1. Electromagnetic Waves
- Transverse waves
- Travel at the speed of light
- Can travel in a vacuum
-Created by alternating electric and magnetic fields.
-James Maxwell created radio waves by passing an
alternating current through a wire.
2. Visible range – from approx 400 nm to 700 nm
- red has the lowest frequency and highest wavelength
3. Properties of EM Waves
Dispersion – white light is separated into the visible spectrum
Red is dispersed the least, violet the most
The lowest
wavelength
refracts the
most.
4. Depending on the frequency of the EM wave and what
substance it is hitting, the wave will be transmitted or
absorbed.
Transmission – the wave will pass through the substance.
Ex. X-rays will pass through most solids
Light will pass through air
Absorption – the wave will be absorbed
and excite the substance
Ex. Microwaves are absorbed by water
molecules
UV is absorbed by the ozone White light hitting a blue filter.
layer Some light is reflected, blue light
is transmitted while the other
colors are absorbed.
The intensity (brightness) of light is indirectly related to
the distance to the source squared.
5. Scattering - light will be scattered by particles in the
air
Red scatters least, blue scatters most
Example – Blue sky, red sunsets
6. LASER
Light Amplification by the Stimulated Emission of
Radiation
Monochromatic (one color) coherent (same frequency, in
phase) light
Population inversion
10. s = D/d
s = how far apart maxima
are
D = distance from slits to
screen
d = distance between slits
11. A source of light of unknown wavelength is
used to illuminate two very narrow slits a
distance 0.15 mm apart. On a screen at a
distance of 1.30 m from the slits, bright
spots are observed separated by a distance
of 4.95 mm. What is the wavelength of the
light being used?
571 nm
12. Two narrow slits 0.01 mm apart are
illuminated by a laser of wavelength 600
nm. Calculate the fringe spacing on a
screen 1.5 m from the slits.
13. Diffraction grating
d sin = n
Light of wavelength 680 nm falls on a diffraction
grating that has 600 lines per mm. What is the
angle separating the central maximum (n=0)
from the next (n=1)?
= 24.1o
14. Red light (wavelength = 700 nm) is shone
through a grating with 300 lines/mm.
a) Calculate the diffraction angle of the first
red line.
b)Calculate the diffraction angle of the
second red line.
15. Optical instruments
Converging lens
A lens that is thicker in the middle than at the edges. Light is
refracted toward the principal axis (the straight line that goes
through the center at right angles to the lens surface).
16. Parallel rays will refract to a point
on the principal axis called the
focal point. The distance from the
lens to the focal point is the focal
length.
17. Optometrists describe the power of a lens as
P= 1/
f
Example: A lens with a focal length of 25 cm would have
a power of 4.0
18. Real image – an image that is created by rays of light
passing through a lens
- can be projected on a screen
- inverted
Virtual image – rays do not actually meet, only their
mathematical extensions do
- cannot be projected on a screen
- upright (erect)
19. Ray diagrams – pictures used to identify what kind of
image will be created by a lens.
Rays to draw
1) Parallel rays refract through the focus
2) Rays through the center continue straight thru
3) Rays thru the focus refract parallel
Where these rays meet describe the image. Only two
rays are necessary.
20.
21.
22. Using graph paper, draw a ray diagram for an object
a) at 2F, b) between F and 2F, c) at F and d) inside F
23.
24. This can also be done mathematically with the thin lens
formula.
1/ = 1/v + 1/u
f u = object distance
v = image distance
Image distance : (+) means real image, (-) means
virtual
Also the magnification can be calculated.
m = hi/ho = -v/
u
25. A converging lens has a focal length of 15 cm.
An object of height 2 cm is placed 60 cm from
the lens. Determine the image.
26. An object is placed 15 cm in front of a converging
lens of focal length 20 cm. Determine the image.
27. A diverging lens causes light to refract AWAY
from the focus.
What kind of image does a diverging lens
create?
Virtual only
28. The Telescope
The refracting telescope works by bending light with two
lenses. The objective lens makes a small real image of the
object while the eyepiece lens acts as a magnifying glass.
The focal points of the two lenses should be at the
same point to produce a focused image.
29. Draw a ray diagram for a telescope. Note where
the objective lens focuses the light from a star and
where the final image is located and in what
direction.
Step 1: Draw the lenses and axis (no foci)
Step 2: Draw a ray passing through the center of
the objective hitting the eyepiece halfway down
Step 3: Draw two more rays entering the objective
at the same angle as the first. The top ray should
hit the bottom of the eyepiece.
30. Step 4: The bottom ray will cross where the other
two intersect. This is the focus. Draw in the first
image.
Step 5: The rays emerge from the eyepiece
parallel. To find the angle draw a dotted line from
the top of the image through the center of the
eyepiece
31. The angular magnification can be worked out by
the simple formula:
M i
o
where i and o are small angles in radians.
•The angle i is the angle subtended by the object
to the unaided eye.
•The angle o is the angle subtended by the image
to the eye.
The magnification can also be shown to be related
to the focal lengths of the lenses by: fo
M
fe
32. In a telescope the eyepiece has a focal length of 2
cm and the objective has a focal length of 220
cm. What is the magnification?
Magnification = fo/fe = 220/2 = 110
If the moon subtends an angle of 8.8 x 10-3 rad to
the naked eye, what would the angle be for the
image of the moon observed through the telescope?
Angle subtended by the Moon = 8.8 x 10-3 rad x 110 =
0.97 rad
33. A telescope is constructed from two lenses: an
objective lens of focal length 100 cm and an
eyepiece of focal length 10 cm. The
telescope is used in normal adjustment.
a) Calculate the angular magnification
b) What is the distance between the lenses?
34. Microscope – similar to a telescope except that
a) The focal points of the two lenses are NOT at the same point
b) The image from the objective lens is formed inside the focal
length of the eye piece.
c) The object is located very close to the objective lens
35. Step 1: Draw the lenses and an axis.
Step 2: Draw a ray through the center of the
objective to a point halfway down the eyepiece.
Draw an object a short distance from the
objective.
Step 3: Draw a parallel ray from the object to the
objective. Continue this ray to the bottom of the
eyepiece. Mark Fo at the point where this ray
crosses the axis. Construct an image at the place
where the two rays intersect.
36. Step 4: Draw a dotted line from the top of the first
image through the middle of the eyepiece.
Choose a point on this line beyond the objective and
draw the rays coming from this point.
37. Aberrations – there are two flaws inherent in all lenses
that must be corrected for perfect images.
Spherical aberration
Rays hitting the very edge of a lens do not focus to the
exact same spot as most of the light.
38. Correcting: Using an aperature to block the
light hitting the edges of the lens
Cameras, eyeballs
