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Light
GCSE Physics
Reflection
Law of Reflection
The angle of incidence (i)
is equal to the angle of
reflection (r)
Note: Both angles are
measured with respect to the
‘normal’. This is a construction
line that is perpendicular to the
reflecting surface at the point
of incidence.
incident ray
normal
angle of
incidence, i
angle of
reflection, r
reflected ray
mirror
The image formed by a plane mirror
image
object
normals
construction lines
(virtual light rays)
plane
mirror
The image produced by the plane
mirror is:
1. The same size as the object
2. The same distance behind the
mirror as the object is in front
3. Upright (the same way up as
the object)
4. Back-to-front compared with
the object (lateral inversion)
5. Virtual
Real and virtual images
REAL images are formed where light rays cross
after reflection by a mirror or refraction by a lens.
Real images can be cast onto a screen.
Example: A projector image
VIRTUAL images are formed where light rays only
appear to come from.
A virtual image cannot be cast onto a screen.
Example: The image formed by a plane mirror
Choose appropriate words to fill in the gaps below:
reflection
The law of reflection states that the angle of __________
is
equal to the angle of incidence.
always _______
normal
Both angles are measured relative to the _________,
a line
ninety degrees to the reflecting surface at the
that is at _______
point of reflection.
A plane mirror forms a _______
virtual image which is unlike a
real image in that it cannot be cast onto a screen. The
_____
image in a plane mirror is also the same ______
size and the same
way up as the object.
WORD SELECTION:
normal
virtual
size
reflection real
equal ninety
Simulations
Optical illusions - by eChalk
BBC KS3 Bitesize Revision:
KS3 Light Contents Page
What is light?
Seeing
Test bite on KS3 Light
Law of Reflection - NTNU - features a movable plane mirror
Reflection in a plane mirror - eChalk
Simulations
Balloon blasting game - eChalk
Height of mirror and image seen - NTNU
Virtual image formation - eChalk
Image formed by a plane mirror - NTNU
Reflection and images from two mirrors at 90 degrees to each other NTNU
Law of Reflection - Crocodile Clip Presentation
BBC KS3 Bitesize Revision:
Reflection & Scattering
Concave mirror
A concave mirror is like the inside of a spoon.
concave
mirror
principal
focus
F
principal axis
O
C
centre
of the
mirror
centre of
curvature
focal length, f
radius of curvature, r
Convex mirror
A convex mirror is like the outside of a spoon.
convex
mirror
principal axis
principal
focus
C
O
F
centre of
curvature
focal length, f
radius of curvature, r
Mirror definitions
The principal axis is a construction line that is perpendicular to
and passes through the centre of the mirror, O.
The principal focus, F is the point through which all rays
travelling parallel to the principal axis before reflection pass
through or appear to come from after reflection.
The focal length, f is the distance from the centre of the mirror,
O to the principal focus, F.
The centre of curvature, C is the centre of the circle of which the
surface of the mirror is a part.
The radius of curvature, r is the distance along the principal
axis between the centre of the mirror, O and the centre of
curvature and equal to TWICE the focal length, f of the mirror.
Standard rays – concave mirror
(a) Rays incident parallel to the principal axis pass
through the principal focus after reflection.
concave
mirror
principal axis
F
(b) Rays passing through the centre of curvature
before reflection are reflected back along their initial
path.
centre of
curvature
F
C
principal axis
(c) Rays striking the centre of the mirror are reflected
as if the mirror was flat.
O
Concave mirror images
1. Object more than the radius of curvature away from the mirror.
object
C
image
F
Use:
Satellite receiver (with microwaves)
The image formed is:
Smaller than the object (diminished)
Between F and C
Inverted (upside down)
Real
O
2. Object between F and C
object
C
F
image
Use:
Satellite transmitter (with microwaves)
The image formed is:
Larger than the object (magnified)
Beyond C
Inverted
Real
O
3. Object nearer than the principal focus
object
image
C
F
observer
Use:
Makeup and shaving mirrors
The image formed is:
Larger than the object
On the other side of the mirror from the object
Upright
Virtual
Standard rays – convex mirror
(a) Rays incident parallel to the principal axis appear
to come from the principal focus after reflection.
convex
mirror
principal
axis
F
(b) Rays heading for the centre of curvature before
reflection are reflected back along their initial paths.
convex
mirror
principal axis
F
C
(c) Rays striking the centre of the mirror are
reflected as if the mirror was flat.
