Everything Science Grade 12

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:

1. 400 nm,


2. 580 nm,


3. 650 nm,


4. 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

1. 400 nm falls into therange for violet light (390 - 455 nm).


2. 580 nm falls into the range for yellow light (577 - 597
   nm).


3. 650 nm falls into therange for red light (622 - 780 nm).


4. 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