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'?




Investigating REFRACTION

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).

1. No refraction when a light ray strikes a different medium at 90o to the surface ie 'down' the normal.

The same applies to 3 and 5 for the central ray in the diagram.

However, do not assume nothing happens! There are changes in the wavelength and speed of light, but NOT the frequency of the light rays.

2. Double refraction through a rectangular glass block at the air/glass interfaces, note that when the ray emerges back into air its path is parallel to the original incident ray.

3. Refraction of light rays at the two surfaces of a diverging concave lens.

4. Refraction of light rays at two of the surfaces of a triangular glass or plastic prism.

5. Refraction of light rays at the two surfaces of a converging concave lens.

 


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:

Vacuum (no material substance) and air (very low density) ~3.00 x 108 m/s.

Glass is ~1.97 108 m/s, Perspex ~2.01 x 108 m/s, diamond 1.24 x 108 m/s

 

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.

Diagram above, and the left of the diagram below. Refraction of light B is discussed later, but it is the opposite situation to refraction of light A.

Comparing refractions A and B

The above diagram illustrates the scientific model of the wave theory of refraction of light.

You can think of the parallel lines as representing a series of points of maximum amplitude of the light waves (rather like the crests of waves eg think waves in a ripple tank, waves on the sea or ripples in a pond on throwing a stone in).

Wave theory of refraction A (light rays-waves passing from a less dense to a more dense medium):

Refraction A happens because the wavefronts of the light rays are NOT parallel to the interface boundary so the first section of a wavefront to hit the interface is slowed down on entering the more dense medium. BUT, the other section of the wavefront is moving at the original faster speed and is skewed around producing the change in direction. So it is this decrease in speed that causes the change in direction, and, in this case, the skewing round causes the refracted ray to bend towards the normal.

You can also see that in refraction A the wavelength is decreased as well as the velocity.

The frequency does NOT change.

speed of light = frequency x wavelength, in 'symbolic shorthand'    v = f x λ   (see wave calculations)

If the frequency (f) does not change, then the velocity (v) is directly proportional to wavelength (λ).

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.

As long as the material is transparent and more dense than air you get refraction of light, as long as the incident light rays strike the interface at any angle other than at 90o (angle of the normal).

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.

The waves slow down in shallower water and if they hit the shallower water at an angle, refraction will occur.

The waves slow down in shallower water because of increased friction with the bottom surface of the ripple tank.

In the ripple tank the refraction of the water waves has nothing to do with density, but is caused by increased friction - increase in the 'drag' effect.

You can observe the change in speed and wavelength of water waves in a ripple tank by placing a rectangular plate in to the water at an angle 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.

For ripple tank experiments see Introduction to Waves - Ripple Tank Experiments

 

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.

This happens because on entering the less dense medium, the light waves can speed up causing the change in wave direction - light rays bent away from the normal.

The obvious examples you see in optics experiments are light rays emerging from transparent plastic blocks or triangular and rectangular glass prisms, and passing out into less dense air.

Refraction of light rays B from a more dense medium to a less dense medium

Diagram above and right of diagram below. Diagram refraction A has been previously discussed, but here refraction B is the opposite situation to refraction A.

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):

Refraction B happens because the wavefronts of the light rays are NOT parallel to the interface boundary so the first section of a wavefront to hit the interface is speeded up on entering the less dense medium. BUT, the other section of the wavefront is moving at the original slower speed and is skewed around producing the change in direction. So it is this increase in speed that causes the change in direction, and, in this case, the skewing round causes the refracted ray to bend away from the normal.

You can also see that in refraction B the wavelength has increased as well as the velocity.

The frequency does NOT change.

speed of light = frequency x wavelength, in 'symbolic shorthand'    v = f x λ  (see wave calculations)

If the frequency (f) does not change, then velocity (v) is directly proportional to wavelength (λ).

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.

The waves speed up in deeper water and if they hit the deeper water at an angle, refraction will occur.

