DEMONSTRATE
UNDERSTANDING OF WAVES
AS 2.3
LIGHT 4 WEEKS
WAVES 4 WEEKS
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Light info
Lenses/mirrors
Concave/convex/plane
Reflection/refraction
Incident/reflected rays
Ray diagrams – how to draw and components
needed
Laws of reflection
LIGHT
Type of electromagnetic radiation
 Oscillating electric and magnetic fields
 Travel together at a speed of 3.0x 108 ms¯¹ (speed
of light)
 Form of energy
 Appears to travel in
straight lines
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A lens is an optical device with perfect or
approximate axial symmetry which transmits and
refracts (bends) light, converging or diverging the
beam.
A mirror is an object with at least one reflective
surface. A plane mirror has a flat surface. Curved
mirrors are also used, to produce magnified or
diminished images or focus light
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Concave:
curves inwards
Convex:
curves outwards
Plane:
flat surface
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Rays and wave fronts can generally be used to
represent light when the light is interacting with
objects. We will mainly use rays to analyse how
light interacts with mirrors and lenses.
Reflection
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Reflection from a mirror:
Normal
Reflected ray
Incident ray
Angle of incidence
Mirror
Angle of
reflection
Incident and reflected rays.......
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The ray of light approaching the mirror is known as the
incident ray
The ray of light that leaves the mirror is known as the
reflected ray
At the point of incidence where the ray strikes the
mirror, a line can be drawn perpendicular to the
surface of the mirror. This line is known as a normal line
The normal line divides the angle between the incident
ray and the reflected ray into two equal angles.
The angle between the incident ray and the normal is
known as the angle of incidence.
The angle between the reflected ray and the normal is
known as the angle of reflection.
Laws of reflection.....
1.
The incident ray,
reflected ray
and the normal
all lie in the same
plane
2.
The angle of
incidence is
equal to the
angle of
reflection
Ray diagrams
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Are diagrams drawn to scale and used to determine
the details of an image produced by either a lens or
mirror
You need to know how to figure out the following
details of the image produced
Position: how far along the principal axis, may be either
behind or in front of the mirror
Size: cm/height
Nature: upright/inverted, virtual or real, magnified or
diminished
Focal point: the point through
which all parallel light rays
are reflected through
 Centre of curvature: centre point
circle
 Radius of curvature: distance between pole and centre
of curvature (same as radius of circle)
 The focal length of a curved mirror is half the radius of
curvature
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A mirror is drawn as a curved line and/or a straight
mirror line to make ray diagrams easier. Dashed
lines or shading on the non-reflective side.
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Draw a ray diagram of:
A concave mirror, with principal axis, focal point of
4cm and centre of curvature of 8cm.
Draw an object (arrow) of 2cm height 5cm from the
pole.
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1. Look at the diagram below.
Which one of the angles (A, B, C, or D) is the angle
of incidence? ______
Which one of the angles is the angle of reflection?
______
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2. A ray of light is incident towards a plane mirror
at an angle of 30-degrees with the mirror surface.
What will be the angle of reflection?
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3. A ray of light is approaching a set of three mirrors as shown
in the diagram. The light ray is approaching the first mirror at
an angle of 45-degrees with the mirror surface. Trace the path
of the light ray as it bounces off the mirror. Continue tracing
the ray until it finally exits from the mirror system. How many
times will the ray reflect before it finally exits?
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4. You can observe the image of the sun in the windows of
distant buildings near the time that the sun is rising or setting.
However, the image of the sun is not seen in the windows of
distant building during midday. Use the diagram below to
explain.
Formula........
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: is the image distance
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: is the object distance
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Magnification factor:
Converging
Diverging
f>0
+ve focal length
f<0
-ve focal length
Concave
Convex
Focal length
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The terms concave and convex don’t tell you
whether the mirror or lens is converging or diverging
and therefore whether the focal length is positive or
negative.
Need to remember the table.
Other variable
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Object distance: for single lens or mirror is always
positive.
Image distance: i is positive for a real image
i is negative for a virtual image
Image location: (reflected rays) where the outgoing
light rays or their tracebacks converge
Real: reflected rays converge
Virtual: traceback rays converge
Real image: same side as the reflected light
positive image distance.
 Virtual: opposite side (to outgoing light)
negative image distance.
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Magnification: >1 magnified
<1 diminished
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So we know that if our mirror or lens is converging or
diverging-this tells us about the focal point
Real or virtual tells us about the image distance
Magnification tells us whether the image is smaller or
larger than the object.
We can then use what we know to help confirm whether
out ray diagram is correct.
