Ray Diagrams
A ray of light is an extremely narrow
beam of light.
All visible objects emit or reflect
light rays in all directions.
Our eyes detect light rays.
We see images when
light rays
converge in our eyes.
converge: come together
Mirrors
It is possible to see
images in mirrors.
image
object
Reflection
• We describe the path of light as straight-line rays
• Reflection off a flat surface follows a simple rule:
– angle in (incidence) equals angle out (reflection)
– angles measured from surface “normal”
(perpendicular)
surface normal
incident ray
same
angle
exit ray
reflected ray
Reflection Vocabulary
• Real Image –
–Image is made from “real” light rays
that converge at a real focal point so
the image is REAL
–Can be projected onto a screen
because light actually passes
through the point where the image
appears
–Always inverted
Reflection Vocabulary
• Virtual Image–
–“Not Real” because it cannot
be projected
–Image only seems to be there!
Plane Mirrors
(flat mirrors)
How do we see images in
mirrors?
Plane Mirrors
(flat mirrors)
object
image
normals
How do we see images in
mirrors?
Light reflected off the mirror converges to form an image in the eye.
Plane Mirrors
(flat mirrors)
object
image
normals
How do we see images in
mirrors?
Light reflected off the mirror converges to form an image in the eye.
The eye perceives light rays as if they came through the mirror.
Imaginary light rays extended behind mirrors are called sight lines.
Plane Mirrors
(flat mirrors)
object
image
normals
How do we see images in
mirrors?
Light reflected off the mirror converges to form an image in the eye.
The eye perceives light rays as if they came through the mirror.
Imaginary light rays extended behind mirrors are called sight lines.
The image is virtual since it is formed by imaginary sight lines, not real light rays.
Hall Mirror
• Useful to think in terms of images
“real” you
mirror only
needs to be half as
high as you are tall. Your
image will the same distance behind the
mirror as you are in front.
“image” you
Complete these ray diagrams to show where the image is formed
A
C
B
D
Curved Mirrors
Spherical Mirrors
(concave & convex)
Concave Mirrors (converging)
(caved in)
•
F
optical axis
Light rays that come in parallel to the optical axis reflect through the focal point.
4 Rules
1. Any ray travelling parallel to the principal
axis into the mirror will pass through the
focal point on the way out of the mirror.
2. Any ray travelling through the focal point
on the way to the mirror will travel parallel
to the principal axis when reflected
3. A light ray that meets the mirror on the
axis will follow rule angle incidence =
angle reflection
Concave Mirror
(example)
•
F
optical axis
Concave Mirror
(example)
•
F
optical axis
The first ray comes in parallel to the optical axis and reflects through the focal point.
Concave Mirror
(example)
•
F
optical axis
The first ray comes in parallel to the optical axis and reflects through the focal point.
The second ray comes through the focal point and reflects parallel to the optical axis.
Concave Mirror
(example)
•
F
optical axis
The first ray comes in parallel to the optical axis and reflects through the focal point.
The second ray comes through the focal point and reflects parallel to the optical axis.
A real image forms where the light rays converge.
Now try Questions 1 and 3 on the curved mirror worksheet
Concave Mirror
(example 2)
•
F
optical axis
Concave Mirror
(example 2)
•
F
optical axis
The first ray comes in parallel to the optical axis and reflects through the focal point.
Concave Mirror
(example 2)
•
F
optical axis
The first ray comes in parallel to the optical axis and reflects through the focal point.
The second ray comes through the focal point and reflects parallel to the optical axis.
Concave Mirror
(example 2)
•
F
optical axis
The first ray comes in parallel to the optical axis and reflects through the focal point.
The second ray comes through the focal point and reflects parallel to the optical axis.
The image forms where the rays converge. But they don’t seem to converge.
Concave Mirror
(example 2)
•
F
optical axis
The first ray comes in parallel to the optical axis and reflects through the focal point.
The second ray comes through the focal point and reflects parallel to the optical axis.
A virtual image forms where the sight rays converge.
Now try question 4 on the curved mirror worksheet
object
•
F
optical axis
concave mirror
• Note: mirrors are thin enough that you just draw a line to represent the mirror
• Locate the image of the arrow
object
•
F
optical axis
concave mirror
• Note: mirrors are thin enough that you just draw a line to represent the mirror
• Locate the image of the arrow
Convex Mirrors (diverging)
(curved out)
•
F
optical axis
Light rays that come in parallel to the optical axis reflect from the focal point.