The waves speed up in deeper water because of decreased friction with the bottom surface of the ripple tank.

In this example the refraction has nothing to do with density, but is refraction caused by decrease in friction - reduction of the 'drag' effect.

You can observe this in a ripple tank by placing a rectangular plate in to the water at an angle 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.

For ripple tank experiments see Introduction to Waves - Ripple Tank Experiments

 

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.

There is no refraction because the wavefronts of the light rays are parallel to the interface boundary so no section of a wavefront is skewed round because another section is NOT being slowed down or speeded up on entering another transparent medium of different density.

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.

angle of incidence i1 > angle of refraction r1

2. glass ==> air: The light beam speeds up in the less dense air, so the ray bends away from the normal.

angle of incidence i2 < angle of refraction r2

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.

I remember it as VIBGYOR sounded phonetically for violet, indigo, blue, green, yellow, orange and red.

Important trend to know: violet === decreasing frequency, increasing wavelength ===> red

(The sequence is preceded by invisible ultra-violet light and succeeded by invisible infra-red light.)

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.

The shorter the wavelength of the light colour, the slower the light colour travels in dense materials.

The slower the light travels the more it is refracted when passing through a media boundary to a more dense medium - the smaller the angle of refraction.

Therefore the shorter wavelength violet light is refracted much more than the longer wavelength red light.

The rule is illustrated by the diagram below and it should correspond to the diagram above on how to produce the spectrum of visible light with a triangular glass/Perspex prism.

It is this difference in the degree of refraction of each colour that allows a prism to separate the colours and produce the 'rainbow' of light we call the visible spectrum.

The wave theory of refraction explains why you can produce the visible spectrum in this way.

Section Refraction A explains the 1st refraction (air to Δ glass prism) and Refraction B explains the 2nd refraction (Δ glass prism to air).

Therefore when white light passes through the ∆ prism all the different colours separate out to give the visible spectrum.

The theory behind the formation of the visible spectrum (refer to diagram below too)

This spreading out into the different colours due to different refraction angles is called dispersion to give what we refer to as the visible spectrum of light.

The shape of the prism allows two sets of refractions to take place and give a greater spread of the different wavelengths of the colours.

When the light enters the prism the rays bend towards the normal after the boundary - the first refraction is from a less dense to a more dense medium (waves slowing down).

When the light exits the prism the rays bend away from the normal after the boundary - the second refraction is from a more dense to a less dense medium (waves speeding up).

At both boundaries, the colours have different speeds and so diffract at different angles - that's what causes them to spread out or disperse. In any liquid or solid material ...

the shorter the wavelength the slower the rays-waves move in the material, and ...

... the shorter the wavelength the greater the change in speed, so the greater the angle the light rays-waves are deviated or diffracted.

-


The formation of a rainbow - you need to refer to the diagram above too.

The formation of a rainbow can be explained by considering a water droplet to behave like a prism. It involves refraction, dispersion and reflection. I've just used a red, green and blue ray to give (I hope!) a clear explanation of 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.

At point B, total internal reflection occurs inside the water drop, but no 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.

The resulting dispersion due to two sets of refractions, creates the rainbow effect from trillions of little rain drops acting in 'harmony'!

If you understand the prism experiment to produce the visible spectrum, you should have no trouble in understanding how a rainbow is formed. There is just a little extra complication with the internal reflection.

A non GCSE science footnote: The photograph is actually of a double rainbow. The brighter primary rainbow on the left is formed as explained above. However, if the sun is at a low angle, you can sometimes get a second internal reflection and the second dispersing refraction produces a fainter secondary rainbow. If you look carefully, the colours are reversed. The two figures were added for human interest when taking the photograph at Blackrock, Co. Louth, in Ireland in 2005.

 

mention prism spectroscope emission spectroscopy?


Investigating DIFFRACTION

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'.

experiments - investigation of refraction using a ray box

carrying out refraction investigations using a glass block or triangular prism

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