Transcript Light is a form of energy

Chapter 29 in textbook.
You’ll want paper for your notes….
Light is a form of energy.
Light travels in a straight line
Light speed is 3.0 x 10
8
m/s
Light is carried by photons
Light can travel through a vacuum
Light is a transverse wave
Light is an electromagnetic wave
ELECTROMAGNETIC WAVES
Electromagnetic waves are
waves that are capable of
traveling through a vacuum.
They consist of oscillating
electric and magnetic fields with
different wavelengths.
The Electromagnetic Spectrum
Increasing frequency
Increasing wavelength
Increasing Energy
The wave speed equation is: c = f λ
where c is the speed of light.
When light strikes an object it can be
REFLECTED, TRANSMITTED or ABSORBED.
Objects are TRANSPARENT,
TRANSLUCENT AND OPAQUE to light
Ultraviolet light oscillates at too high a frequency
for electrons in glass molecules, while infrared is
too low. Visible light is just right.
Making glass transparent to visible light and opaque
to infrared and ultraviolet
REFRACTION
The bending of a ray of light as it passes from one
medium to another is called refraction.
Reflection and Refraction at an Interface
The speed of light c in a material is generally less than
the free-space velocity c of 3 x108 m/s. In water light
travels about three-fourths of its velocity in air. Light
travels about two-thirds as fast in glass.
The ratio of the velocity c of light in a vacuum to the
velocity v of light in a particular medium is called the
index of refraction, n for that material.
c
n
v
Light bends toward the
normal when entering
medium of higher index
of refraction
Light bends away from the
normal when entering
medium of lower index
of refraction
SNELL’S LAW
The ratio of the sine of the incident angle to the sine of
the refracted angle is constant.
n1 sinθ1 = n2 sinθ2
n1 = index of refraction of the incident medium
n2 = index of refraction of the second medium
Example A ray of light travels from air into liquid. The ray is
incident upon the liquid at an angle of 30°. The angle of refraction
is 22°.
a. What is the index of refraction of the liquid?
n1 = 1
n1 sin 1 = n2 sin 2
1 = 30
2 = 22
n1 sin 1 1sin 30 
n2 


sin
22
sin 2
= 1.33
Critical Angle
n1 sin 1 = n2 sin 2
= n2 sin 90
sin 1 = n2 / n1
Example Find the critical angle for an air-crown glass boundary.
ni= 1.52
nr= 1
nr
sin  c 
ni
nr
 c  sin
ni
1
1
 sin
152
.
1
= 41˚
THIN LENSES
Lenses are an essential part of telescopes, eyeglasses,
cameras, microscopes and other optical instruments. A
lens is usually made of glass, or transparent plastic.
The two main types of lenses are convex and concave
lenses.
The focal length (f) of a lens depends on its shape and
its index of refraction.
A converging (convex) lens is thick in the center and
thin at the edges.
A diverging (concave) lens is thin in the center and thick
at the edges.
A real image is a representation of an object (source) in which the perceived location
is actually a point of convergence of the rays of light that make up the image.
A real image is
visible on the
screen and inverted
And formed on the
opposite side of
a lens..
A virtual image is an image in which the outgoing rays from a point on the object never
actually intersect at a point. Virtual images cannot be seen on a screen and form on
the same side of a lens.
IMAGE FORMATION BY LENSES
There are three principal rays to locate an image.
Ray 1. A ray parallel to the axis passes through the
second focal point F2 of a converging lens or appears
to come from the first focal point F1 of a diverging
lens.
Ray 2. A ray which passes through the first focal
point F1 of a converging lens or proceeds toward the
second focal point F2 of a diverging lens is refracted
parallel to the lens axis.
Ray 3. A ray through the geometrical center of a lens
will not be deviated.
Principal Rays
A real image is always formed on the side of the lens
opposite to the object. A virtual image will appear to be
on the same side of the lens as the object.
23.7
a. Find the images formed by the following lenses
using the Ray Tracing method.
b. Write the characteristics of each image:
-real or virtual,
-larger, smaller or same size as object and
-upright or inverted.
CASE 1: Object is beyond 2F
Image is real, reduced, inverted and located
between f and 2f on the opposite side of the lens
CASE 2: Object at 2F
Image is real, same size, inverted and
located at 2f
CASE 3: Object between F and 2F
Image is real, magnified, inverted and
located beyond 2f
CASE 4: Object is at the focal point
No image is formed.
CASE 5: Object is between f and the lens
The image is virtual, upright, magnified and
located on the same sides as the object
DIVERGING OR CONCAVE LENS
Case 1: Object outside of 2f
Image is virtual, reduced, upright and located on
the same side of the lens as the object
Case 2: Object between f and the lens
Image is virtual, reduced, upright and located on
the same side of the lens as the object
What if there are TWO lenses?
THE LENS EQUATION
The lens equation can be used to locate the image:
1 1 1
 
d o di
f
Where do is the object’s distance, di is the image
distance and f is the focal length.
hi
di
M

ho
do
The ratio M is called the magnification, ho is the object’s
size and hi is the image size.
23.8 A 5 cm tall object is located 30 cm from a convex lens of 10
cm focal length.
a. Find the location and nature of the image.
do = 30 cm
f = 10 cm
30(10)
do f

di 
= 15 cm, real
30  10
do  f
b. What is the height of the
image?
di ho
hi  
do
ho = 5 cm
hi
di

ho
do
15(5)

= - 2.5 cm, inverted
30
R
radius of
curvature
+ converging
- diverging
f
focal
length
+ converging
- diverging
do
object
distance
+ real object
+ real object
di
image
distance
+ real
images
- virtual
images
ho
object size
+ if upright
- if inverted
hi
image size
+ if upright
- if inverted
VISION PROBLEMS:
• MYOPIA is when image is formed in front
of retina and is also known as
nearsightedness and is corrected with a
concave lens
VISION PROBLEMS:
• HYPEROPIA is when image is formed
behind the retina and is also known as
farsightedness and is corrected with a
convex lens
VISION PROBLEMS:
• ASTIGMATISM is when the eye is shaped
like a football rather than the normal eye
that has a round shape similar to
basketball. It causes certain amounts of
distortion or pitched images because of
the uneven bending of light rays entering
the eye.