Colours in light

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Lesson notes

White Light and the Spectrum

White light is made up of **seven different colours**.
Passing white light through a **prism** splits it into a **spectrum** of colours.
This splitting is called **dispersion** and is what causes rainbows.
The colours in order from **longest to shortest wavelength** are: **red, orange, yellow, green, blue, indigo, violet** (ROY G. BIV).

Dispersion and Refraction

Dispersion happens because different colours of light have **different refractive indices** in a medium.
Each colour **refracts by a different amount** when entering a prism.
**Violet** refracts most because it experiences the **highest refractive index** ("violet bends most violently").
**Red** refracts least because it experiences the **lowest refractive index**.

Colour Filters

A colour filter **transmits only one colour** of light and **absorbs all other colours**.
For example, a red filter transmits only red light; a green filter transmits only green light.

Colour of Objects

An object appears a certain colour because it **reflects that colour** of light into our eyes.
A **green object** reflects only green light and absorbs all other colours.
A **red object** reflects only red light.
A **white object** reflects **all seven colours** of the spectrum.

Combining Filters and Object Colour

If white light passes through a **yellow filter**, only yellow light is transmitted.
If that yellow light shines on a **blue object**, the object appears **black** because it can only reflect blue light, but no blue light is present.

Refraction of light entering a prism: different colours refract by different amounts (violet most, red least).

Reflection of light from a surface: angle of incidence equals angle of reflection.

Slides

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Practice questions

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1.What is the splitting of white light into a spectrum called?
Easy
- ADispersion
- BReflection
- CRefraction
- DAbsorption
2.How many colours are in the spectrum of white light?
Easy
- A7
- B5
- C6
- D3
3.Which colour of light refracts the most in a prism?
Easy
- AViolet
- BRed
- CBlue
- DGreen
4.Which colour of light has the longest wavelength in the visible spectrum?
Easy
- ARed
- BViolet
- CBlue
- DGreen
5.A red filter is placed in front of a white light source. What colour light is transmitted?
Medium
- ARed
- BWhite
- CBlue
- DNo light
6.Why does a green object appear green?
Medium
- AIt reflects green light into our eyes.
- BIt transmits green light.
- CIt absorbs green light.
- DIt emits green light.
7.White light is passed through a yellow filter and then shines onto a blue object. What colour does the object appear?
Medium
- ABlack
- BBlue
- CYellow
- DGreen
8.Which of the following statements about dispersion is true?
Hard
- ADispersion is caused by different colours refracting by different amounts.
- BDispersion is caused by reflection.
- CAll colours refract by the same amount in a prism.
- DDispersion only occurs in rainbows.

Revision notes

## Colours in Light - White light is made of different colours. Passing it through a prism splits it into a spectrum (dispersion). Dispersion causes rainbows. - Order from longest to shortest wavelength: Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROY G. BIV). - Dispersion happens because different colours have different refractive indices, so they refract by different amounts. Violet refracts most (highest refractive index); red refracts least (lowest refractive index). - A filter transmits only its own colour; other colours are absorbed. - An object appears coloured because it reflects only that colour into our eyes. A white object reflects all colours. ## Key Terms

Dispersion: The splitting of white light into its component colours (spectrum) due to different refractive indices.
Spectrum: The seven colours of visible light: red, orange, yellow, green, blue, indigo, violet.
Refractive index: A measure of how much light bends when entering a medium; higher index means more bending.
Filter: A material that transmits only one colour of light and absorbs others.

Flashcards

What is white light made of?

White light is made from different individual colours.

What happens when white light passes through a prism?

It is split up into seven different colours called a spectrum.

What is dispersion?

Dispersion is the splitting of white light into its component colours.

What natural phenomenon is caused by dispersion?

Rainbows are caused by dispersion.

What is the order of colours in the visible spectrum from longest to shortest wavelength?

Red, orange, yellow, green, blue, indigo, violet.

What mnemonic is used to remember the order of colours in the spectrum?

ROY G. BIV (Red, Orange, Yellow, Green, Blue, Indigo, Violet).

Why does dispersion occur?

Different colours of light experience a different refractive index and so refract by different amounts.

Which colour refracts most in a prism?

Violet refracts most because it experiences the highest refractive index.

Which colour refracts least in a prism?

Red refracts least because it experiences the lowest refractive index.

What is a useful phrase to remember which colour bends most?

"Violet bends most violently."

What does a filter do to light?

A filter only allows one colour of light to be transmitted through it; other colours are absorbed.

What colour of light is transmitted through a red filter?

Red light is transmitted through a red filter.

What colour of light is transmitted through a green filter?

Green light is transmitted through a green filter.

What colour of light is transmitted through a violet filter?

Violet light is transmitted through a violet filter.

Why does a green object look green?

It reflects only green light into our eyes.

Why does a red object look red?

It reflects only red light into our eyes.

What colours does a white object reflect?

A white object reflects all seven colours of the spectrum.

True or false: All colours of light experience the same refractive index when entering a prism.

False.

True or false: Dispersion is caused by refraction.

True.

True or false: Red light refracts most in a prism.

False.

True or false: Of all the colours, red light has the longest wavelength.

True.

What colour is transmitted through an orange filter?

Orange light is transmitted through an orange filter.

Why does a blue object look blue?

It reflects only blue light into our eyes.

White light is passed through a yellow filter. What colour is transmitted?

Yellow light is transmitted.

If yellow light from a filter is shone onto a blue object, why does the object look black?

The blue object reflects only blue light, but yellow light contains no blue, so no light is reflected into our eyes, making it appear black.

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