Transcript Lenses - Heritage High School

Thin Lenses
Converging and Diverging
Images
• If you see an object, then light from that
object must enter your eye, be redirected
onto the retina and then interpreted by
your brain.
• These rays of light do not have to come
directly from the object. The light could be
reflected off of a mirror or some other
object.
Real Images
• Real images can be formed on a surface,
such as a card or movie screen.
• The existence of these images do not
depend on you seeing them and they are
present even if you are not.
• A real image will exist at a point in space
where rays converge
Virtual Image
• An image which exists only within the brain and
only exists if it is observed.
• If you see an penguin in a mirror, the penguin
looks as if it is behind the mirror.
• Your eyes intercept the light rays from the
direction of the mirror so that is where you
perceive the penguin to be.
• No rays actually leave the object that we see,
but the light acts as if it left from where we see
the image
Thin Lens Assumptions
• The thickness of the lens is small
compared to the distance to the image and
the object.
• The light rays make small angles with the
central axis (although in diagrams they are
exaggerated for clarity).
Why does a lens, lens?
• A lens can produce an image because
light will refract as it passes through the
lens.
• The index of refraction of the lens has to
be different from that of the surrounding
medium (a pyrex lens in cooking oil won’t
make an image)
Types of Lenses
• Converging Lens –
A lens that causes light rays that are initially
parallel to the central axis lens to converge
• Diverging Lens –
A lens that causes light rays that are initially
parallel to the central axis lens to diverge
Don’t be lame (or like DeStefano), show the demo…
Types of Lenses
Notice that there are two focal points,
one on each side of the lens
Each focal point is the same distance from the lens.
Image Formation
• Real images form when the light rays cross
– Opposite side of lens
• Virtual images form when the projected rays
cross – Same side as object
Real
Virtual
Virtual
Thin Lenses and Ray Tracing
The focal point of a
converging lens
The focal point of a
diverging lens
Slide 18-20
Three Important Sets of Rays: Converging Lenses
Slide 18-21
Ray Tracing: Real Images
Slide 18-22
Three Important Sets of Rays: Diverging Lenses
Notice where the image (I) forms in relation to the object (O)
Slide 18-28
The Three Special Rays
♥♥♥
Three Rays
1. A ray that is initially parallel to central axis
will go through a focal point
2. A ray that initially passes through a focal
point will emerge from the lens parallel to
central axis
3. A ray that is initially directed towards the
center of the lens will emerge from the
lens with no change in direction.
Use your textbooks
Slide 18-23
Use your textbooks
Slide 18-29
Checking Understanding
Which of these ray diagrams is possibly correct?
Slide 18-24
Answer
Which of these ray diagrams is possibly correct?
Slide 18-25
Checking Understanding
In this figure the image is produced by a lens. At
which position A–E is the lens?
A
B
C
E
Slide 18-30
Answer
In this figure the image is produced by a lens. At
which position A–E is the lens?
What type of lens must it be?
Converging
Slide 18-31
The Three Special Rays
♥♥♥
Three Rays
1. A ray that is initially parallel to central axis
will go through focal point
2. A ray that initially passes through a focal
point will emerge from the lens parallel to
central axis
3. A ray that is initially directed towards the
center of the lens will emerge from the
lens with no change in direction.
The Thin-Lens Equation
1 1 1
 
s s f
Thin-lens equation (also works for mirrors)
relating object and image distance to focal length
Slide 19-10
Magnification
hi
si
M  
ho
so
The equations for the mirrors and lenses are
identical.
Slide 18-26
Sign Conventions for Lenses and Mirrors
Slide 19-11