The focal point is considered virtual since sight lines, not light rays, go through it.
Convex Mirror
(example)
•
F
optical axis
Convex Mirror
(example)
•
F
optical axis
The first ray comes in parallel to the optical axis and reflects through the focal point.
Convex Mirror
(example)
•
F
optical axis
The first ray comes in parallel to the optical axis and reflects through the focal point.
The second ray comes through the focal point and reflects parallel to the optical axis.
Convex Mirror
(example)
•
F
optical axis
The first ray comes in parallel to the optical axis and reflects through the focal point.
The second ray comes through the focal point and reflects parallel to the optical axis.
The light rays don’t converge, but the sight lines do.
Convex Mirror
(example)
•
F
optical axis
The first ray comes in parallel to the optical axis and reflects through the focal point.
The second ray comes through the focal point and reflects parallel to the optical axis.
The light rays don’t converge, but the sight lines do.
A virtual image forms where the sight lines converge.
•
F
object
optical axis
convex mirror
• Note: mirrors are thin enough that you just draw a line to represent the mirror
• Locate the image of the arrow
object
image
•
F
optical axis
convex mirror
• Note: mirrors are thin enough that you just draw a line to represent the mirror
• Locate the image of the arrow
Magnification Equation
Magnification = image height
object height
hi
m
ho
m = magnification
hi = image height
ho = object height
If height is negative the image is upside down
if the magnification is negative
the image is inverted (upside down)
You try…
• Complete the 4 ray diagrams on the
“curved mirror worksheet”
Lesson 4 – Refraction
Objective –
To describe how light is refracted when it
passes from one material to another
To describe how light is refracted by a prism
Activity
Use a rectangular prism
Make a diagram like this
Measure the angles
Work on scrap paper
Refraction
(bending light)
Refraction is when light bends as it
passes from one medium into another.
normal
air
θi
When light traveling through air
passes into the glass block it is
refracted towards the normal.
glass
block
θr
When light passes back out of the
glass into the air, it is refracted away
from the normal.
Since light refracts when it changes
mediums it can be aimed. Lenses are
shaped so light is aimed at a focal
point.
θi
θr
normal
air
You try
•P3.12b
Lenses
The first telescope, designed and built by Galileo, used lenses to focus light from
faraway objects, into Galileo’s eye. His telescope consisted of a concave lens and a
convex lens.
light from
object
convex
lens
concave
lens
Light rays are always refracted (bent) towards the thickest part of the lens.
Convex Lenses
Convex lenses are thicker in the middle and focus light rays to a focal point in front of
the lens.
The focal length of the lens is the distance between the center of the lens and the
point where the light rays are focused.
Convex Lenses
•
F
optical axis
Convex Lenses
•
F
optical axis
Light rays that come in parallel to the optical axis converge at the focal point.
Convex Lens
(example)
•
F
optical axis
The first ray comes in parallel to the optical axis and refracts through the focal point.
Convex Lens
(example)
•
F
optical axis
The first ray comes in parallel to the optical axis and refracts through the focal point.
The second ray goes straight through the center of the lens.
Convex Lens
(example)
•
F
optical axis
The first ray comes in parallel to the optical axis and refracts through the focal point.
The second ray goes straight through the center of the lens.
The light rays don’t converge, but the sight lines do.
Convex Lens
(example)
•
F
optical axis
The first ray comes in parallel to the optical axis and refracts through the focal point.
The second ray goes straight through the center of the lens.
The light rays don’t converge, but the sight lines do.
A virtual image forms where the sight lines converge.
Your Turn
(Convex Lens)
•
F
object
optical axis
image
convex lens
• Note: lenses are thin enough that you just draw a line to represent the lens.
• Locate the image of the arrow.
Your Turn
(Convex Lens)
•
F
object
optical axis
image
convex lens
• Note: lenses are thin enough that you just draw a line to represent the lens.
• Locate the image of the arrow.
Concave Lenses
Concave lenses are thin in the middle and make
light rays diverge (spread out).
•
F
optical axis
If the rays of light are traced back (dotted sight lines),
they all intersect at the focal point (F) behind the lens.
Concave Lenses
•
F
optical axis
Light
Therays
light that
rayscome
behave
in parallel
the same
to the
wayoptical
if we ignore
axis diverge
the thickness
from the
offocal
the lens.
point.
Concave Lenses
•
F
optical axis
Light rays that come in parallel to the optical axis still diverge from the focal point.
Concave Lens
(example)
•
F
optical axis
The first ray comes in parallel to the optical axis and refracts from the focal point.
