Reflection and Refraction

Recall Huygen’s work:

Each point on a wavefront acts like a source of point waves… The constructive and destructive interference of these waves continually recreates the wavefront as it moves at the wave speedLINK

Law of Reflection

Angle of incidence = Angle of reflection

Normal line

Principle Rays for Curved Mirror

•Ray parallel to axis reflects through focus

•Ray through focus reflects parallel to axis

•Ray through C reflects straight back along path

•Ray striking center point of mirror uses axis as normal line LINK

// + f

Formulas

oif

111

o

i

h

hm

o

i

Rf2

1

Concave vs. Convex Mirrors

• Images behind mirror are virtual

• Images in front of mirror are real (actual light rays cross)

• Can have real and virtual images

• Can have inverted and upright images

• Can have images larger or smaller in size than object

• i < 0 for virtual; f > 0 for all

• Images behind mirror are virtual

• Only virtual images• Only upright

images• Images smaller

than object size• i < 0 for all• f < 0 for all

Recall Huygen’s work (again):

Each point on a wavefront acts like a source of point waves… The constructive and destructive interference of these waves continually recreates the wavefront as it moves at the wave speed

LINK

Law of Refraction

n1

n2 2

1

)sin()sin( 2211 nn

v

cn

Prisms

Light refracts toward “fat” side; different frequencies have slightly different speeds, so refract differently

Principle Rays for Convex Lenses

•Ray parallel to axis refracts through focus on other side (also remember kleenex rule)

•Ray through focus refracts parallel to axis

•Ray through center of lens does not bendLINK

// + f

Principle Rays for Concave Lenses

•Ray parallel to axis refracts away from focus on incoming side (also remember kleenex rule)

•Ray toward focus on other side refracts parallel to axis

•Ray through center of lens does not bendLINK

// + f

Concave vs. Convex Lenses

• Images on same side as object are virtual

• Images on opposite side to object are real (actual light rays cross)

• Can have real and virtual images

• Can have inverted and upright images

• Can have images larger or smaller in size than object

• i < 0 for virtual; f > 0 for all

• Images on same side as object are virtual

• Only virtual images

• Only upright images

• Images smaller than object size

• i < 0 for all• f < 0 for all

Principle of Least TimeFermat discovered that the path a

light ray takes (reflection or refraction) is the path that takes the least time… This idea is used a lot in advanced physics to formulate

the physical laws in a different way… For instance, the path that a

projectile takes is the one that minimizes the function f = KE – PE…

Food for thoughtLINK