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Optics and Photonics Technology EEEE4127
Example sheet – Light Interactions with matter – Week 2
1. Why is the sky blue?
2. What happens at sunset?
3. Consider a ray moving from air to water. The incident angle is 20 degrees, what
angle is the refracted ray in water from the normal? nwater =1.33, nair=1
4. Consider ray moving from water to air. The incident angle is 60 degrees, what is the
angle for the ray in air when it leaves the water?
5. What proportion of light intensity will be reflected when working at normal incidence
and shining light on glass with n=2? How much will be transmitted? NOTE – this is high for
glass.
6. A solution with a concentration of 0.14M is measured to have an absorbance of 0.43.
The same apparatus is used to measure the same chemical and an absorbance of 0.37 is
measured. What is the new concentration? State any assumptions.
7. Why do we prefer to express the Beer-Lambert law using absorbance as a measure
of the absorption rather than %T ?
8. Calculating energy levels is often very complex – apart from hydrogen (single proton,
single electron) where it is relatively straight forward. The energy levels in a hydrogen atom
are given by:
Where E0 is the energy of the ground state (n=1). n represents the state with n=1 being the
ground state. The energy of states n=1 to n=4 can be seen in the following table:
Energy (eV) State
n1 13.6 Ground
n2 3.4 1st
n3 1.5 2nd
n4 0.9 3rd
n(infinity) 0 ionization
What would be the wavelength of the photon emitted by a hydrogen atom if the electron
goes from the 1st state to the ground state?
9. A highly scattering tissue is illuminated with green light at 532nm. The scattering
coefficient is µs=80cm-1 and the absorption coefficient is µa=0.25cm-1. 40% of the intensity is
lost as the light travels through the tissue.
=
0
2
a) How thick is the tissue?
b) At this thickness will the photon path be random? Will the output direction of the photons
resemble the input direction? Assume g=0.9
10. Explain the formation of a mirage.
