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Lesson 3
1. To understand the effect that convex
and concave lenses have on light.
2. To appreciate real world applications
of concave and convex lenses.
Experiment
Aim
To look at how convex and concave lenses
refract light.
Method
Sheet of
Paper
Ray box
with
triple
slit
Convex Lens
1. Draw around the convex lens.
2. Switch on the ray box.
3. Sketch over the rays of light.
4. Repeat using a concave lens.
Results – Convex (Converging) Lens
Focal Length
Copy!
Focus
Point
A thicker lens will cause the focal length (measured in
metres) to decrease.
Focal Length
Focus
Point
Results – Concave (Diverging) Lens
Copy!
Focal Length
Virtual
Focus
There is no natural focal point. Therefore, the rays are
extended backwards to show the virtual focus.
The focal length of a CONCAVE lens is always a
NEGATIVE value.
Applications of lenses
Lenses are used in a range of important
devices used by humans.
Lens inside a PS3.
Lens from inside a
photocopier.
Magnifying Glass
The Eye
The structure of the eye is as shown below:
Iris
Retina
Pupil
Cornea
Lens
Optic
Nerve
The Eye
The Cornea
The cornea refracts light as it enters the eye.
It is made of hardened tissue that protects
the inner parts of the eye.
The Eye
The Iris/Pupil
The pupil is merely an opening in the iris,
through which light enters the eye.
The iris controls the size of the pupil, allowing
different amounts of light to enter the eye.
The Lens
The Eye
The lens is used to fine tune the focussing
process, further refracting light as it passes
through the eye.
It becomes thicker and shorter to focus on
nearby objects and thinner and longer to focus
on far objects.
The Retina
The Eye
The retina has special light sensitive cells
called rods and cones, which change the
incoming picture into electrical impulses.
The Optic Nerve
The Eye
The optic nerve takes electrical signals from
the retina and transfers them to the brain.
The optic nerve creates a “blindspot” at the
back of the eye.
Applications of lenses – The Eye
Both the cornea and the lens inside
the human eye act as convex lenses, causing
light to refract and come to a focus. Retina
Cornea
Light from
faraway objects.
Lens
In the perfect eye (emmetropia), the light
will focus on the retina.
Applications of lenses – The Eye
Light from nearby objects does not
travel in parallel rays. Therefore, the lens
would not normally be powerful enough to
focus it on the retina. Cornea
Retina
Light from
near objects.
Lens
In order to compensate for this, the lens will
change shape.
Short Sightedness
Copy!
Myopia, or short sightedness, affects
millions of people worldwide. Sufferers can see
nearby objects clearly but those far away
Retina
appear blurry. Cornea
Lens
Short sightedness is usually caused by the eye
being too long, so the light focuses short of
the retina.
Short Sightedness – The Cure
Copy!
In order to alter the eyesight of someone who
is short sighted, a CONCAVE lens is used.
The concave lens diverges the rays, meaning
they now focus on the retina.
Long Sightedness
Copy!
Hyperopia, or long sightedness, also
affects millions of people worldwide.
Sufferers can see faraway objects clearly but
those nearby appear blurry.
Cornea
Lens
Retina
Long sightedness is usually caused by the eye
being too short, so the light focuses long of
the retina.
Long Sightedness – The Cure
Copy!
In order to alter the eyesight of
someone who is long sighted, a CONVEX lens is
used (like in “reading glasses”).
The convex lens converges the rays, meaning
they now focus on the retina.
2000
2007 (Int. 1)