Colour of an object

Question 1

What is the colour an object appears related to?

Answer 1

transmission, reflection, and absorption of different wavelengths of light

Question 2

specular reflection

Answer 2

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

Question 3

What is the requirement for specular reflection?

Answer 3

the waves must be reflected from a smooth surface

Question 4

diffuse reflection

Answer 4

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

Question 5

What is the requirement for diffuse reflection?

Answer 5

the surface isn’t smooth and contains many irregularities

Question 6

the colour spectrum

Answer 6

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

Question 7

What does the colour an object appears to be depend on?

Answer 7

how the atoms at its surface respond to the light being shone on them

Question 8

differential absorption

Answer 8

atoms absorb some wavelengths and reflect others

Question 9

Why does a material appear green?

Answer 9

the atoms reflect green wavelengths and absorb all the others

Question 10

filters

Answer 10

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

Question 11

What happens if waves are absorbed at the boundary between two materials?

Answer 11

the energy of the wave is transferred to the particles in the surface

this increases the internal energy of the particles

Question 12

internal energy

Answer 12

The total kinetic energy and potential energy of the particles in an object.

Question 13

white light and objects

Answer 13

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

Question 14

Why are materials transparent?

Answer 14

they transmit light with very little absorption

translucent materials transmit some light but are not completely clear

Question 15

white and black objects

Answer 15

an object will appear to be black if it absorbs all wavelengths of light, and white if it reflects them all

Question 16

coloured objects in coloured light

Answer 16

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

Question 17

Explain which type of reflection you would associate with a plane mirror or a calm lake surface [2]

Answer 17

specular reflection

the surface is smooth, so parallel rays or wavefronts that are incident on these surface will be reflected as paralle rays/wavefronts

Question 18

Explain which type of reflection you would associate with a gravel path [2]

Answer 18

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

Question 19

Explain how a filter works to let through red light [2]

Answer 19

red light is let through the filter (transmitted) and all of the other colours are absorbed

Question 20

Explain what you would see when white light travels through a red filter and then a green filter [2]

Answer 20

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