O
Convex mirror images
Objects at all distances from a convex mirror
object
image
F
C
observer
Use:
Security mirrors
The image formed is:
Smaller than the object
On the other side of the
mirror from the object
Upright
Virtual
Convex mirrors give
a wide field of view
Magnification
magnification = image height
object height
Question:
Calculate the magnification if a mirror produces an
image of 40cm from an 8cm sized object.
magnification = image height / object height
= 40cm / 8cm
magnification = 5 x
Answers
Complete:
object height
image height
magnification
3 cm
24 cm
8x
15 cm
45 cm
3x
20 cm
5 cm
0.25 x
300 mm
0.10 m
3x
Choose appropriate words to fill in the gaps below:
concave
There are two types of curved mirror, __________
and
convex. Concave mirrors look like the ________
of a spoon.
inside
converge
Concave mirrors __________
light parallel to the principal
principal focus.
axis so that all the rays pass through the __________
Makeup mirrors can be concave in order to provide a
magnified
__________
view of the face.
convex
Light is diverged by _________
mirrors. These always
virtual images and are used to provide a wide field
produce _______
car rear view mirrors.
of view which is especially useful for ____
WORD SELECTION:
converge
magnified concave principal car convex
inside virtual
Simulations
Lens / mirror effect on a beam of light - NTNU
Tiger image formation by a plane or curved mirror
- NTNU
Mirage of pig formed by a concave mirror includes UTube clip - NTNU
Curved mirror images / ray diagrams - NTNU
Refraction
Refraction occurs when a wave
changes speed as it passes
from one region to another.
This speed change usually
causes the wave to change
direction.
Water waves slow down as they
pass over from a deeper to a
shallower region.
Light slows down as it passes
from air into glass, perspex or
water.
Refraction of light at a plane surface
(a) Less to more optical dense transition (e.g. air to glass)
AIR
GLASS
normal
angle of
incidence
angle of
refraction
Light bends TOWARDS the normal.
The angle of refraction is LESS than the angle of incidence.
(b) More to less optical dense transition (e.g. water to air)
angle of
refraction
normal
angle of
incidence
WATER
AIR
Light bends AWAY FROM the normal.
The angle of refraction is GREATER than the angle of incidence.
Refraction experiment
Typical results:
angle of
incidence / °
angle of
refraction / °
deviation / °
0
0
0
15
10
5
30
19
11
45
28
17
60
35
25
75
40
35
No deviation occurs when the angle of incidence is zero.
Increasing the angle of incidence increases the deviation.
Why a pool appears shallow
normals
observer
AIR
WATER
image
object at the
bottom of a pool
Complete the paths of the RED light rays:
A
B
D
C
E
F
Dispersion
A prism splits the colours of white
light into the spectrum.
This is called dispersion.
Violet light deviates the most,
red the least.
white
light
prism
spectrum
Choose appropriate words to fill in the gaps below:
Refraction occurs when a wave changes ______
speed as it crosses
direction of the
the boundary between two regions. The _________
wave also usually changes.
towards the normal when they pass from
Light rays deviate ________
perspex
less dense air to more dense _________.
The greater the
deviation
angle of incidence the greater is the _________.
colours of light deviate by different amounts. Violet
Different ______
deviates the _____.
most A prism can be used to split the colours of
dispersion
white light into a spectrum. This is called _________.
WORD SELECTION:
towards deviation most dispersion direction perspex speed colours
Simulations
Refraction - Powerpoint presentation by KT
Light Refraction - Fendt
Reflection & Refraction at a boundary - NTNU
Refraction animation - NTNU - Does not show TIR effect
Law of Refraction - Crocodile Clip Presentation
Prism - non dispersive reflections and refractions - NTNU
Prism/Lens - non dispersive refraction and reflections - NTNU
BBC KS3 Bitesize Revision:
Refraction
Light moving from water to air - NTNU
Where is the fish? - refraction by water - NTNU
The appearance of an object under water / ray diagram - NTNU
How a fish sees the world - NTNU
Fibre optic reflection - NTNU
BBC Bitesize Revision:
Optical fibres
Dispersion - Powerpoint presentation by KT
Dispersion of light using a prism - NTNU - prism apex angle can be
changed
Prism showing light dispersion for different colours - Explore Science
Dispersion - Crocodile Clip Presentation
Sequential Puzzle on Colour Spectrum order- by KT - Microsoft WORD
Prism - multishape prism and single light ray - no extra reflections netfirms
BBC KS3 Bitesize Revision: Dispersion
Lenses
Converging lens
With glass and plastic lenses a converging lens
has a convex shape.
