1. Refraction – Snell’s Law

3. WAVE PROPERTIES Refraction
Interactive spreadsheet
Refraction is the change in direction at a boundary when a wave travels from one medium to another.
Amount of refraction depends on the refractive index of the different media (materials).
Normal
Air
All waves can be Refracted.
Refraction is a change of direction due to a change of speed.
The example I used involves light rays hitting a glass block. Glass is a medium which is more dense than air. so the light will travel more slowly within the glass block than it would do in air. As the light ray crosses the boundary from one medium to another, it bends.
Glass Block
Normal 3

4. Snell’s law Note: medium two is less dense
v= speed of wave in the medium

5. Example 1
A wave travelling at 12 cm s-1 is incident upon a surface at an angle of 55° from the normal.
If the angle of refraction is 40°, determine the speed of the wave in the second medium.
If the initial wavelength is 6 cm determine the frequency of the wave in the second medium.
sinθr = vr
sinθi vi
sin = vr
sin
vr = 9.4 cm s-1
In first medium: v = fλ f = v/λ = 0.12 / = 2.0 Hz
Frequency does not change during refraction  f = 2.0 Hz

6. For light travelling from water into glass, r = 20°.
Example 2
For light travelling from water into glass, r = 20°.
If nw = 1.33 and ng = 1.50, determine i (θi).
video
sinθr = ni
sinθi nr
sin20 = 1.33
sinθi
sinθi = / 0.89
= 0.38
θi = sin
= 22.7°

7. Total Internal Reflection
The Endoscope

8. You will need… A semicircular block A power pack A ray box
A single slit
A protractor
A ruler
A pencil

9. Total internal reflection
Draw on your normal line
half way up the block at
90o
Shine an incident ray into P
You should see a refracted ray P i
You should also see a partially
reflected ray r
The angle of reflection should be
equal to the angle of incidence

10. Gradually increase the angle
of incidence.
The angle of refraction will
increase until the refracted ray
emerges along the boundary
We call the angle of incidence
at this position the critical angle c
We label the critical angle, c

11. The sine of the angle in air = n x the sine of the angle in the block
Remember the law of refraction when the light ray is leaving the glass block
The sine of the angle in air = n x the
sine of the angle in the block

12. The sine of the angle in air = n x the sine of the angle in the block
In this case the angle in the air is 90o
The angle in the block is c
sin 90o = n x sin c
Because sin 90o = 1,
the equation becomes:
1 = n x sin c c r
How can this equation be rearranged?
n = 1
sin c
sin c = 1 n or

13. What is the refractive index for your block?

14. Calculate the critical angle for glass with a refractive index of 1.59
sin c = 1 n
sin c = 1/1.59 sin c = c = 39.0o

15. Total internal reflection
If the angle of incidence is increased
beyond the critical angle, the light ray
is totally internally reflected i r
The angle of reflection, r is
equal to the angle of incidence, i

16. Total Internal Reflection allows light to travel around corners
Normal i r

17. The Endoscope

18. The Endoscope Two bundles of optical fibres next to each other
One bundle shines light into the cavity
The second bundle of optical fibres has a lens over the end of the fibres allowing an image to be formed using a digital camera

19. Endoscope laser surgery
A laser on the endoscope can carry out surgical procedures
It can be used to cauterise blood vessels
The colour of the laser can be matched to the tissue to ensure maximum absorption
Retinal eye surgery can be carried out by laser by shining a laser through the pupil for a very short time