Download Mirrors and Lenses

Mirrors and Lenses
Chapter 23
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Go AP Physics Students!
Mirrors

Mirrors form images using the property of
light called reflection, unlike lenses which
form images using refraction.

Mirrors are smooth reflecting surfaces.

A plane mirror is a flat surface. Usually it
is glass coated with a reflective metallic
substance.
Plane Mirrors

A ray diagram is used to
determine the location of the
image in a mirror or lens.

The image in a plane mirror
appears to be behind the
mirror.
The rays of light diverge at
the location of the image.
When the rays diverge, the
image is called a virtual
image.


Plane Mirrors

Notice the distance of
the object and image
from the mirror. For a
plane mirror,
d o = di

The height of the
image is another
important feature.
For a plane mirror,
ho = h i
The ratio of hi/ho is
called magnification.

Plane Mirrors

Plane mirrors form virtual images.

Image distance is equal to object
distance. do = di

Magnification = 1
Example

What is the minimum vertical length
of a plane mirror needed for a
person to see a complete head to
toe image of himself?
Spherical Mirrors

Spherical mirrors are reflecting
surfaces with spherical geometry.

For reflections on the inside surface,
the mirror is called concave.

For reflections on the outside
surface, the mirror is called convex.
Concave Mirrors

Concave mirrors focus light
at a single point.

Light rays that travel parallel
to the mirror reflect through
the focal point.

The focal point is half of the
radius of curvature.

Since light rays converge, the
image formed is real. You
could project an image on a
carefully placed card.
Concave Mirrors – Ray
Diagrams

Optical Axis - a line through the center of
the mirror that intersects the surface of
the mirror.

Center of Curvature – center of the circle

Focal point – the point at which reflected
rays intersect
Ray Diagrams

Draw the mirror, the optical axis, the center of
curvature,and the focal point.

Draw the object at the appropriate position.
1.
2.
3.

Draw the first ray from the object to the mirror parallel to the
optical axis, and reflecting through the focal point.
Draw the second ray through the center of curvature.
A third ray travels from the object through the focal point
and to the mirror. It reflects parallel to the mirror.
An image will be formed where the rays converge.
Concave Mirror Ray Diagram



Notice the object is
placed beyond C.
Three rays are
drawn.
The image is real,
inverted, located
between C and F,
and reduced.
Concave mirrors – Three
situations

If do >C, then f<di<C
and is real, reduced,
inverted.

If f<do<C, then di>C
and is real, inverted,
and enlarged. (no
picture)

If do <f, then image is
virtual and enlarged.
Mirror Equations

The image and object distances are
related by

The magnification can be found
using
Sign Conventions for
Spherical Mirrors
Example

A concave mirror has a radius of
curvature of 30cm. If an object is
placed a)45cm b) 20 cm c) 10 cm
from the mirror, where is the image
formed and what are its
characteristics?
Example

An object is placed 20cm in front of
a diverging mirror that has a focal
length of -15cm. Use a ray diagram
to determine whether the image
formed is real or virtual. Find the
location of the image using
equations.
Spherical Aberrations


Spherical mirrors
focus light well for
small angles of
incidence (and
reflection) but
produce blurry images
for larger angles of
incidence.
Parabolic mirrors
focus parallel rays
from distant objects
at one focal point.
Lenses
Lenses focus light by refracting light
to form an image.
 Biconvex lenses are convex on both
surfaces and cause rays to
converge.
 Biconcave lenses are concave on
both surfaces and cause light to
diverge.

Lenses
Three Rays to Draw!
First ray: parallel to optical axis and
refracting through focal point.
 Second ray: called the chief ray
passes from the object through the
center of the lens un-refracted.
 Third ray: through the focal point
and refracting parallel to optical
axis.

Lens Ray Diagram


If object is beyond
the focal point, a
real inverted
image if formed.
If the object is
between the focal
point and the lens,
a magnified
virtual, upright
image is formed
Concave lenses

Concave lenses
form virtual
images.
Lens Equations

Are exactly the same as mirror
equations!
Example

An object is 30 cm in front of a
biconvex lens of focal length 20 cm.
Use a ray diagram to locate the
image. Discuss the characteristics
of the image.
Homework

Pg 755 # 45, 49, 54, 55, 59, 62, 63,
69 – 71, 75

Begin to prepare for Ch 22,23 exam
on MONDAY.
Combinations of Lenses

The image of the first lens becomes the
object of the second lens!

If the image of the first lens is on the
opposite side of the second lens, consider
the image of the first lens to be a virtual
object for the second lens and do
becomes negative.

Magnification of the total Mtot = M1M2
Example

Consider two lenses similar to those
illustrated in fig 23.19. Suppose the
object is 20 cm in front of lens L1
which has focal length of 15 cm.
Lens L2, with focal length of 12 cm,
is 26 cm from L1. What is the
location of the final image?
Homework

Pg 757 # 75, 78- 81, 100, 103