Transcript Document

WAVES : Optics
(triple science)
By the end of this presentation you should be able to:
 Identify the differences between converging and diverging
lenses.
 Draw diagrams showing the effect each lens on a parallel
beam of light.
 Identify the positions of the focus of both a converging and a
diverging lens.
 Know the difference between a real and virtual image.
 Construct ray diagrams to show the formation of real and
virtual images by a converging lens.
 Explain the use of a converging lens as a magnifying glass, a
projector and in a camera.
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Reviewing refraction
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Remember that when light crosses into a
different medium, its speed is affected
and the direction of the wave changes as
a result.
Two exceptions
- If the wave is incident on the surface at
90o , its direction will not change.
- If light is travelling from glass into air,
and hits the boundary at an angle
greater than the critical angle, it will
undergo total internal reflection.
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Refraction by non-rectangular prisms.
Can you predict the path that this ray would follow
through the glass prism shown?
First draw the normal to the prism where the ray is incident
on it.
Then, considering that the ray will bend towards the normal…
Now repeat the process, remembering that the ray will bend
the other way as it emerges from the glass into the air.
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Lenses
The effect that lenses have on light can be explained
by considering them to be constructed of different
arrangements of prisms.
1. A converging lens
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2. A diverging lens
How a parallel beam is affected by different lenses
F
F
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principal axis
A parallel beam
will converge
and be focussed
at the focal
point: F.
A parallel beam
will diverge. A
virtual focal
principal axis point, F, can be
found by
projecting the
rays backward.
Drawing ray diagrams for a convex lens
Remember that light is coming from the object in all directions
and from all points on the object, but we only need to consider a
few rays to be able to determine the location, orientation, and
size of the image.
Light rays do change directions at each air/glass interface; but to
make the ray diagrams easier to draw, we will only draw the rays
changing direction at the imaginary line going vertically through
the centre of the lens.
Consider these rules:
1.Rays parallel to the principal axis are refracted through the
focal point on the other side of the lens.
2.Rays through the focal point are refracted parallel to the
principal axis.
3.Rays through the optical centre are undeflected.
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Applying the rules to scale diagrams
Choose the largest scale
possible, given the data
supplied (like object
height, distance, focal
length) and write it down.
Do a rough sketch to
determine where to
position the lens and
principal axis.
F
F
Draw the principal axis and the lens.
Use the scale to mark the focal point on each side of the principal
axis.
Now use the scale to accurately draw in the position and height of
the object.
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Using the rules for drawing ray diagrams
1.
2.
3.
Rays parallel to the principal axis are refracted through the focal
point on the other side of the lens.
Rays through the focal point are refracted parallel to the
principal axis.
Rays through the optical centre are undeflected.
Only two rays need to be
drawn to determine
where all rays would
focus.
Once this point is found,
draw in the image from
the principal axis to that
point.
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F
F
Uses of converging (or convex) lenses
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Cameras
Projectors
Magnifying glasses
Telescopes (not on syllabus)
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Cameras
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In a camera, and in the eye, a converging lens is
used to produce a real, inverted image of an
object on a film/retina. The image is smaller than
the object (diminished) and nearer to the lens
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A simple camera
the film
Rays from the top of the
object will be focussed at
the bottom of the film (and
vice versa)
The lens will focus the rays
from the object on the film.
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The
brightness
sharpness of
the image is
controlled by
thethe
shutter
stop,
speed.
which can
Taking
be
pictures
opened to
of
make
moving
the
aperture
objects
larger
requireand
short
let
in more
exposures.
light.
the outer case
A Projector
In a projector, a converging lens is used to produce a real,
inverted image of an object on a screen. The image is
larger than the object (magnified) and further from the
lens.
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The Projector
This arrangement of mirrors
and lenses simply focuses all
the light from the lamp onto
the slide
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The slide is placed on one side
of the convex lens, and the
screen is on the other side
slide
convex
lens
screen
A magnifying glass
A magnifying glass uses a convex lens to produce a “virtual
image”.
A virtual image is one where rays of light do not actually pass
through the image (so a virtual image cannot be formed on a
screen).
This is because the object is very close to the lens (between the
focal point and the lens). The rays diverge after being refracted.
Your eye will bring these rays to a focus on the same side as
the object. It will be upright and magnified.
F
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F
A Magnifying glass
When the object is close to the lens a virtual, upright image is
formed. In this mode, the converging lens acts as a magnifying
glass.
Unlike real images, virtual images cannot be projected onto a screen
or film; light only appears to come from a virtual image, as indicated
by the dotted lines.
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More ray diagrams.
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A moving visual aid