Honors Physics
Chapter 23 Notes
Mr. Fedell
Chapter 23 Problems: 1, 2, 7, 23, 24, 27, 28, 43, 45
23.1 The Ray Model of Light
A wave front is like the _________________. The distance between the ripples is the
_____________. The wave fronts are curved.
Light travels in straight-line paths called __________________.
Rays are lines drawn in the direction the wave front is ________________.
Rays are ______________ to the wave front.
A long distance from the source, the wave fronts are flat (i.e.
). The rays are all
___________________to each other. We use this idea in optics. (e.g. Rays from the sun are all
parallel to each other.)
23.2 Reflection: Image Formation by a Plane Mirror
The law of reflection says:
1. The incident ray (a.k.a
) and the reflected ray are all in the same plane.
2. The angles are equal ________ . Always measure the angles from the normal to the surface.
Do not measure the angle from the surface face itself.
Light reflects off a _______________surface. If you see a reflection, the surface demonstrates
“__________________”. Many surfaces are microscopically rough, like the pages of a book. We
don’t see any reflection off of the pages.
Smooth
Rough
What would the reflection off of a sphere look like? The angle of reflection is determined by
drawing a plane ________________ to the surface of the sphere, and having the reflected and
incident angles the same.
Images from a Plane Mirror
We have all used plane mirrors and know the properties of them.
1. The image is ____________.
2. The image is located as far behind the mirror as the object is _____________ of the mirror.
3. The image is the _______________ as the object.
4. The image _____________ front and back compared to the object.
We can demonstrate this using ray tracing.
We see that do = di.
This is called a __________________ because the light rays do not really come from the image,
but we think that they do. A photographic plate placed at the location of a ______________
image will not detect the image because _____ light rays go through it.
However, our eye does see the image and we could photograph the virtual image or project it on
a screen. Just by looking at an image you____________ tell if it is ________ or ___________.
Does the image from a plane mirror reverse left and right?
Example: Mirrors normally reverse left and right. But if we put two plane mirrors precisely at
right angles to each other, they will form an image which is not left right reversed.
We can draw how this works. Something on the left side appears to be coming from the left.
23.3 Spherical Mirrors
There are two kinds of spherical mirrors:
1. _____________: Reflecting surface is inner surface. Like a cave
2. Convex: Reflecting surface is _____________surface.
What happens to light rays incident on concave or convex mirrors?
Concave Mirrors
Rays coming from a distant source hit a spherical mirror.
The lines hitting the mirror may be ____________________. Paraxial Rays are rays that are
close to the principal axis. They are not necessarily parallel to the principal axis.
An image is formed at the ___________________ for paraxial rays. The focal length is
where R is the radius of curvature of the mirror. So usually a spherical mirror is small compared
to its diameter. A better mirror is actually a _____________________, but harder to make. We
can see where the image is by drawing three rays.
1. Ray 1 is ______________to principal axis and goes through the focal point.
2. Ray 2 goes through the focal point and bounces back parallel.
3. Ray 3 goes through the ______________ of curvature and bounces straight back.
We get different images depending on where the object is relative to C (
and f (
).
There are three places the object could be.
1. ______________ of C.
The image is inverted, smaller and real
2. ___________ C and f.
)
The image is larger, inverted, and real. (It’s the same picture as above, but the image just
becomes the object.)
3. ____________ of f.
The image is upright, larger, and virtual
It is nice to draw these pictures, but we need to figure out a quantitative way to solve these
problems. This is analogous to the situation we had with vectors. We could just draw them, but
to get a quantitative number we needed a quantitative method, breaking the vector up into
components. So we now develop a quantitative method to do mirrors.
The mirror equation : ________________________
The image is __________the same height as the object but has a magnification given by
The magnification equations: ____________________
The _____________ sign is there because inverted images have a negative magnification.
Mirror Sign Conventions:
1. _____________ Distance
do is + if the object is in front of the mirror (
do is - if the object is behind the mirror (
object)
object)
2. _____________ Distance
di is + if the image is in front of the mirror (
di is - if the image is behind the mirror (
image)
image)
3. Focal Length
f is + for a _____________ mirror.
f is - for a _______________ mirror.
4. Magnification
m is + for an image that is _______________ with respect to the object
m is - for an image that is inverted with respect to the _______________
Example:
A clown is using a concave makeup mirror to get ready for a show and is 27 cm in front of the
mirror. The image is 65 cm behind the mirror. Find (a) the focal length of the mirror and (b) its
magnification.