Concave Lens
(example)
•
F
optical axis
The first ray comes in parallel to the optical axis and refracts from the focal point.
The second ray goes straight through the center of the lens.
Concave Lens
(example)
•
F
optical axis
The first ray comes in parallel to the optical axis and refracts from the focal point.
The second ray goes straight through the center of the lens.
The light rays don’t converge, but the sight lines do.
Concave Lens
(example)
•
F
optical axis
The first ray comes in parallel to the optical axis and refracts from the focal point.
The second ray goes straight through the center of the lens.
The light rays don’t converge, but the sight lines do.
A virtual image forms where the sight lines converge.
Your Turn
(Concave Lens)
object
•
F
optical axis
concave lens
• Note: lenses are thin enough that you just draw a line to represent the lens.
• Locate the image of the arrow.
Your Turn
(Concave Lens)
object
•
Fimage
optical axis
concave lens
• Note: lenses are thin enough that you just draw a line to represent the lens.
• Locate the image of the arrow.
Sound
Objective - To recognise the key features of
a sound wave
To be able to describe a sound wave from
an oscilloscope trace
What is sound?
• Sound travels as waves from vibrating
objects
• Sound waves are a series of
compressions and rarefactions called
longitudinal waves
• Sound relies on particles and so can not
travel through a vacuum.
• Humans can hear sound between 20 –
20000Hz
Basic wave form
Oscilloscope - Wave
diagrams
• The pitch of a note increase with
frequency
• The loudness of a sound increases as the
amplitude of the wave increases
• The quality of a note depends upon the
waveform. Smooth clear sound has a
smooth sine wave.
Other wave forms
Equations from other units
• Frequency of sound is measured in hertz
Hz
• 1 Hz is 1 wave per second.
• The higher the frequency the shorter the
wavelength and the higher the pitch of the
sound
• Frequency (Hz) = 1/ Time period (unit 1b)
• A duck which bobs on a pond 5 x in 10
seconds has a frequency of 0.5 Hz
Speed of sound
• Speed (m/s) = Frequency (Hz) x
wavelength (m) (Unit 2)
If the length of the waves on the duck pond
is 2m then
0.5 Hz x 2m = 1m/s
Ultra sound
Objective – To understand what ultrasound
is and be able to identify some uses of
ultrasound
Sound waves that have too high a frequency
for us to hear are called ultrasound. They
are higher than 20000Hz
Using ultrasound in medicine
Ultrasound is the name given to a medical technique.
It uses high frequency sound waves to produce images of inside the body
without opening up the body.
fetus at 10 weeks
fetus at 20 weeks
ultrasound
for scanning
fetuses instead
of Xa rays
which
would givethey
a
XWhy
raysisare
more energetic
and penetrating
and are
lot more
dangerous,
could cause clearer
damagepicture?
to the growing baby.
How does ultrasound imaging work?
Ultrasound, like all sound, is reflected when it meets different boundaries. So
how is this used for imaging?
An ultrasound machine transmits high-frequency sound waves into the body.
These sound waves are reflected different amounts by different tissues.
The reflected waves are
detected by a receiver.
A computer turns the distance
and intensities of these echoes
into a two-dimensional image.
Breaking up kidney stones
Kidney stones occur in the Nephrons of
kidneys they are very tiny yet cause
blockages and pain.
Traditional treatment relied on the stones
being passed – this was very painful, caused
bleeding and could lead to infections or
surgery.
Ultrasound can break up kidney stones so
that they are easy and harmless to pass.
This prevents surgery and is an non invasive
and safe alternative
Cleaning
Ultra sonic sound can be
used to clean metals, the
tarnish/dirt is blasted off by
the vibrations – this can be
used to clean teeth.
It is very good at cleaning
hard to reach places and
causes very little damage.
Questions
1
Ultrasound is a longitudinal wave with a frequency
greater than 20,000 Hz.
(a)
What is a longitudinal wave?
(b)
Describe the motion of the particles in both a
longitudinal wave and a transverse wave.
(c)
What is meant by frequency?
2
(a)
Apart from body scans, write down one other
medical use of ultrasound.
(b)
Explain how ultrasound is used for body scans.
(c)
Original ultrasound scans were done under water
with the scanner in a plastic bag to keep it dry. Today, a clear gel
is used on the patient’s skin and the head. Why this is
necessary?
3
In many cases ultrasound is used for scans instead of Xrays. Explain the advantages of using ultrasound.
Sound multiple choice