Download Object-Image Real Image Virtual Image and more Lecture notes Physics in PDF only on Docsity!
3.1 Images formed by Mirrors and Lenses
• Images
• Image formation by mirrors
• Images formed by lenses
Object-Image
• A physical object is usually observed by
reflected light that diverges from the
object.
• An optical system (mirrors or lenses) can
produce an image of the object by
redirecting the light.
– Real Image
– Virtual Image
Real Image
Object (^) real Image
Optical System
diverging (^) converging diverging
Light passes through the real image Film at the position of the real image is exposed.
Virtual Image
Object
virtual Image
Optical System diverging diverging
Light appears to come from the virtual image but does not pass through the virtual image Film at the position of the virtual image is not exposed.
Image formed by a plane mirror.
The virtual image is formed directly behind the mirror.
Light does not pass through the image
Object Image
Each point on the image can be determined
by tracing 2 rays from the object.
mirror
B (^) B’
A (^) A’
object image
p q
A virtual image is formed by a plane mirror at a distance q behind the mirror.
q = -p
Parabolic Mirrors
Parallel rays reflected by a parabolic mirror are focused at a point, called the Focal Point located on the optic axis.
Optic Axis
Parabolic Reflector
Parabolic mirrors can be used to focus incoming parallel rays to a small area or to direct rays diverging from a small area into parallel rays.
Spherical mirrors
•Spherical mirrors are much easier to fabricate than parabolic mirrors
- A spherical mirror is an approximation of a parabolic mirror for small curvatures. (i.e. for paraxial rays –close to parallel to the optic axis.
- Spherical mirrors can be convex or concave
light light
concave convex
Parallel beams focus at the focal point of
a Concave Mirror.
Focal point
Ray tracing with a concave spherical mirrors
- A ray parallel to the mirror axis reflects through the focal point, F which is at a point half the radius distance from the mirror along the optic axis.
- A ray passing through the focal point reflects parallel to the mirror axis
- A ray striking the center of the mirror reflects symmetrically around the mirror axis
- A ray that passes through the center of curvature C reflects and passes back through itself
Mirror C F axis
R
F
Law of Reflection
The position of the image can be
determined from two rays from the
object.
F
C
The image is real, inverted, reduced
When object distance > C
O<F
Virtual Image
Upright
Enlarged
Convex Mirror
Image is virtual, upright, reduced
Ray parallel to the optic axis reflects so that the reflected ray appears to pass through the focal point.
Focal Point
A Convex Mirror always forms
virtual images
virtual, upright, reduced
virtual, upright, reduced
Question
Describe how your image would appear as
you approach a convex mirror?
Virtual Image
Upright
The image is
reduced in size
and the field of
view is larger.
Virtual Image
Upright
Virtual Image
Upright
Mirror Equation
• f
O
I
p
q
p q f
p is positive for real objects.
f is positive if the light from infinity goes through the focal point. f positive for concave mirrors, f negative for convex mirrors q is positive if the light goes through the image – real image q is negative if light does not go through image – virtual image
p – object distance q – image distance f - focal length
Magnification
f
O
I
p
M =^ h^ '= − q q
h p
h
h’
q –positive – image is real
M is negative - the image is inverted.
Magnification
f
O
I
p
' q
h q
M
h p
h
h’
q is negative – the image is virtual
M is positive – the image is upright.
Question
A boy stands 2.0 m in front of a concave mirror with a focal length of 0.50 m. Find the position of the image. Find the magnification. Is the image real or virtual? Is the image inverted or erect? p
O
I q (^1) + 1 = 1 p q f (^1) = 1 − 1 q f p
q = (^) p^ fp − f^ 0.5(2.0)^ 0. 2.0 0.
= = m −
m = − qp^ = −^ 0.672.0^ = −0.
Real image
inverted
Image formed by refraction
• Light rays are deflected by refraction through
media with different refractive indexes.
• An image is formed by refraction across flat or
curved interfaces and by passage through
lenses.
Ray diagram for a converging
lenses
Object Image
A converging lens can form real
and virtual images
converging light
converging light
diverging light
Real Inverted reduced
Real Inverted Enlarged
Virtual Upright Enlarged At the focal point the image changes from real to virtual
Question
How will an object viewed through a
converging lens appear as the lens is
brought closer to the object?
Real Image
Inverted
Real Image
Inverted
Magnified
Virtual Image
Upright
Magnified
Virtual image
Upright
Parallel light though a diverging
lens appears to go through the
focal point.
A virtual image is formed.
Image formed by a Diverging lens
Virtual Upright Reduced
A Diverging lens always forms a
virtual image
Question
How will the image of an object formed by a
diverging lens change as the lens is
brought closer to the object?
Virtual Image
Upright
Reduced
Example
F
30 cm
10 cm
An object is placed 30 cm in front of a diverging
lens with a focal length of -10 cm. Find the
image distance and magnification
p q f
q f p
q fp p f
( 10)(30) (^) 7.5cm 30 ( 10) = − = −
− − Virtual image
M q^ 7.5 0. p 30
= − = − − = Upright image
reduced
