into swimming pools or off rocks at the seaside - the bottom might not be quite where you think it is!
You can observe both refraction situations A and B when doing the ray box light experiments with a transparent rectangular block of glass or Perspex.
the green dotted vertical lines are the two normals.
angles 1 and 3 are angles of incidence
angles 2 and 4 are angles of refraction
Remember, when a ray enters a more dense medium (air ==> glass), the ray bends towards the normal, and on entering a less dense medium (glass ==> air) the ray bends away from the normal
there maybe a little reflection of incident rays 1 and 3, but most of the rays are refracted.
If the waves hit the interface at an angle of 90o (perpendicular, diagram 1 above) to the interface between the two mediums, there is still a change in speed and wavelength, but there is NO change in direction, NO refraction and the wave frequency remains the same. In all the other cases 2 to 4, refraction can occur.
Wave theory to explain what happens and what doesn't happen.
A: When the waves pass from a less dense medium to a more dense medium the waves decrease in velocity at the media boundary and the wavelength also decreases.
B: When the waves pass from a more dense medium to a less dense medium the waves increase in velocity at the media boundary and the wavelength also increases.
In both cases the frequency of the light remains unchanged and in both cases no refraction takes place - no change in direction.
You can observe this in a ripple tank by placing a rectangular plate in to the water parallel to the waves and you can see these changes in wavelength and speed. BUT, by using a stroboscope, you can show the frequency does not change.
The visible spectrum of light and triangular prism experiments
The refraction of a single wavelength light ray by a 600 triangular prism.
You get refraction twice as the laser beam passes through two boundaries.
From the diagram on the right:
1. air ==> glass: The light beam slows down in the more dense glass, so the ray bends towards the normal.
2. glass ==> air: The light beam speeds up in the less dense air, so the ray bends away from the normal.
Note that when using a single wavelength of light from a laser beam there is no splitting of the colour - contrast the above diagram with the diagram below showing the dispersion of white light into all its constituent colours - the visible spectrum.
The production of the visible spectrum with a triangular prism - white light is dispersed into all its colours.
The different colours we experience are due to differences in photon energy, wavelength and frequency (all of which are related), and this is irrespective of what medium the light travels through - vacuum, air, glass, anything transparent.
However, in a vacuum or in air (very low density) all the colours have the same speed.
BUT, in dense transparent materials like glass the speed of each colour actually varies.
A nice visible light spectrum from a glass pendant hanging up by a brightly sunlit window.
In the past 60o triangular prisms have been used in emission spectrometers for analysing light from high temperature sources like stars. However, these days diffraction gratings are used to separate the different wavelengths of visible light.
The formation of a rainbow - you need to refer to the diagram above too.
The formation of a rainbow can be partly explained by considering a water droplet to behave like a prism. It involves refraction and reflection. I've just used a red, green and blue ray diagram to give (I hope!) the basic ideas to explain how a rainbow is formed.
Imagine a ray of sunlight entering the water drop at point A. On going from less dense air to more dense water refraction occurs at the boundary. The shorter wavelength blue light slows down more and refracts at a greater angle - the order being blue > green > red. You may of course get some reflection too, but lets concentrate on the refracted rays.
At point B, some internal reflection occurs inside the water drop (and maybe some refraction).
At point C a second refraction takes place as the rays move from a more dense medium to a less dense medium. A second dispersion takes place to produce the final rainbow effect of the visible spectrum. You may also get internal reflection too.
If you understand the prism experiment to produce the visible spectrum, you should have no trouble in having some idea on how a rainbow is formed - but it is not a true visible spectrum - there are many complications which we don't need to go into in detail. BUT, the ray diagram explains the general idea of why you get a separation of white light into the colours of a rainbow - due to different angles of refraction of the different colours.
A ray box system is no good for investigating the diffraction of light.
Instead, you can use water waves in a ripple tank.
Remember both light and 'ripples' are transverse waves.
So, in effect, you are using water waves to model light waves.
(You can of course investigate reflection and refraction with the ripple tank too.)
Diffraction is the effect of waves spreading out when passing through a gap or passing by a barrier. In effect, waves go round corners! and it doesn't matter if its sound, light or water waves - they all diffract and bend round corners! The effect is so small with light (tiny wavelength), you don't notice it, but you see water waves bending around walls of a harbour and you can hear sounds from round a corner.
You should appreciate that significant diffraction only occurs when the wavelength of the wave is of the same order of magnitude as the size of the gap or obstacle.
A: There is a relatively small diffraction effect when waves pass through a wide gap that is much bigger than the wavelength of the wave.
B: You get the maximum spreading or diffraction when the light waves pass through a gap of similar size to the wavelength of the incident waves.
You can see these effects with transverse water waves at the seaside as waves hit the protective walls of a harbour BUT you need a very tiny slit to observe diffraction with light waves because of their tiny wavelength.
Can you observe the diffraction of light?
When you hold up a fine needle towards a bright light, the edges aren't quite sharp because the light rays are diffracting (bending) around the pin's surface.
Check out your practical work you did or teacher demonstrations you observed, all of this is part of good revision for your module examination context questions and helps with 'how science works'.
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