White Light Is Seven Colours Travelling Together
Learn how a prism disperses white light into VIBGYOR, how a spinning disc and a second prism recombine it, the primary and secondary colours of light, and why a red object looks black in green light.
Is white light really a single colour?
No — white light is a mixture of seven colours travelling together, and a glass prism can separate them. Sunlight passing through a prism spreads into a band of colours on the wall behind it.
Once you know that, the colour of everything you see becomes a question of which of those seven reach your eye. This page covers everything in the ICSE Class 7 Physics chapter's second half: dispersion by a prism, recombining the spectrum, the primary and secondary colours of light, and the colour of objects.
Once you know that, the colour of everything you see becomes a question of which of those seven reach your eye. This page covers everything in the ICSE Class 7 Physics chapter's second half: dispersion by a prism, recombining the spectrum, the primary and secondary colours of light, and the colour of objects.
What happens when white light passes through a prism?
The prism disperses it — splitting white light into its component colours, which emerge as a band called the spectrum.
The seven colours in order, remembered as VIBGYOR, are:
- Violet
- Indigo
- Blue
- Green
- Yellow
- Orange
- Red
The order is not arbitrary. Each colour is bent, or deviated, by a different amount as it passes through the glass: violet is bent the most and red the least, so violet appears at one edge of the band and red at the other.
A rainbow is the same effect on a huge scale, with raindrops doing the work of the prism. Oil films on a puddle and the flash of colour from a cut-glass bangle are everyday versions too.
The usual misconception is that the prism adds the colours. It does not create anything — the colours were present in the white light all along, and the glass only sorts them by how much each is deviated.
So dispersion is a separating process, and that is why the order of the colours is always the same for a given prism.
The seven colours in order, remembered as VIBGYOR, are:
- Violet
- Indigo
- Blue
- Green
- Yellow
- Orange
- Red
The order is not arbitrary. Each colour is bent, or deviated, by a different amount as it passes through the glass: violet is bent the most and red the least, so violet appears at one edge of the band and red at the other.
A rainbow is the same effect on a huge scale, with raindrops doing the work of the prism. Oil films on a puddle and the flash of colour from a cut-glass bangle are everyday versions too.
The usual misconception is that the prism adds the colours. It does not create anything — the colours were present in the white light all along, and the glass only sorts them by how much each is deviated.
So dispersion is a separating process, and that is why the order of the colours is always the same for a given prism.
How can you prove the seven colours recombine into white light?
Two standard demonstrations show the process running backwards.
Newton's disc. A circular disc is divided into seven sectors painted with the seven spectrum colours. Spun rapidly, the disc appears white or greyish-white, because the eye cannot follow the colours separately and blends them into one impression.
Two prisms. Pass white light through one prism to produce a spectrum, then place a second, inverted prism in the path of that spectrum. The second prism bends the colours back together, and white light emerges on the far side.
A spinning top with coloured segments shows the same blending in a toy, and a fan blade painted in sectors does it as soon as the fan reaches speed.
The second prism must be inverted relative to the first — this is the detail most often left out of the answer. Placed the same way up, it deviates the colours further apart and widens the spectrum instead of collapsing it.
Together the two experiments settle the argument. Dispersion separates, recombination reassembles, and nothing is added or lost either way.
Newton's disc. A circular disc is divided into seven sectors painted with the seven spectrum colours. Spun rapidly, the disc appears white or greyish-white, because the eye cannot follow the colours separately and blends them into one impression.
Two prisms. Pass white light through one prism to produce a spectrum, then place a second, inverted prism in the path of that spectrum. The second prism bends the colours back together, and white light emerges on the far side.
A spinning top with coloured segments shows the same blending in a toy, and a fan blade painted in sectors does it as soon as the fan reaches speed.
The second prism must be inverted relative to the first — this is the detail most often left out of the answer. Placed the same way up, it deviates the colours further apart and widens the spectrum instead of collapsing it.
Together the two experiments settle the argument. Dispersion separates, recombination reassembles, and nothing is added or lost either way.
What are the primary and secondary colours of light?
The primary colours of light are red, green and blue. They cannot be produced by mixing other coloured lights, and mixing all three in equal proportion gives white light.
Mixing them in pairs gives the secondary colours:
- red + green = yellow
- green + blue = cyan (peacock blue)
- blue + red = magenta
The screen of a television or a phone works exactly this way, using tiny red, green and blue dots whose brightnesses combine to make every colour you see on it.
Here is the point that confuses almost everyone, and it is worth stating plainly: mixing coloured lights is not the same as mixing paints. Red and green light make yellow, while red and green paint make a muddy brown. Lights add — each one contributes what it sends to your eye. Paints subtract — each pigment removes some colours from the light falling on it.
The chapter is about light, so the addition rules are the ones to apply. A stage lit by red and green lamps together looks yellow, which no painter's palette would predict.
Mixing them in pairs gives the secondary colours:
- red + green = yellow
- green + blue = cyan (peacock blue)
- blue + red = magenta
The screen of a television or a phone works exactly this way, using tiny red, green and blue dots whose brightnesses combine to make every colour you see on it.
Here is the point that confuses almost everyone, and it is worth stating plainly: mixing coloured lights is not the same as mixing paints. Red and green light make yellow, while red and green paint make a muddy brown. Lights add — each one contributes what it sends to your eye. Paints subtract — each pigment removes some colours from the light falling on it.
