Colour of an object Flashcards
What is the colour an object appears related to?
transmission, reflection, and absorption of different wavelengths of light
specular reflection
when parallel rays of light are incident, on a smooth, plane surface (e.g. mirror) the reflected rays of light will also be parallel
the sizes of any irregularities on the surface are much smaller than the wavelength of the wave
What is the requirement for specular reflection?
the waves must be reflected from a smooth surface
diffuse reflection
parallel incident rays are reflected at many different angles and the reflected rays will not be parallel (e.g. a painted wall)
the size of irregularities is comparable with the wavelength of the wave
What is the requirement for diffuse reflection?
the surface isn’t smooth and contains many irregularities
the colour spectrum
visible light makes up a very small part of the electromagnetic spectrum
the colours that we see can be split apart by a prism
these colours all have a different wavelength
red has the longest wavelength
violet has the shortest
What does the colour an object appears to be depend on?
how the atoms at its surface respond to the light being shone on them
differential absorption
atoms absorb some wavelengths and reflect others
Why does a material appear green?
the atoms reflect green wavelengths and absorb all the others
filters
filters let different colours of light through and absorb all the other colours
only the transmitted colour is observed by the human eye
e.g. a green filter will transmit green light but absorb other wavelengths
What happens if waves are absorbed at the boundary between two materials?
the energy of the wave is transferred to the particles in the surface
this increases the internal energy of the particles
internal energy
The total kinetic energy and potential energy of the particles in an object.
white light and objects
when white light is shone on an object, we see the parts of the visible light spectrum that are reflected back
other wavelengths we can’t see are reflected or absorbed
Why are materials transparent?
they transmit light with very little absorption
translucent materials transmit some light but are not completely clear
white and black objects
an object will appear to be black if it absorbs all wavelengths of light, and white if it reflects them all
coloured objects in coloured light
an object that is a colour in white light may appear black/a different colour in differently coloured light because there isn’t any light of a specific wavelength to reflect
the object will appear black because all the wavelengths of light are absorbed
an object that appears blue in white light appears black in red light because there is no blue light to reflect
an object that is purple in white light will appear blue in blue light because there is only blue light to reflect
Explain which type of reflection you would associate with a plane mirror or a calm lake surface [2]
specular reflection
the surface is smooth, so parallel rays or wavefronts that are incident on these surface will be reflected as paralle rays/wavefronts
Explain which type of reflection you would associate with a gravel path [2]
specular reflection involves the reflection of parallel rays from a smooth surface
diffuse reflection is the reflection of rays that will not be parallel, often from a rough surface hence it is diffuse reflection from a gravel path
Explain how a filter works to let through red light [2]
red light is let through the filter (transmitted) and all of the other colours are absorbed
Explain what you would see when white light travels through a red filter and then a green filter [2]
you would see no colour (it would appear black) since only red would be let through the green filter, and it would not then be let through the green filter
