Refraction

• When light passes from one medium to another, it bends.• The bending of light rays between two different media is

called refraction. • Refraction is due to changes in the speed of light. The more

light slows down, the more light is refracted.

• The index of refraction is the amount by which a medium decreases the speed of light.

• The index of refraction of the speed of light in a vacuum is assigned a value of 1.00.

• The larger the index of refraction, the more the medium decreases the speed of light.

• The more the medium decreases the speed of light, the more optically dense the medium.

Snell’s Law

PHET

• The angles of the refracted light rays are measured from the normal

• When light travels from a medium with a low refractive index (less optically dense) to a medium with a high refractive index (more optically dense), it bends towards the normal.

• When light travels from a more optically dense medium to a less optically dense medium, it bends away from the normal.

Total Internal Reflection

• Recall that when light passes from a denser material into a less dense material the light refracts away from the normal.

• As the angle of incidence increases, the angle of refraction increases.

• As we increase the angle of incidence, eventually the light will refract so far away from the normal that it follows a path exactly along the surface of the water.

• The angle at which this occurs is called the critical angle.

• What if the angle of incidence is increased even farther? (What if the angle of incidence is larger than the critical angle?)

• If the angle of the incident ray is larger than the critical angle, the light will be completely reflected back into the water.

• This is called Total Internal Reflection:

In TIR, light reflects completely off the inside wall of a denser medium (higher index of refraction) rather than passing through the wall into a less dense medium (lower index of refraction).

• Application: Fiber Optics

LENSES

CONCAVE LENS USES

• Peepholes to provide a panoramic view• In glasses to correct nearsightedness• Binoculars and Telescopes to help focus

images more clearly• In flashlights to increase the beam of the light

source• To modify laser beams in medical equipment,

scanners and CD players

CONVEX LENS USES

• The Eye to focus an image on the retina in the back of the eye

• Glasses and contact lenses to correct farsightedness

• Microscope, Telescope and Binoculars• Cameras to focus an image on a film or a

sensor in a digital camera

TERMINOLOGY

Optical Center, O

Principal FocusSecondary Focus

Secondary Focus

Principal Focus

DRAWING A RAY DIAGRAM

• The index of refraction of a lens is greater than the index of refraction of air.

• Therefore, when a light ray passes through the lens two refractions occur.

PA

Axis of Symmetry

Normal 1 Normal 2

DRAWING A RAY DIAGRAM

• We will assume we are working with a thin lens.

• The thickness of a thin lens is small compared to its focal length.

• You can simplify drawing a ray diagram of a thin lens by assuming that all refraction takes place at the axis of symmetry.

CONCAVE LENS

RAY 1: From the tip of the object parallel to the principal axis (When this ray refracts, it appears to come from the principal focus.)

RAY 2: From the tip of the object toward the secondary focus (This ray refracts parallel to the principal axis.)

RAY 3: From the tip of the object through the optical center (This ray is not refracted.)

Draw the image where the rays appear to intersect.

CONVEX LENS

• RAY 1: From the tip of the object parallel to the principal axis (When this ray refracts, it passes through the principal focus.)

• RAY 2: From the tip of the object through the secondary focus (This ray refracts parallel to the principal axis.)

• RAY 3: From the tip of the object through the optical centre (This ray is not refracted.)

Draw the image where the rays appear to intersect.

F F’

RAY 1: From the tip of the object parallel to the principal axis (When this ray refracts, it appears to come from the principal focus.)

CONCAVE LENSESObject placed in front of lens

RAY 2: From the tip of the object toward the secondary focus (This ray refracts parallel to the principal axis.)

F F’

RAY 3: From the tip of the object through the optical center (This ray is not refracted.)

F F’

Draw the image where the rays appear to intersect.DON’T FORGET SALT!

F’ F

RAY 1: From the tip of the object parallel to the principal axis (When this ray refracts, it passes through the principal focus.)

CONVEX LENSESObject is more than two focal lengths away from the lens

RAY 2: From the tip of the object through the second focus (This ray refracts parallel to the principal axis.)

F’ F

RAY 3: From the tip of the object through the optical centre (This ray is not refracted.)

F’ F

Draw the image where the rays appear to intersect.

DON’T FORGET SALT!

F’ F

RAY 1: From the tip of the object parallel to the principal axis (When this ray refracts, it passes through the principal focus.)

CONVEX LENSESObject is between one and two focal lengths

F’ F

RAY 2: From the tip of the object through the second focus (This ray refracts parallel to the principal axis.)

F’ F

RAY 3: From the tip of the object through the optical centre (This ray is not refracted.)

Draw the image where the rays appear to intersect.

DON’T FORGET SALT!

CONVEX LENSESObject is less than one focal length away from the lens

RAY 1: From the tip of the object parallel to the principal axis (When this ray refracts, it passes through the principal focus.)

F’ F

RAY 2: From the tip of the object through the second focus (This ray refracts parallel to the principal axis.)

F’ F

RAY 3: From the tip of the object through the optical centre (This ray is not refracted.)

F’ F

Draw the image where the rays appear to intersect.

DON’T FORGET SALT!

Thin Lens Equations

POWER (Diopters)