Refraction

1
Refraction Lenses

• Chapter 18

2
Refraction of Light

• Look at the surface of a swimming pool
• Objects look distorted
• Light bends as it goes from one medium into
  another\
• Why?

3

• When a medium causes a wave to slow down it is
  more optically dense

4
Entering more optically dense

• Waves moving into a more optically dense medium
  will cause the wave speed to slow down and then
  bend toward the normal
• Therefore, the angle of refraction is smaller
  than the angle of incidence

5
Entering less optically dense

• Waves moving into a less optically dense medium
  will cause the wave speed to increase and then
  bend away from the normal
• Therefore, the angle of refraction is larger than
  the angle of incidence

6
Snells Law

• Dutch Scientist Willebrord Snell discovered that
  a ray of light bends in such a way that the ratio
  of the sine of the angle of incidence to the sine
  of the angle of refraction is a constant, index
  of refraction (n)

7

• ni sin? i ??nr sin?r
• ni index of refraction for the incident medium
• ? i angle of incidence
• nr index of refraction for the refracting medium
• ? r angle of refraction

8
Index of refraction

• ratio of speed of light in a vacuum, c, to
  another medium, v
• nc/v
• index of refraction in a vacuum is 1.00
• table on page 486

9
Problem Solving Strategy

• Draw a diagram showing the two media. Label
  them, indicating the two indices of refraction,
  ni and nr
• Draw a normal to the surface
• Use Snells law to calculate the angle of
  refraction

10
Total Internal Reflection

• If ray travels from a more optically dense into a
  less optically dense medium the angle of
  refraction is larger than the angle of incidence
• Eventually the incidence angle is so great that
  all the light reflects back into the medium
• No refraction takes place

11
Critical Angle

• The incident angle that causes the refracted ray
  to lie right along the boundary of the substance
• Unique to each substance

12
Critical Angle
air
critical ray
water
qc
Calculate the critical angle
13
Total Internal Reflection

• When light enters a thin glass rod light is
  internally reflected (fiber optics)
• Telephone, computer and video signals
• Explore the human body
• Plants use internal reflection

14
Effects of Refraction

• Mirages
• Summertime - the sun hits to road and causes the
  air above the road to heat up
• The index of refraction for warm air is 1.00026
• The index of refraction for cool air is 1.00028

15

• This small change in index of refraction causes
  the rays to bend
• This bending makes the road look like there is a
  puddle on it

16

• Because light travels slightly slower in Earths
  atmosphere than in outer space the sun rays bend
  causing the sun to reach us before the sun is
  actually above the horizon
• Same in the evening, the rays bend and reach us
  after the sun has actually set

17
Dispersion of Light

• Light of all wavelengths travels at the same
  speed in a vacuum
• Other media causes the speed to slow down
• The wavelength also determines the speed index
  of refraction

18

• In most materials red light travels fastest
• It also has the smallest index of refraction
• Violet is the slowest and has the largest index
  of refraction
• Red is bent the least, violet the most

19
Dispersion

• The separation of light into a spectrum by
  refraction
• Diamond
• Rainbow (water)