2. Alexander Pope
• Nature and Nature’s laws lay hid in night;
• God said, Let Newton be!
• And all was light.
• Epitaph intended for Sir Isaac Newton
3. Light
• Light is an electromagnetic disturbance
whose propagation allows the
transportation of light energy.
• Light is considered to travel as a wave
AND as particle or photon.
• As a wave, light has different
frequencies or cycles per second (cps)
• The unit of frequency is a hertz (Hz).
4. Transmission of light
• Recti-Linear propagation: light travels in
straight lines;
• Ray: the path taken by light energy;
• Ray diagrams: pictures that show the
paths of light rays;
• Diverge: light rays become farther apart;
• Converge: light rays come closer together;
• Parallel rays: rays that travel the same
distance apart;
• Speed of light: c = 3x108
m/s
5. Speed of light
• Light travels at a definite speed (c);
• c = 3x108
m/s = 300000 km/s
• Distance = speed x time
• Calculate the distance (km) travelled in:
• 1 minute
• 1 hour
• 1 day
• 1 year - 365.25 days – 1 lightyear
7. Pinhole camera
• Pinhole cameras demonstrate that
light travels in straight lines;
• An image is formed inverted
vertically and horizontally on the
back of the
camera
• The image will
be inverted,
smaller and real.
8. Pinhole camera
• Magnification = height of image
height of object
• M = h i/ h o
• As well, M = distance to image
distance to object
• Or, M = d I/d O
• So h i/ h o = d I/d O
9. QuestionA pinhole camera is used to show the image of a turtle on its screen as shown below.
Which of the following is the image of the turtle that will appear on the screen of the pinhole
camera?
A) C)
B) D)
10. Question
Alex places a 5.00 cm tall candle 30.0 cm in front of a pinhole camera
that has a length of 50.0 cm.
What is the magnification of the candle?
A) 1.66
B) 0.166
C) 0.63
D) 0.1
11. Shadow
• Umbra: the area of total
shadow – this area receives no light;
• Penumbra: the area of
partial shadow – this area
receives some light;
• E.g. lunar/solar eclipses
12. Question
When only the Moon's penumbral shadow strikes Earth, we see a partial eclipse of the Sun from
that region.
In which region(s) on Earth do we see a partial eclipse of the Sun?
A) Region A only
B) Region B only
C) Regions A and C
D) Regions A, B and C
14. Summary
• A luminous light source emits its own light.
• A luminous light source can produce its light
by incandescence, luminescence,
fluorescence, or phosphorescence.
• Light striking an object can be reflected,
refracted, absorbed or any combo of the 3.
• Other light phenomena: diffraction, diffusion,
scattering, interference, dispersion.
• Light travels in straight line ray paths –
rectilinear propagation.
15. Summary
• When an object creates a shadow, the
area of total shadow is the umbra, the
area of partial shadow is the penumbra.
• For a pinhole camera, M, the
magnification is given by
• M = hi/ho = di/do.
17. Vocabulary
• Reflection in a plane mirror,
• Incident ray – ray coming into
surface
• Point of incidence – where the
ray hits the surface
• Reflected ray – the reflected ray
• Normal – line at 90 ⁰ to point of
incidence.
18. Vocabulary
• Image Type
• Virtual only appears in the eye of
the beholder
• Real can be put on a screen
• Attitude or Orientation – upright
or inverted (upside down)
• Size – smaller, same, bigger
19. Types of Reflection
• Diffuse reflection occurs when
the reflected rays of light are
not parallel.
• The surface is usually rough and
uneven.
• E.g. paper, snow
20. Types of Reflection
• Specular (regular) reflection
occurs when the reflected rays
of light emerge parallel from the
surface.
• The surface is usually smooth
and polished.
• E.g. mirror, glass
22. Angle of Incidence &
Reflection
• The angles of incidence and
reflection are measured relative
to the NORMAL –
• NOT THE SURFACE.
• The angle of incidence equals
the angle of reflection.
• The incident ray, the reflected
ray and the normal are all in the
same plane.
23. Formation of Image by a
Plane Mirror – Ray
Diagram
• A ray of light incident on a plane mirror at
90o
gets reflected from the mirror along the
same path.