CONCAVE MIRRORS
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Used to converge light and focus it on a point
Law 1: Any light ray that is parallel to the principal axis will
be reflected through the focal point
Law 2: Any light ray that passes through the focal point will be
reflected back parallel to the principal axis
Converging mirror..........
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Real vs Virtual image
Real means that the image is formed by converging
reflected light rays.
If the object gets too close to the mirror (closer than the
focal point), the reflected rays diverge. The human eye
can still see this image but it is virtual and cannot be
projected onto a screen.
Ray diagram-if the rays diverge, continue on the
reflected rays behind the mirror to the point where they
converge
LIGHT……
Review mirrors/lenses/ray diagrams
• Ray diagrams cont…..
• Use formula to find unknowns
•
1.
2.
3.
An incoming ray of light parallel to the principal
axis will be ___________________________
Any light ray that passes through the focal point
will be ________________________________
How is a virtual image formed?
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What is this and what will it do to the incoming light
rays?
Draw a convex mirror-what will it do to the incoming
light rays?
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An incoming ray of light parallel to the principal axis
will be …
reflected back through the focus
Any light ray that passes through the focal point will
be ….
reflected back parallel to the principal axis
How is a virtual image formed?
The reflected light rays converge and so tracebacks
need to be continued to form a virtual image.
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What is this and what will it do to the incoming light
rays?
Concave lens that will diverge the light rays.
Draw a convex mirror-what will it do to the incoming
light rays?
Diverge the light rays.
Formula example…..
•FIND THE FOCAL POINT OF A
MIRROR IF THE OBJECT DISTANCE IS
9CM AND THE IMAGE DISTANCE IS
18CM
CONCAVE MIRRORS...
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Don’t need to use negative number for
magnification equation
If you need to work out the size of the image, you
need to work out the magnification and then
multiply the object size by this magnification.
CONVEX MIRRORS
Laws
Formulas
Ray diagrams
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Law 1: any incoming light ray that is parallel to the
principal axis is reflected back so it is aligned with
the far focal point
Law 2: any incoming light ray directed towards the
focal point is reflected back parallel to the
principal axis
CONVEX MIRRORS
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Image is always virtual i.e. Formed from traceback
rays.
And always behind the mirror.
Image is formed between the focus and the mirror.
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Same formulas apply
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The focal point is always negative as it is behind the
mirror.
CONVEX MIRRORS
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ALWAYS:
Located behind the convex mirror
A virtual image
An upright image
Reduced in size
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Practice:
Using a ray diagram find the image location and
size.
Convex mirror, focal length of 8cm, 3cm high, and
placed 4cm away from the mirror.
Convex Mirror Practice
Ray
diagrams
Pg 108
Formula
questions
Pg 109
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Real image video.......................
CONVEX MIRRORS…….
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Name 2 features of an image formed by convex
mirror.
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What does a convex mirror do to light rays?
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Why is the focal point negative?
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What is the formula for magnification?
CONCAVE PRAC 2 ...
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Investigate the formula
Set up the mirror as you did for the last experiment.
Repeat step 2 to find the COC and F points for your mirror.
Then measure 6-10 different values for object & image
distance in a table.
Complete the table as it is set up in your workbook.
What happens if you try distances less than the focal point?
Write this up with conclusion about what you have found-you
need to be able to explain your processes.
REFRACTION OF LIGHT
REFRACTION:
When a light ray refracts, it bends.
 The light rays will only bend at the
boundary between 2 different substances. Prior to
and after that the light rays will travel in a straight
line.
 The refraction is caused by the light changing speed
as it enters a new medium.
 The amount of refraction depends on the refractive
index of the substance.
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n= speed of light in a vacuum
speed of light in medium
SNELLS LAW.....
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The refractive index of a medium is given by Snell’s
law..........
If a ray of light passes across the boundary from a
material in which it travels fast into a material in which
travels slower, then the light ray will bend towards the
normal line.
If a ray of light passes across the boundary from a
material in which it travels slowly into a material in
which travels faster, then the light ray will bend away
from the normal line.
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http://www.upscale.utoronto.ca/PVB/Harrison/Flas
h/Optics/Refraction/Refraction.html
A ray of light strikes a glass block (n=1.5) at 40 deg to the normal.
Find the angle through which the ray is deviated. Draw a diagram to
model this situation.
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1. Assume
= 1 sin (40)
1.50
2.
θ= 25.4 deg
3. The angle of deviation will therefore be
40-25.4 = 14.6 deg.
Refractive
index/Snells
Law Questions
Pages 100102