1. Refraction and Lenses

2. Refraction
The bending of light when it enters a different medium at an angle due to a change in speed.
Light slows down and bends toward the normal when entering a more optically dense medium (greater n).
Light speeds up and bends away from the normal when entering a less optically dense medium (smaller n).
Air (n = 1.00)
Glass (n=1.50)
Glass (n=1.50)
Air (n = 1.00)

3. Snell’s Law
normal
incident ray
Air (ni =1.00) i
n = index of refraction for the medium
Boundary
refracted ray r
Water (nr = 1.33)
Angles are always measured from the normal, never the surface

4. ni = index of refraction of incident medium
θi = angle of incidence, degrees
nr = index of refraction of refracting medium
θi = angle of refraction, degrees

5. Index of Refraction Light changes speed (v) as it enters a new medium
In a vacuum the speed of light (c) is 3.0 x 108m/s
The index of refraction (n) of a material is the ratio of the speed of light in a vacuum to the speed of light in the material.
Index of refraction has no units! The larger the n,
the slower the light speed.

6. Critical Angle
The angle of incidence that causes the angle of refraction to be 90o
The refracted ray is tangent to
the boundary between mediums
Only possible when going from a more optically dense (high index of refraction) to less optically dense medium (low index of refraction) ni > nr c
r=900
n=1
n=1.5

7. Critical Angle

8. Total Internal Reflection
When the angle of incidence exceeds the critical angle, the light does not cross the boundary into the new medium or refract.
All of the light is reflected back into the incident (denser) medium according to the Law of Reflection (angle of incidence = angle of reflection)
Application – fiber optic cables

9. Total Internal Reflection
Angle of incidence > critical angle and ni > nr
Angle of incidence = Angle of reflection

10. Prisms Makes white light split up or disperse into the color spectrum
The index of refraction depends on the wavelength of light so different colors of light bend or refract at different angles.

11. Dispersion
The process of separating white light into its component wavelengths (or colors).
As wavelength decreases, the index of refraction increases.
Blue light refracts more than red light since it has a shorter wavelength and thus a larger index of refraction.

12. Diffraction
The bending or spreading out of a wave (including light) in the region behind an obstruction as the waves go around it or through an opening between obstructions.

13. Concave Lenses Thinner in the middle than at the edges.
Parallel rays of light from a far object will refract through the lens and diverge as if they came from the focal point in front.
Concave lenses are also called “diverging lenses”.
Light may come in from either side of lens so there will be a focal point on both sides equal distances from the lens (assuming symmetrical lenses).

14. Convex Lenses Thicker in the center than at the edges
Parallel rays of light from a far object will refract through the lens and converge at the focal point on the other side of the lens.
Convex lenses are “converging lenses”.
Light may come in from either side of lens so there will be a focal point on both sides equal distances from the lens (assuming symmetrical lenses).

15. Formulas f = focal length do = object distance di = image distance
hi = image height
ho = object height
M = magnification

16. Interpreting Calculations
Focal length (f)
convex or converging lens, f = +
concave or diverging lens, f = -
Image distance (di)
di=+ , image is real & on opposite side of object
di= -, image is virtual & on same side as object
Magnification (M)
M & hi = +, image is upright and virtual
M & hi = - , image is inverted and real

17. Image is real & inverted
Ray Diagram
Convex Lens (do>f) – primary
f on opposite side of lens
Draw rays from tip of object:
1) parallel, then through primary f
2) through the center of the lens
3) through f ‘(same side), then parallel
4) Image is located where the refracted rays converge
Image is real & inverted
object f
image
f ’

18. Convex Lens (Inside f)- primary f on opposite side
Draw 2-3 rays from tip of object:
1) parallel, then through primary f
2) through the center of the lens
3) from f ‘on same side through tip of the object, then parallel
4) Extend the refracted rays back (dashed lines) to locate the image
Ray Diagram
Convex Lens (Inside f)- primary f on opposite side
image
Image is virtual,
Upright, & larger f
f ‘
object

19. Concave Lens – primary f on same side
Ray Diagram
Concave Lens – primary f on same side
Draw 2-3 rays from tip of object & refract at vertical line:
1) parallel, then refracted ray from f on same side of lens
2) through the center of lens
3) Toward f ‘ on the other side of lens, then parallel
4) Extend refracted rays back (dashed lines) to locate image
concave lens (axis)
object
image f f’
Image is virtual,
upright, & smaller

20. Polarizing filters Only allows light to pass through in one plane
Light is reduced by one-half
Sunglasses – polarized vertically, cuts out the horizontal components of light to reduce glare from horizontal surfaces such as sand or water

21. Photoelectric Effect
When light of certain frequencies shine on a surface, electrons are emitted from the surface.
Accounted for by the photon or particle theory of light.
Many applications such solar panels, photomultiplier tubes (or PMTs used in high energy physics to study particle collisions), and photocells (which operate switches or relays for automatic door openers and burgular alarms).