Transcript Ray Diagrams

By
Mark Jordan
©
The Professional Development Service for Teachers is funded by the
Department of Education and Skills under the National Development Plan
OUTLINE OF THE DAY
To Be Completed by Class Teacher
The Professional Development Service for Teachers is funded by the Department of
Education and Science under the National Development Plan
What do you think?
•
Light is stated to travel at a of 299 792 458 m / s. so how long does it take
light to come from the Sun to Earth?
•
How often could light travel around the earth in one second?
•
With the advent of new innovative technology is it likely that light will be
made travel faster in the future?
•
Dave Grennan, Irish astronomer, recently discovered a supernova that
exploded nearly 300,000 years ago yet the light from that explosion is now
only reaching Earth. How is this possible?
(see notes for more information)
Light is part of
Electromagnetic Spectrum
– the part we can see, i.e. the visible spectrum
Shortest 
Longest 
Electromagnetic waves (including light)
travel at a speed of 3 x 108 ms-1
(see notes for more information)
The visible spectrum is made up of seven colours.
• Can you explain why we can see these different
colours.
• Is black a colour?
Light bounces of surfaces. Click the link below (must have Quicktime
installed) to find more about bouncing light and ……. photons.
http://www.teachersdomain.org/asset/lsps07_vid_lightreflect/
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.
Light can be reflected. Reflection is the
bouncing of light of a solid object
It is possible to see
images in mirrors.
image
object
Mirrors are good at reflecting light rays.
Plane Mirrors
How do we see images in
mirrors?
Light reflected off the mirror converges to form an image in the eye.
image
The eye perceives light rays as if they came from the mirror.
The image is virtual since it is formed by the apparent intersection of light rays.
(apparent rays are indicated on the diagram as broken lines and actually don’t exist)
Laws of Reflection Exp.- Follow steps in animation
The normal is a line right angles to the mirror where the ray of light hits it.
(A ray of light striking the mirror at 900 is reflected back along the same path).
normal
Law 1
When light is reflected off
a mirror, it hits the mirror
at the same angle (the
incidence angle, θi) as it
reflects off the mirror (the
reflection angle, θr).
Angle of
reflection
Angle of
incidence
reflected
ray
incident
ray
Law 2
The incident ray,
the reflected ray
and the normal
are all lie on the
same plane.
θr θi
Mirror
Points to ponder
•
A driver in a parked car has 2 views of the car parked behind him – ‘rear view
mirror’ (right) & in the ‘side mirror ‘(left).
o How is it that each mirror gives a different view?
o Which view represents the true distance the parked car is from the drivers car?
(see notes for more information)
Concave Mirror- Part of
a sphere reflective
surface on inside
•
C
r
•
F
f
C: the center point of the sphere
r: radius of curvature (just the radius of the sphere)
F: the focal point of the mirror (halfway between C and the mirror)
f: the focal distance, f = r/2
Concave Mirrors
(caved in)
•
F
optical axis
•Light rays that come in parallel to the optical axis reflect through the focal
point
•Light rays that come in along the optical axis strike the mirror at 90 so reflect
back along optical axis through the focal point.
Concave Mirror
Image formed in a concave mirror object placed outside centre of curvature
Centre of Curvature
Focus
v
Object
•
c
•
F
f
Principal axis
u
Image:- Real, Inverted & diminished
Concave Mirror
Image formed in a concave mirror when object placed at centre of curvature
Centre of Curvature
uFocus
Object
•
c
f
•
F
Principal axis
v
Image:- Real, Inverted & diminished
Concave Mirror
Image formed in a concave mirror when object placed between centre of
curvature & focus
Centre of Curvature
Focus
u
Object
•
c
•
F
f
Principal axis
v
Image:- Real, Inverted & Enlarged
Concave Mirror
Image formed in a concave mirror when object placed at focus
Centre of Curvature
Focus
Object
•
c
•
F
u
f
Principal axis
Image:- At Infinity
Concave Mirror
Image formed in a concave mirror when object placed inside focus
Centre of Curvature
u
Focus
Object
•
c
•
F
Principal axis
f
v
Image:- Virtual, Erect & Enlarged
Equation
1 1 1
 
f u v
ƒ = focal length
u = object distance
v = image distance
if distance is negative the image is behind the mirror
Magnification Equation
v
m
v
m = magnification
v = image height
u = object height
if the magnification is negative
the image is inverted (upside down)
Sign Convention for Mirrors
Quantity
Positive (+)
Negative (--)
Object location (u)
Object is in front of Object is behind
the mirror
the mirror
Image location (v)
Image is front
mirror
Image is behind of
mirror
Focal length (f)
Mirror is concave
Mirror is convex
Magnification (M)
Image is upright
Image is inverted
TO FIND THE FOCAL LENGTH OF A CONCAVE
MIRROR
Crosswire
Concave
mirror
Lamp-box
Screen
u
v
Procedure
•
Get the approx. focal length of mirror by focusing distant object on screen – why?
•
Place the lamp-box well outside the approximate focal length – why?
•
Move the screen until a clear inverted image of the crosswire is obtained.
•
Measure the distance u from the crosswire to the mirror, using the metre stick.
•
Measure the distance v from the screen to the mirror.
•
Calculate the focal length of the mirror using - - - - - •
•
Repeat this procedure for different values of u.
Calculate f each time and then find an average value.
1 1 1
 
f u v
Convex Mirrors
•
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 Mirrors
Focus
Centre of Curvature
v
Object
•
F
u
•
C
f
principal axis
Image:- Virtual, Erect & Diminished