Download Thin Lenses

Thin Lenses
Any lens that is thicker in the center than at
the edges will make parallel rays converge
to a point and is called a converging lens.
Lenses that are thinner in the center than at
the edges are called diverging lenses
because they make parallel light diverge.
Finding the Image Position
Formed by a Thin Lens
-ray 1 is drawn parallel to the axis; therefore
it is refracted by the lens so that is passes
along a line through the focal point;
-ray 2 is drawn on a line passing the other
focal point F’ and emerges from the parallel
to the axis;
-ray 3 is directed toward the very center of
the lens, this ray emerges from the lens at the
same angle as it entered.
The Lens Equation
Converging Lens:
1 1 1
 
d 0 di f
Diverging Lens:
1 1
1
 
d 0 di
f
1. The focal length is positive for converging lens and
negative for diverging.
2. The object distance is positive if it is on the same side
of the lens from where the light is coming; otherwise
it is negative.
3. The image distance is positive if it is on the opposite
side of the lens from where light is coming.
4. The height of the image is positive if the image is
upright and negative if the image is inverted relative
to the object.
Lateral Magnification
hi
di
m

ho
do
1. You find a diverging (converging) lens in the storeroom and
wish to define its focal lens. Describe how you could use
two lasers to accomplish this.
2. What type of lens would you use to construct an overhead
projector? Explain your reasoning.
3. Two converging lenses with identical shapes are made from
glasses with different indices of refraction. Which one has
the shorter focal length? Why?
4. What type of lens would be helpful in starting of a campfire?
Why?
Optical Instruments
Pinhole Cameras
Cameras
Our Eyes
Normal Eye
Lens relaxed,
focused at infinity:
Lens thickened,
focused on nearby
object:
Correcting eye defects with lenses
Test for Astigmatism
The Magnifying Glass
Telescopes
Newtonian
Cassegrainian
1. In a refracting telescope, parallel light is collected from a
distant source to form a real image that is then magnified
by the eyepiece. To achieve high magnification, this real
image in located near the eyepiece’s focal point. Use this
information to determine how the length of a refracting
telescope compares with the local lengths of the lenses.
2. Sometimes when a person develops a cataract, the lens of
the eye is surgically removed and replaced with a plastic
one. Would you expect this lens to be converging or
diverging one? Explain.
Baby-Quiz
1. Why is the diamond more brilliant than a clear piece of glass
having the same shape?
2. A surface appear yellow under the white light. How it will appear
under red light? Under green light? Under blue light?
3. Most of us find that we really have to strain our eyes to focus on
objects located close to our noses. You hold two mirrors 1 foot in
front of your face. One is a plane mirror, and the other is a concave
mirror with a 3-inch focal length. In which case are you more likely
to have to strain your eyes to see the image of your nose?
4. When you mix red and green light from separate projectors, you get
yellow spot on the wall. However, if you mix red and green paint,
you get a muddy brown color. How do you account for this
difference?