Concave mirrors:

1

• Concave mirrors
• When an object is at C (centre of curvature) you
  get a real, upside down image the same size as
  the object.
• Between C and F, you get a real, upside down
  image bigger than the object.
• An object in front of F makes a virtual image,
  the right way up.

2

• LENSES
• L.O.
• Construct ray diagrams to show the formation of
  images by diverging lenses and converging lenses
• Describe the nature of an image produced by
  diverging ad converging lenses.

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Lenses
Lenses REFRACT light and are usually used to form
IMAGES
2 types
convex
concave
bi-convex
plano-convex
plano-concave
bi-concave
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Actual
Draw as
Actual
Draw as
In practice, light is refracted at both surfaces
of the lens but for simplicity we draw only one
refraction as if it happened at the centre line
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Convex
FOCUS
axis
F
PARALLEL rays from distant object
focal length
Convex lenses bring the rays together
(converge) at a focus. Convex lenses are
CONVERGING LENSES
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Concave
PARALLEL rays from distant object
F
F
FOCUS (Virtual)
focal length
Concave lenses spread the rays out (diverge).
The rays seem to come from a virtual focus on
the other side. Concave lenses are DIVERGING
LENSES
Jump back to ray diagrams
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Ray diagrams
Light is reflected off ALL POINTS of a
non-luminous object in LOTS OF DIFFERENT
DIRECTIONS
To work out what sort of image a lens will
produce, we select 2 rays only from the top point
of the object one parallel to the axis one to
the centre of the lens
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RAY DIAGRAMSRULES
OBJECT (simplified)
Image diagram gives position and size
F
F
2F
2F
focal length
2 x focal length
1. A ray parallel to the axis is refracted
through the focus
2. A ray to the centre of the lens passes through
undeflected
(3. A ray through the focus is refracted parallel
to the axis)
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RAY DIAGRAMSIMAGES
If image LARGER than object MAGNIFIED If image
SMALLER than object DIMINSHED MAGNIFICATION
IMAGE HEIGHT
OBJECT HEIGHT
F
F
2F
2F
This is MAGNIFIED INVERTED REAL Mag 1.25
If image SAME WAY UP as object UPRIGHT If image
UPSIDE DOWN INVERTED
If rays pass through object REAL If rays only
seem to come from object (see diverging lens
) VIRTUAL
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IMAGE TYPES
Object
Image
Image
Object
Image MAGNIFIED UPRIGHT VIRTUAL
Image DIMISHED INVERTED REAL
REAL images can be PROJECTED ON A SCREEN VIRTUAL
images cannot be projected
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1. Draw the following on graph paper
2cm large square
2. Draw an object outside 2F at the position
shown and at the size shown
3. Apply the ray diagram rules and draw in the
image
4. Classify the image by filling in the table
below. Repeat for other positions
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1. OBJECT OUTSIDE 2F
IMAGE REAL, INVERTED, DIMINSHED
uses
IMAGE POSITION between F and 2F
next diagram
Arrow key to animate slide
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2. OBJECT AT 2F
IMAGE REAL, INVERTED, SAME SIZE
uses
IMAGE POSITION at 2F
next diagram
Arrow key to animate slide
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3. OBJECT BETWEEN F AND 2F
IMAGE REAL, INVERTED, MAGNIFIED
uses
IMAGE POSITION outside 2F
next diagram
Arrow key to animate slide
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4. OBJECT AT F
IMAGE NO IMAGE FORMED (rays dont meet)
uses
IMAGE POSITION none (or at infinity)
next diagram
Arrow key to animate slide
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5. OBJECT INSIDE F
Arrow key to animate slide
uses
VIRTUAL Image Light does NOT actually pass
through it cannot be projected onto a screen
end show
IMAGE VIRTUAL, UPRIGHT, MAGNIFIED
IMAGE POSITION inside 2F SAME SIDE AS OBJECT
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(No Transcript)
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6. CONCAVE (DIVERGING) LENS
uses
end show
F
F
2F
2F
Same type of image for all object positions
IMAGE VIRTUAL, UPRIGHT, DIMINSHED
IMAGE POSITION INSIDE F SAME SIDE AS OBJECT
STOP
Arrow key to animate slide
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1. An object 2 cm high is placed 10 cm from a
diverging lens of focal length 10 cm. Draw a ray
diagram on graph paper to find the position,
size and nature of the image.   2. An object 5 cm
high is placed 15 cm from a diverging lens of
focal length 10 cm. Draw a ray diagram on graph
paper to find the position, size and nature of
the image.
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1. OBJECT OUTSIDE 2F
IMAGE REAL, INVERTED, DIMINSHED
IMAGE POSITION between F and 2F
Cameras, eyes
Back to diagram
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2. OBJECT AT 2F
IMAGE REAL, INVERTED, SAME SIZE
IMAGE POSITION at 2F
Copying camera
Back to diagram
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3. OBJECT BETWEEN F AND 2F
IMAGE REAL, INVERTED, MAGNIFIED
IMAGE POSITION outside 2F
Projectors
Back to diagram
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4. OBJECT AT F
IMAGE NO IMAGE FORMED (rays dont meet)
IMAGE POSITION none (or at infinity)
Searchlights
Back to diagram
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5. OBJECT INSIDE F
IMAGE VIRTUAL, UPRIGHT, MAGNIFIED
IMAGE POSITION inside 2F SAME SIDE AS OBJECT
Magnifying glass
Back to diagram
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6. CONCAVE (DIVERGING) LENS
Used in spectacles
Short sight Myopia
Long sight Hyperopia
Eye lens too weak spectacle lens converges light
to bring back into focus
Eye lens too strong spectacle lens diverges
light to bring back into focus