• A ray of light falling on a plane mirror at
any angle gets reflected from the mirror
such that the angle of incidence is equal to
the angle of reflection.
• P. 45, Q. 1, 3
24. Images formed in a Plane
Mirror
• A plane mirror is a flat mirror.
• Images are the same attitude – upright
or inverted.
• Images are the same distance “behind”
the mirror as the object is in front of it.
• Images formed are virtual – they
cannot be projected on a screen.
• A line joining the image and object is
perpendicular to the plane mirror.
25. QuestionQuestion
An object is placed in front of a plane mirror.
Which of the following diagrams correctly represents the image formed in the plane mirror?
A) C)
B) D)
26. QuestionQuestion
An object is placed in front of a plane mirror. Which statement correctly describes the
characteristics of the image?
A) The image is real, upright, smaller than the object and located in front of the mirror.
B) The image is virtual, inverted, larger than the object and located behind the mirror.
C) The image is real, inverted, the same size as the object and located in front of the
mirror.
D) The image is virtual, upright, the same size as the object and located behind the
mirror.
27. Exercise
• Using a light beam, ruler and
plane mirror, draw the incident,
normal and reflected ray coming
off the mirror.
• Compare the angles.
• __________________________________
28. Field of Vision
• The field of vision from a mirror
is determined by the width of
the mirror, the distance of the
observer to the mirror and its
curvature.
• In stores curved mirrors are
used to give a wider field of
vision.
30. Field of Vision
• Our view in a mirror depends on how we
position ourselves in front or to the side of
the mirror.
• The field of vision can be determined
through ray diagrams:
• a. Draw two normals, one at either end of
the mirror,
• b. Draw an incident ray from your eye to
each normal.
• c. Draw a reflected ray from each normal.
• d. Remember the laws of reflection when
drawing both sets of rays (i = r)
• Make a diagram – ask teacher.
31. Activity: Determine FieldActivity: Determine Field
of Vision:of Vision:
for a Plane Mirrorfor a Plane Mirror
________MirrorMirror__________
OO
33. QuestionQuestion
An observer is standing in front of a reflective window in which he can see the images of some
shrubs.
Based on the above diagram, the images of which shrubs can be seen by the observer?
A) III only
B) II and III only
C) III and IV only
D) I, II, III and IV
Good, now answer: p. 47, Q. 1-3
35. 35
The Inside of A Spoon
• Why is it when you look into a soup spoon
that your image turns upside down
sometimes?
• The inside of a spoon is a concave mirror.
• If the inside of the spoon is held close to
the eye, a magnified upright view of the
eye will be seen (in this case the eye is
closer than the focal point of the mirror).
• If the spoon is moved farther away, a
smaller upside-down view of the whole
face will be seen.
36. 36
Concave Mirrors
• The concave mirror has a reflection
surface that curves inward, like a
portion of the interior of a sphere.
• When light rays that are parallel to
the principal or optical axis reflect
from the surface of a concave mirror,
they converge on the focal point
(black dot) in front of the mirror.
38. 38
Rules for Con/Di-Verging
Mirrors
• A ray that is parallel to the principal
axis is reflected through the
principal focus – real or virtual.
• A ray passing through the principal
focus is reflected parallel to the
principal axis.
• A ray passing through the centre of
curvature is reflected back along the
same path.
• The centre of curvature or radius is
twice the focal length.
39. QuestionQuestion
Look at the diagram below. A 12 cm tall object is placed in front of a concave mirror. The focal
length is 30 cm. The object is located 70 cm from the top of the mirror.
What will be the height of the image reflected by the mirror?
A) 5.1 cm
B) 9.0 cm
C) 16 cm
D) 28 cm
40. Concave Mirrors
Object’
s
Positio
n
Image Characteristics
Type Orientati
on
Size Position
At ∞ Real Point Image At F
Beyond
C
Real Inverted Smaller Between F and C
At C Real Inverted Same size At C
Between
C and F
Real Inverted Bigger Beyond C
At F No Image
Between
F and V
Virtual Upright Larger
than the
object
Between F and V
behind the mirror,
farther from the
mirror than the
object.
41. 41
Convex Mirrors
• The convex mirror has a reflecting
surface that curves outward like a
portion of the exterior of a sphere.
