Conceptual Physics: pp. 448-459; Chapter 30

Refraction-The bending of a wave as it enters a new medium

Medium-The material the wave travels through

Why does light refract? It enters the new medium at an angle It changes speed

Angle of incidence (i or θi) -The angle between the incident ray and the normal line

Angle of refraction (r or θr) -The angle between the refracted ray and normal line

Normal line – an imaginary line perpendicular to where the light strikes the interface.

i

r

Imaginary line showing the path that the light would take if it could go straight. (used as a reference line)

Interface – the boundary between substances

Incident ray

Refracted ray

Index of refraction (n): the ratio of the speed of light in a vacuum to the speed of light in that medium

Example: nglass = speed of light in vacuum speed of light in glass

nglass = 3.00 x 108 m/s 2.00 x 108 m/s

nglass = 1.50

The higher the n value (index of refraction) The slower the light will travel through

the material The greater the optical density The more light will refract (bend)

A few index values that we will be using often in class: (Pg. 15 in workbook)

  nglass = 1.50 nwater = 1.33 nair = 1.00 ndiamond = 2.42

Light will NOT refract when;1. the two materials have the same

index If the materials have the same index,

the speed of light is the same in each, so no refraction takes place

n=1.3

n=1.3

Light will NOT refract when;2. the light strikes perpendicular

(normal) to the surface

n=1.2n=1.4

Critical Angle-The angle of incidence that produces a 90º angle of refraction

(can ONLY be produced going from more to less optically dense)

1. Light passing from less to greater index Light bends toward the normal line

n = 1.2

n = 1.5

Angle of incidence (measured from the normal)

i

r

Angle of refraction

2a. Light passing from greater to lesser index Light bends away from the normal line

n = 1.6

n = 1.3

(i is less than critical angle)

As the angle of incidence increases, the angle of refraction also increases and the refracted ray moves closer to the interface.

2b. Light passing from greater to lesser index Light travels along interface

n = 1.6

n = 1.3

(i equals / at critical angle)

2c. Light passing from greater to lesser index Total internal reflection

n = 1.6

n = 1.3

(i is greater than critical angle)

The light doesn’t refract. Instead, it reflects back into the first substance. This is how fiber optics work.

Two Types of Lenses1. Convex (converging)2. Concave (diverging)

Lenses are simply rectangular and triangular prisms put together.

When light passes through a triangular prism, it bends toward the thicker portion of the prism.

n = 1.0 n = 1.5

n = 1.0 n = 1.5

Light bends when it enters and unbends by the same amount when it exits the prism

The path is unchanged- it is simply displaced sideways.

Putting the triangular and rectangular prisms produce the 2 types of lenses (double concave and double convex)

Focal point-The point where refracted rays from parallel incident rays intersect

Focal length-The distance between the focal point and the lens

Real image-An image formed from the intersection of actual light rays

Virtual image-An image that is NOT formed by the intersection of actual light rays

Have real foci Two focal points at equal distances

on each side of the lens

FF

FF

1. Draw the ray parallel to the axis, hit the lens, and refract through the focal point on the other side.

2. Draw the ray through the center of the lens without bending.

3. Draw the ray through the first focal point, hit the lens, and refract parallel to the axis.

F

F

The image is real, inverted and smaller

Have virtual foci.

F F

FF

1. Draw the ray parallel to the axis, hit the lens, and refract away from the first focal point.

2. Draw the ray through the center of the lens without refracting.

3. Draw the ray toward the focal point on the other side of the lens, hit the lens, and refract parallel to the axis.

The image with a concave lens will always be smaller, upright, and virtual

It will also always be found between the focal point and the lens. (similar to a convex mirror)

Rules to determine whether the image is real or virtual: 1. Real images are always inverted 2. Virtual images are upright.

(Except for combinations of lenses)

2 Types 1. Double convex 2. Double concave

Converging lensExamples:

Magnifying glasses Microscopes Correct farsightedness (hyperopia)

Diverging LensExamples:

Peep hole Correct nearsightedness (myopia)

Light traveling through air hits glass at a 28 degree angle. What is the refracted angle?

1. Draw a picture 2. List variables 3. Write formula 4. Substitute 5. Show some work 6. Solve and label 7. Check if the answer is reasonable!!!

1. Light strikes the surface of a piece of glass from air with an incident angle of 70◦. What is the angle of refraction?

Person #1: Draw a diagram Person #2:List the variables Person #1: Write the formula and

substitute Person #2: Solve

2. Calculate the critical angle for the substances water and glass.

Person #2: Draw a diagramPerson #1: List the variablesPerson #2: Write the formula and

substitutePerson #1: Solve

Object Distance

Image Distance

Larger/smaller/

same

Erect/Inverted

Real/Virtual