The chapter is about light, so the addition rules are the ones to apply. A stage lit by red and green lamps together looks yellow, which no painter's palette would predict.
Why does a red object look black in green light?
Because an object's colour is the colour it reflects, and a red object absorbs everything else — including green. With only green light available, there is nothing for it to reflect, so it looks black.
The general rule is that an opaque object reflects its own colour and absorbs the rest:
- A red shirt in white light reflects red and absorbs the other six colours.
- A white object reflects all colours, which is why it looks white.
- A black object absorbs all colours and reflects almost none — which is also why dark clothes feel hotter in the sun.
So the appearance of a coloured object changes with the light you view it under:
- Red object in red light — looks red (there is red to reflect).
- Red object in green light — looks black.
- White object in red light — looks red, since it reflects whatever falls on it.
- Green leaf in red light — looks black.
Sodium street lamps make the case in real life. Under their yellowish light, a red car and a black car can be almost impossible to tell apart, because the lamp provides so little of what a red surface needs to reflect.
That is the reasoning to reproduce in an answer: name the colour the object can reflect, then check whether the illuminating light contains it.
The general rule is that an opaque object reflects its own colour and absorbs the rest:
- A red shirt in white light reflects red and absorbs the other six colours.
- A white object reflects all colours, which is why it looks white.
- A black object absorbs all colours and reflects almost none — which is also why dark clothes feel hotter in the sun.
So the appearance of a coloured object changes with the light you view it under:
- Red object in red light — looks red (there is red to reflect).
- Red object in green light — looks black.
- White object in red light — looks red, since it reflects whatever falls on it.
- Green leaf in red light — looks black.
Sodium street lamps make the case in real life. Under their yellowish light, a red car and a black car can be almost impossible to tell apart, because the lamp provides so little of what a red surface needs to reflect.
That is the reasoning to reproduce in an answer: name the colour the object can reflect, then check whether the illuminating light contains it.
Exam tip
Exam tip: giving the reason in terms of reflect and absorb
Colour questions are answered in two moves, and stating both is where the marks are.
Say what the object reflects and what it absorbs, then check what the light supplies. A red object reflects only red and absorbs the rest; green light contains no red; therefore it appears black. Three clauses, full marks.
Write VIBGYOR in order every time, and remember which end is which: violet is deviated most, red least. Reversing that loses the mark even if the seven colours are all named.
For the two-prism experiment, state that the second prism is inverted. For Newton's disc, state that it must be spun rapidly.
And keep light-mixing separate from paint-mixing. In this chapter, red + green = yellow, and any answer built on mixing pigments will be wrong.
Say what the object reflects and what it absorbs, then check what the light supplies. A red object reflects only red and absorbs the rest; green light contains no red; therefore it appears black. Three clauses, full marks.
Write VIBGYOR in order every time, and remember which end is which: violet is deviated most, red least. Reversing that loses the mark even if the seven colours are all named.
For the two-prism experiment, state that the second prism is inverted. For Newton's disc, state that it must be spun rapidly.
And keep light-mixing separate from paint-mixing. In this chapter, red + green = yellow, and any answer built on mixing pigments will be wrong.
Did you know
Why is violet bent more than red by a prism?
Because the glass slows the colours by different amounts, and the more a colour is slowed, the more its path bends.
Violet light is slowed most as it enters the glass, so it turns through the largest angle. Red is slowed least and turns least. The other five fall in between, in the fixed VIBGYOR order.
That single difference is what spreads a narrow white beam into a wide band — and why the rainbow always stacks its colours in the same sequence, with red on the outside of the arc.
Violet light is slowed most as it enters the glass, so it turns through the largest angle. Red is slowed least and turns least. The other five fall in between, in the fixed VIBGYOR order.
That single difference is what spreads a narrow white beam into a wide band — and why the rainbow always stacks its colours in the same sequence, with red on the outside of the arc.
Key takeaways
Dispersion and colour: quick revision
- A prism disperses white light into a spectrum of seven colours, VIBGYOR, proving white light is a mixture rather than a single colour.
- Violet is deviated most and red least, which fixes the order of the band; the prism separates colours, it does not create them.
- A rapidly spun Newton's disc appears white, and a second inverted prism recombines the spectrum into white light.
- The primary colours of light are red, green and blue; in pairs they give yellow, cyan and magenta, and all three together give white.
- Mixing lights adds and mixing paints subtracts, so red + green light is yellow while red + green paint is not.
- An opaque object reflects its own colour and absorbs the rest, so a red object looks black in green light and a white object takes the colour of the light on it.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Violet is deviated most and red least, which fixes the order of the band; the prism separates colours, it does not create them.
- A rapidly spun Newton's disc appears white, and a second inverted prism recombines the spectrum into white light.
- The primary colours of light are red, green and blue; in pairs they give yellow, cyan and magenta, and all three together give white.
- Mixing lights adds and mixing paints subtracts, so red + green light is yellow while red + green paint is not.
- An opaque object reflects its own colour and absorbs the rest, so a red object looks black in green light and a white object takes the colour of the light on it.
You will remember all of this far better after answering five questions on it than after reading it twice.