16.4 CHAPTER 16. OPTICAL PHENOMENA; PROPERTIES OF MATTER
Example 5: Colours and energies of light
QUESTION
I have some sources which emit light of the following wave-
lengths:
- 400 nm,
- 580 nm,
- 650 nm,
- 300 nm.
What are the colours of light emitted by the sources (see ta-
ble 16.2)? Which sourceemits photons with the highest energy
and which with the lowest energy?
SOLUTION
Step 1 : What information is given, and what do we need to do?
Four wavelengths of light are given and we need to find their
colours.
We also need to find which colour photon hasthe highest
energy and which one has the lowest energy.
Step 2 : To find the colours of light, we can compare the wavelengths to
those given in table 16. 2
- 400 nm falls into therange for violet light (390 - 455 nm).
- 580 nm falls into the range for yellow light (577 - 597
nm). - 650 nm falls into therange for red light (622 - 780 nm).
- 300 nm is not shown in the table. However, this wave-
length is just a little shorter than the shortest wavelength
in the violet range. Therefore 300 nm is ultraviolet.
Step 3 : To find the colour of the light whose photonshave the highest
and lowest energies respectively, we need to calculate the energies of all
the photons
We know E =hcλ