Convex Mirrors
The mirror equation and the magnification equation hold for convex mirrors as long as you
remember that the focal point has a ________ sign. This implies that the image is __________
and _____________.
(3rd line not drawn goes through C and back out, and would intersect with tip of arrow).
Example: The image of a distant car (passenger side mirror) is located 12 cm behind a convex
mirror. What is the radius of curvature of the mirror?
The Refraction of Light
23.4 Index of Refraction
In different media, light travels at a __________________ than in a vacuum. The speed slows
down, the wavelength ______________________ but the frequency stays the same in the
medium compared to in the vacuum. The ratio of the speed of light in a vacuum to the speed of
light in a medium is called the __________________________ ( ).
n = c/v
It is always greater than or equal to one. The index of refraction is for diamond is _________.
This leads to diamonds characteristic sparkle.
23.5 Refraction: Snell’s Law
________________________________
Always use θ from the _____________
In this diagram n2 > n1
This has lots of consequences.
Suppose I am looking at an object that is under water.
Light from the object is __________________ at the water, air surface
___________ is d. ____________ depth is d’.
Length along bottom is l.
______________ which is the apparent depth of an object when the observer is directly above it.
Example: An aquarium has a back wall which is 30 cm from the front glass which is negligibly
thin. A fish half way between the front and the back is swimming.
(a) How close to the front does it appear. (n for water is 1.33)
(b) If the back wall of the aquarium is a mirror and an image of the fish appears behind the
mirror. How far is this image from the front wall?
Example: A submarine is 300 m horizontally from the shore and 100 m beneath the surface. A
laser beam is sent from the submarine so that it strikes the surface of the water at a point 210 m
from shore. If the beam just strikes the top of a building standing directly at the water’s edge,
find the height of the building.
Lenses
Lenses are optical instruments like mirrors. There are two kinds of lenses, diverging and
converging. We will use equations to determine where real and virtual images will be.
1. Lenses have ______________ focal points, one on each side of the lens.
2. Converging lenses are mathematically treated very much like concave mirrors
3. Diverging lenses are mathematically treated very much like convex mirrors
4. For _______________ lenses, the thin lens equation and the magnification equation hold.
5. _______________________ are similar to mirrors.
23.7 Thin Lenses; Ray Tracing
Ray Tracing for Converging Lenses
1. Light ________________ after exiting the lens.
2. ______________________ in the middle than at the edges.
Use three rays to see where image is:
1. ____________ and through the far f.
2. Through the near f and exit _____________.
3. Through the center and ____________.
Like concave mirrors, the image formed depends on ______________ the object is located with
respect to the ________________.
1. Farther than 2F, the image is real, inverted and smaller
2. Between 2F and F, the image is real, inverted and larger.
3. Closer than F, the image is virtual, upright, and larger.
For thin lenses (the lens is thin compared to f, so we can measure from ___________on the lens),
the thin lens equation and the magnification equations hold.
Lens Sign Conventions:
1. Object Distance
do is
(
) if the object is on the same side of the lens as the original object
do is
(
) if the object is on the opposite side of the lens from the original object
2. Image Distance
di is the same sign as that of the object if the image is formed on the _______________ side of
the lens from the object.
di is the ___________ sign as that of the object if the image is formed on the same side of the
lens as the object.
(So if the object is + (
) an image formed on the opposite side of the lens as the
object will be + (real), and an image formed on the same side of the lens as the object will be
-(
).)
3. Focal Length
f is + for a converging lens.
f is - for a diverging lens.
4. ____________________
m is + for an image that is upright with respect to the object
m is - for an image that is __________________ with respect to the object
Example: The movie film in a theater has a width of 70 mm. The projector lens has a focal length
of 305 mm. If the screen in the theater is 60 m from the projector, how wide is the image?
Ray Tracing for Diverging Lenses
1. Light ______________ after exiting the lens.
2. The lens is ___________________ in the middle than at the edges.
We use three rays to see where the image will be focused.
1. ____________ parallel and exits in a direction that appears to have originated from the first f.
2. Is incident on the second f and _____________ parallel to the principal axis.
3. Goes _______________through the middle.
Like a convex mirror, a diverging lens only has one possible image that can form, independent of
the location of the object. A diverging lens always forms an image that is virtual, upright, and
smaller than the object. Use the _____________ thin lens equation with the appropriate signs,
and the same magnification equation.