Light · Model

Colour and the spectrum

A piece of plain, colourless glass throws a band of every colour there is onto a wall. It is not making any of them.

Start here

White light is not a colour. It is all of them at once.

A triangular block of glass in a beam of white sunlight throws a band of colour across the wall behind it — red at one end, violet at the other, and every shade in between with no gaps.

Where did the colours come from?

• AThe glass added them — that is what a prism does
• BThey were all in the white light already, and the prism separated them
• CThe white light was turned into coloured light by passing through glass
• DThe colours came off the coloured edges of the prism

What we call white light is a mixture of light of every visible frequency arriving together. Each frequency is seen as a different colour: the lowest frequencies of visible light look red, the highest look violet, and the familiar band of red, orange, yellow, green, blue and violet runs between them. The band is called a spectrum, and it is continuous — the six names are places along it rather than six separate things.

A prism separates them because refraction depends slightly on frequency. All the colours slow down on entering the glass, but the higher frequencies slow a little more than the lower ones, so violet is bent a little further than red. One bend on the way in, another on the way out, and by the time the light reaches the wall the colours have fanned apart. That fanning is called dispersion.

Nothing has been added to the light and nothing has been made. Put a second prism the other way up in the fanned-out beam and the colours come back together into white. The prism sorts; it does not manufacture.

At the bench · a ray box, a prism and a white screen

Send white light in. Sort what comes out.

Change a control to begin

A narrow beam of white light through a triangular glass prism onto a screen. Set what goes in, and set whether a second prism is placed in the beam on the far side.

Commit first. A prism spreads white light into a band of colour on a screen. Where were the colours before the light reached the prism?

• AThey were not there yet — the glass makes them
• BThey were all in the white light already, mixed together
• CThey were in the screen, and the prism brings them out
• DThey form in the air between the prism and the screen

The figure

The band, and which end is which

The two arrows point the same way, and that is the whole of dispersion: the higher the frequency, the more the prism bends it. The six names are handy labels on a band that has no joins in it — between yellow and green there is no line, only a gradual change, and how many names a language uses for it is a matter of custom.

!

Think again

“The prism adds the colour to the light.”

It sorts what was already there. Nothing about the prism is coloured, and the same block of glass turns a red beam into a red patch — no extra colours appear from anywhere. The strongest evidence is the second prism: put it the other way up in the fanned-out beam and the colours run back together into a white patch. If the glass were making colour, a second piece of it would make more, not less. Newton did exactly this experiment for exactly this reason.

“A rainbow has seven colours with lines between them.”

It has as many as you can distinguish, and there are no lines anywhere. The spectrum is continuous: frequency changes smoothly from one end to the other and so does the colour, with no boundary between yellow and green any more than there is a boundary between warm and hot. Seven is a historical count — Newton wanted the number to match the notes of a musical scale, which is why indigo is on the list at all — and different languages divide the same band up differently. What is real is the frequency; the names are ours.

Mastery ladder

Not started yet.

Rungs 3 and 4 you mark yourself.

Rung 1 · Apply the rule

White light passes through a prism and fans out onto a screen. Which colour lands closest to where the undeviated beam would have gone, and why?

• ARed, because it has the lowest frequency of the visible colours and is refracted the least.
• BViolet, because it has the highest frequency and high-frequency light is bent least.
• CGreen, because it is in the middle of the spectrum and so is deflected least.
• DThey all land in the same place, because refraction is the same for every colour.

Rung 2 · The one that catches people

A student says the prism must be adding the colours, because white light goes in and coloured light comes out. Which statement is right?

• AThe student is right — the glass is what makes the colours, which is why plain glass is used and not coloured glass.
• BA second prism the other way up in the fanned beam recombines the colours into white, which shows they were in the white light all along.
• CThe prism does not add colour, but it does turn some of the white light into red and violet as it passes through.
• DThe colours come from the surface of the glass, which is why only the edges of the beam are coloured.

Rung 3 · Explain

Explain how a prism produces a spectrum from white light, using the words frequency, refraction and dispersion.

Your explanationComplete

Rung 4 · Take it somewhere new

A rainbow appears when the Sun is behind you and rain is falling in front of you, and red is always on the outside of the arc. Explain what each raindrop is doing, and why the order of the colours is the same in every rainbow.

Your answerComplete

Key note

White light is a mixture of light of every visible frequency, and each frequency is seen as a different colour — the lowest visible frequencies red, the highest violet. A prism refracts higher frequencies slightly more than lower ones, so the colours fan apart into a continuous spectrum; that fanning is dispersion. A second prism the other way up recombines them into white, which shows the prism separates light rather than creating colour.

Going further

The visible band is a narrow strip of something much wider. Below red in frequency come infrared, microwaves and radio waves; above violet come ultraviolet, X-rays and gamma rays. All of them are the same kind of wave as light, all travel at 300 000 000 m/s in a vacuum, and the only thing separating them is frequency. Our eyes respond to about one octave of it — roughly a doubling of frequency from red to violet — and are blind to everything else, in exactly the way our ears stop at 20 000 Hz.

Never look directly at the Sun — not with your eyes, not through a lens, not through a pinhole camera. A lens gathers sunlight to a point hot enough to scorch paper, and it will do the same to the back of your eye. The damage is painless while it happens and it does not heal.

The bench is a teaching model. The prism angles and the spread of the fan are drawn for clarity rather than calculated: real dispersion through a glass prism is a spread of a couple of degrees, far narrower than the drawing, and the exact spread depends on the glass. The colours are drawn as six separate rays because six is what can be told apart on a screen; the real spectrum is continuous with no gaps and no boundaries. Screen colours can only approximate spectral colours, and the band in the figure is an illustration rather than a measurement. The recombining prism is shown putting the beam back together exactly — to white when the whole spectrum went in, and to one blended colour when only part of it did. A real pair of prisms manages that only approximately.

Lesson content © MrBadmusAI.