1. Chapter 23
MEASUREMENT INVOLVING LIGHT:
PRINCIPLES AND INSTRUMENTATION

2. LIGHT IS PART OF THE ELECTROMAGNETIC
SPECTRUM
 γ rays
 X rays
 UV light
 Visible light
 IR light
 Microwave
 Radio waves

3. UNITS
 Light progresses in waves (lambda or λ) that
produce photons – the basic unit of light
 Light is measured in nanometers (nm= 10-9
meter)
 Visiblelight between 380 and 780nm. Table 23.1
page 405
 UV light between 280 and 315nm
 Ranges from UV-A (tanning, fluorescence) to UV-B –
used to visualize DNA in gels, to UV-C (germicidal
lamps)

4. LIGHT AND MATTER
 Types of interactions of light and matter:
 Transmittance (pass through)
 Reflection (change direction i.e. mirror)
 Scatter - (deflection in many directions , i.e.
prism)
 Absorbance – energy transferred to the material
(i.e. sunlight absorbed by earth)
Fig. 23.3 on page 405

5. CONT.
 White light is a mixture of many wavelength and it is
called polychromatic
 When one color is removed our eyes see the
complementary color
 If a solution absorb light at 400nm, what will be the color
that we see? (table 23.3 page 406)

6. COLOR WHEEL

7. ANSWER
 400nm is violet. The complementary color of
violet is yellow.
 The solution will look yellow to our eyes.

8. SPECTROPHOTOMETERY
 Use of instruments design to measure
 Transmittance
And
 Absorbance
Of light

9. TRANSMITTANCE
 t= light transmittance through a sample
light transmittance through water blank
It is common to calculate %T
%T=t x 100%

10. ABSORBANCE (A)
 Represents the amount of light absorbed by a
sample
 Absorbance calculation is based on
transmittance
 Also called optical density or OD
 A=OD=log10(t)
 Absorbance does not have units
 Spectrophotometers convert transmittance
values to absorbance

11. QUESTIONS
 If the transmittance is 100%. What is the
absorbance?
 If the transmittance is 0, what is the
absorbance?

12. SPECTROPHOTOMETER
 Contain
 Lightsource
 wavelength selector (filter or monochromator)
 Sample chamber holds a cuvette. The cuvette
dimensions determines the path length
 Detector converts light into a readable form.
Common detector is photomultiplier
 For accuracy t is important to work in the
midrange of the of the instrument

13. CUVETTES
 Made of glass or plastic. Not all
transmit/absorb in UV range
 Quartz glass are commonly used for UV
range
 There are disposable plastic cuvettes
suitable for UV
 UV cuvettes are more expensive

14. SPEC 20