REFRACTION AND LENSES
Refraction:
Label the diagram:
Refraction is the bending of light as it moves from one medium to a medium with a different
optical density. This bending occurs as a result of the speed of light changing at the interface
between the two media. In the picture notice the spoon appears to bend where it enters the
water.
Draw and label Refraction:
 The light ray that hits the interface is called
the incident ray.
 At the point where the incident ray hits the interface,
a normal (perpendicular) to the surface should be
drawn.



The angle between the incident ray and the normal
is called the angle of incidence.
The light ray that passes into the new medium is
called the refracted ray.
The angle between the refracted ray and the normal
is called the angle of refraction.
As light strikes the interface between two media with different optical densities at an oblique
(not 90o) angle, it changes speed and is refracted.
Less to More Dense
Medium:
More to Less Dense
Medium:
Striking Medium at a
90o angle:
As it moves from a less dense medium to a more
dense medium, it bends toward the normal
(perpendicular to the interface) and slows down.
As it moves from a more dense medium to a less
dense medium, it bends away from the normal
and speeds up.
If the light strikes the interface at a 90o angle, it is
not refracted and continues moving in a straight
line but its speed will change.
Less
More
More
Less
-2Striking a Parallel
Sided Figure:
When light passes through a parallel sided glass figure, the emergent ray will be parallel to the
incident ray because the amount it is bent toward the normal as it enters the glass is the same
amount it bends away from the normal as it leaves the glass.
Light rays that strike the parallel sided glass figure perpendicular to the side will pass straight
through the piece of glass without bending.
Striking a Non-Parallel
Sided Figure:
Light is also refracted by the same rules when it goes through an object that does not have
parallel sides. However, in this case, the emergent ray will not be parallel to the incident ray.
 As the light ray enters the prism, it is moving from a less dense to a more dense

Striking a Prism:
substance so it is bent toward the normal.
As the light ray leaves the prism, it is moving from a more dense to a less dense
substance so it is bent away from the normal.
Notice the bending as the light travels through the prism, when it leaves the prism the white
light has been separated into its component colors. This separation is due to the fact that each
different wave length of light moves at a slightly different speed in glass and is therefore
refracted at slightly different amounts.
We are able to see most objects not because they are emitting light but because they reflect
light. When you are looking into a pond, at many angles you are able to see the fish below the
water but he is not exactly where you appear to see him.
Internal Reflection:
When light is reflected from a fish and it hits the surface of the water at an angle greater than
the critical angle all of the light is reflected back into the water and none is allowed to escape.
This is called internal reflection.
 Light is transmitted along a fiber optic cable due to the phenomenon of total internal
reflection.
Lenses:
The most common application of refraction in science and technology is lenses.
The kind of lenses we typically think of are made of glass. The basic rules of refraction still
apply but due to the curved surface of the lenses, they create images.
-3Types of Lenses:
Convex Lenses:
Convex lenses are also known as converging
lenses since they bring light rays to a focus.
Draw a Convex Lens:
Concave Lenses:
Concave lenses are also known as diverging
lenses since they spread out light rays.
Draw a Concave Lens:
Focal Point:
All lenses have a focal point (f).
 In a convex lens, parallel light rays all come together at a single point called the focal
point.

Focal Length:
In a concave lens, parallel light rays are spread apart but if they are traced backwards,
the refracted rays appear to have come from a single point called the focal point.
The distance from the lens to the focal point is called the focal length. Typically, a point is
also noted that is 2 focal lengths from the lens and is labeled 2f.
The principle axis is a line which connects the focal point and the 2f point and intersects the
lens perpendicular to its surface.
Refracted Images:
Rules for Locating Refracted Images:
1. Light rays that travel through the center of the lens (where the principle axis intersects
the midline) are not refracted and continue along the same path.
2. Light rays that travel parallel to the principle axis, strike the lens, and are refracted
through the focal point (f).
3. Light rays that travel through the focal point (f), strike the lens, and are refracted
parallel to the principle axis.
All three of these light rays will intersect at the same point if they are drawn carefully.
However, the image can be located by finding the intersection of any two of these light rays.
Real Images:
Real images are images that form where light rays actually cross. In the case of lenses, that
means they form on the opposite side of the lens from the object since light can pass through a
lens. Real images are always inverted (flipped upside down).
-4Virtual Images:
Eye:
Virtual images are images that form where light rays appear to have crossed. In the case of
lenses, that means they form on the same side of the lens as the object. Virtual images are
always upright.
The eye contains a convex lens. This lens focuses images on the back wall of the eye known
as the retina.
What the brain “sees”:
The distance from the lens to the retina is fixed by the size of the eyeball. For an object at a
given distance from the eye, the image is in focus on the retina. Although the image on the
retina is inverted, the brain interprets the impulses to give an erect mental image.
How the eye works:
If the object moved closer to the eye and nothing else changed the image would move behind
the retina the image would therefore appear blurred. Similarly if the object moved away from
the eye the image would move in front of the retina again appearing blurred. To keep an object
in focus on the retina the eye lens can be made to change thickness. This is done by
contracting or extending the eye muscles. We make our lenses thicker to focus on near objects
and thinner to focus on far objects.
Nearsightedness:
Someone who is nearsighted can see near objects more clearly than far objects. The retina is
too far from the lens and the eye muscles are unable to make the lens thin enough to
compensate for this. Diverging glass lenses are used to extend the effective focal length of the
eye lens.
Farsightedness:
Someone who is farsighted can see far objects more clearly than near objects. The retina is
now too close to the lens. The lens would have to be considerable thickened to make up for
this. A converging glass lens is used to shorten the effective focal length of the eye lens.
Today’s corrective lenses are carefully ground to help the individual eye but cruder lenses for
many purposes were made for 300 years before the refractive behavior of light was fully
understood.