Converging lens with a parallel beam of light
principal
focus
centre
of the lens
principal axis
O
F
converging lens
focal length, f
Diverging lens
With glass and plastic lenses a diverging
lens has a concave shape.
Diverging lens with a parallel beam of light
principal focus
F
O
diverging
lens
focal length, f
principal axis
Lens definitions
The principal axis is a construction line that is
perpendicular to and passes through the centre
of the lens.
The principal focus, F is the point through which
all rays travelling parallel to the principal axis
before refraction pass through or appear to
come from after refraction.
The focal length, f is the distance from the
centre of the lens, O to the principal focus, F.
Standard rays – converging lens
(a) Rays incident parallel to the principal axis pass through the principal
focus after refraction.
principal
focus
principal axis
F
(b) Rays passing through the centre of the lens are not deviated.
centre
of the
lens
O
(c) Rays passing through the principal focus before refraction are
refracted parallel to the principal axis.
principal axis
F
F
Converging lens images
1. Object more than twice the focal length distant from a converging lens
object
O
2F
F
Uses: Camera and Eye
The image formed is:
Smaller than the object
(diminished)
Between the F and 2F
Inverted (upside down)
Real
F
2F
image
2. Object between F and 2F
object
2F
Use: Projector
The image formed is:
Larger than the object
(magnified)
Beyond 2F
Inverted
Real
F
F
2F
image
3. Object nearer than the principal focus
image
F
object
F
observer
Uses: Magnifying glass
The image formed is:
Larger than the object
On the same side of the lens as the object
Upright
Virtual
Standard rays – diverging lens
(a) Rays incident parallel to the principal axis appear to come from
the principal focus after refraction.
diverging lens
principal axis
F
(b) Rays passing through the centre of the lens are not deviated.
O
(c) Rays heading for the principal focus before refraction are
deviated parallel to the principal axis.
O
F
principal axis
Diverging lens images
Objects at all distances from a diverging lens
object
F
image
Use:
Correction of short sight
The image formed is:
Smaller than the object
On the same side of the lens as the object
Upright
Virtual
observer
The Camera
The camera uses a
converging lens to produce
an image on a light detecting
surface such as a CCD
(Charge Coupled Device)
chip or photographic film.
In most cases the subject (object) is well more than twice
the focal length of the lens away from the camera.
The image produced is therefore:
diminished, inverted and real.
subject
(object)
2F
image
produced on
CCD or film
F
F
Structure of a simple camera
shutter
CCD
or
film
lens
aperture
Focussing is achieved by moving the lens away from or towards
the light detector. The further the subject is away from the camera
the closer the lens is moved towards the CCD / film.
The amount of light reaching the light detector is controlled by
adjusting the size of the ‘stop’ aperture and the length of time that
the shutter opens.
Choose appropriate words to fill in the gaps below:
refraction A converging lens
Lenses work by the process of _________.
convex shaped piece of glass or perspex.
is made by using a _______
nearer
A magnifying glass requires the object to be placed _______
virtual
than its focal length. The image formed is _______.
magnified and inverted image
A projection lens produces a __________
diminished
whereas a camera lens normally produces a _________
real
image. In both cases the image is ______.
short
Diverging lenses are used to correct ______sight.
WORD SELECTION:
convex magnified short refraction nearer real virtual diminished
Simulations
Geometric Optics with Lenses - PhET - How does a lens
form an image? See how light rays are refracted by a
lens. Watch how the image changes when you adjust the
focal length of the lens, move the object, move the lens, or
move the screen.
Prism/Lens - no dispersive refraction and reflections NTNU
Lens images / ray diagrams - NTNU
How an image is formed by a convex lens / effect of
stopping down lens - NTNU
Lens / mirror effect on a beam of light - NTNU