Section 1 of 4White Light and the Spectrum
White light is a mixture of all the colours of the visible spectrum.
Spectrum
- The colours run from red, with the longest wavelength and lowest frequency, to violet, with the shortest wavelength and highest frequency.
Dispersion
- A prism splits white light because each colour is refracted by a different amount.
Order
- Violet is bent the most and red is bent the least, which produces the band of colours seen in a rainbow.
Frequency
- Each colour has its own frequency, and colour is not changed by refraction or reflection.
Section 2 of 4Why Objects Have Colours
The colour of an opaque object depends on the wavelengths it reflects.
Why Objects Have Colours
Coloured object
- A red object reflects mostly red light and absorbs most of the other colours in white light.
White object
- A white surface reflects all the colours of visible light about equally.
Black object
- A black surface absorbs nearly all the colours of visible light and reflects almost none.
Coloured light
- A red object seen in blue light looks dark or black, because the blue is absorbed and there is no red to reflect.
| Object | Reflects | Absorbs |
|---|---|---|
| Green leaf | Green | Other colours |
| White paper | All colours | Very little |
| Black cloth | Almost nothing | Almost all colours |
Section 3 of 4Filters, Transmission and Reflection Types
A transparent coloured material works in a similar way but for transmitted light.
1
White light shines onto a red filter, which is a sheet of transparent plastic or glass.
2
The filter absorbs most wavelengths but allows red light to pass through, so red is transmitted.
3
Looking through the filter, only red light reaches the eye, so a white scene appears red.
4
Smooth surfaces such as mirrors give specular reflection, with parallel rays reflected in one direction.
5
Rough surfaces give diffuse reflection or scattering, with rays sent in many directions, which is why most objects can be seen from any angle.
Section 4 of 4Exam Checklist & Tips
5.3.4 – Key Points for the Exam
- Spectrum The range of colours into which white light can be split.
- Dispersion The separation of white light into colours by refraction.
- Filter A transparent material that transmits some colours and absorbs the rest.
- Specular reflection Reflection from a smooth surface in a single direction.
- Diffuse reflection Reflection from a rough surface in many directions.
Common Mistakes
- Saying a red object gives off red light; it reflects red light that falls on it.
- Writing that a prism adds colour to white light, when it separates colours that are already present.
- Forgetting that a coloured filter absorbs the other colours rather than creating colour.
- Claiming that diffuse reflection breaks the law of reflection; each ray still obeys it at its own point.