• Light rays parallel to the optical axis
are reflected from the surface in a
manner that diverges from the focal
point, which is behind the mirror.
• Images formed with convex mirrors are
always right side up and reduced in
size.
• These images are also termed virtual
images, because they occur where
reflected rays appear to diverge from a
focal point behind the mirror.
42. 42
Diverging Mirrors
• Diverging mirrors are convex.
• Regardless of the position of the
object reflected by a convex mirror,
the image formed is always virtual,
upright, and reduced in size.
• When the brain retraces the rays,
they appear to come from behind the
mirror where they would converge,
producing a smaller upright image
• The image is upright since the
virtual image is formed before the
rays have crossed the focal point.
44. Exam Question – on board
A) + 1.60
B) + 0.63
C) - 0.63
D) - 1.63
The teacher will draw this on the board, please.
Alex places a 5.00 cm tall candle 30.0 cm in front of a
mirror that has a focal length of ‑50.0 cm.
What is the magnification of the candle?
Good, now do, page 54, Q. 4, 7, 9
45. Mirror Equations
• do is the distance to the object
• di is the distance to the image
• f is the focal length
• hi is the image height
• ho is the object height
• N.B. the negative sign
46. Conventions for the
Equations
• Distances are measured from the vertex.
• Focal lengths are positive for converging
mirrors and negative for diverging mirrors.
• Radii of curvature are positive for converging
mirrors and negative for d.m.
• Image and object distances are positive for
real images and objects.
• Image and object distances are negative for
virtual images and objects.
• Image and object heights are positive when
upright and negative when inverted.
47. Exam Question
A) 20.0 cm C) 90.0 cm
B) 60.0 cm D) 120.0 cm
An object, 45.0 cm high, is placed in front of a
convex mirror.
A virtual image, 15.0 cm high, is formed.
The focal length of the mirror is -30.0 cm.
At what distance is the object from the mirror?
Good, now do: p. 70, Q. 1,2,9,10 and p. 76, Q. 1,2,9
48. 48
Uses for Concave Mirrors
• Concave mirrors are used in optical
telescopes to collect the faint light
emitted from very distant stars.
• The curved surface concentrates
parallel rays from a great distance
into a single point for enhanced
intensity.
• This mirror design is also commonly
found in shaving or cosmetic mirrors
where the reflected light produces a
magnified image of the face.
49. 49
Uses for Convex Mirrors
• Convex mirrors are often used in
automobile right-hand rear-view
applications where the outward mirror
curvature produces a smaller, more
panoramic view of events occurring
behind the vehicle.
• Convex mirrors are also used as wide-
angle mirrors in hallways and
businesses for security and safety.
• Carnival Fun House mirrors often
incorporate a mixture of concave and
convex surfaces, or surfaces that
gently change curvature, to produce
bizarre, distorted reflections when
50. Other Reflections
• Non-optical reflection can occur
as:
• Echoes – sounds reflected from
surface;
• Radio and microwaves;
• Ocean waves when they recede;
• Strings/Springs – as in guitar
strings;
51. Summary
• A normal is a line drawn at right
angles to a reflective surface at
the point of incidence.
• The Laws of Reflection are:
– The incident ray, the reflected ray
& the normal are all in the same
plane;
– The angle of incidence equals the
angle of reflection
52. • Specular reflection is the reflection of
light from smooth plane surfaces. Rays
parallel.
• Diffuse reflection is the reflection of
light from irregular surfaces. Rays not
parallel.
• The field of vision using a plane mirror
depends on the size of the mirror and
the distance of the observer and the
object-to-be-seen from the mirror.
• Curved mirror terms – centre of
curvature, principal axis, vertex, radius
of curvature, principal focus, focal plane
& focal length.
53. • All rays parallel to each other and
striking a converging mirror meet
at the focal point.
• The focal length is the distance
between the principal focus and
the vertex.
• In a curved mirror, the focal
length is ½ the radius.
• Rays from distant objects are
considered parallel.
54. • Spherical aberration occurs in
curved mirrors when parallel rays
do not meet at the focal point. A
parabola can solve this.
• The Laws of Reflection apply to
non-optical phenomena, like
sound.
• Lenses are used to correct vision
problems.
