Download CHAPTER 5 Light and Vision 2015

Chapter 5: Light and Vision
CHAPTER 5: LIGHT AND VISION
╞╡§¥ Physics
SPM 2015
These notes have been compiled in a way to make it easier for revision. The topics are
not in order as per the syllabus.
5.1
Mirrors and Lenses
5.1.1 Image Characteristics
Image characteristics are described using the following three categories:
Same
Image is exactly the same size as the object
Size
Magnified Image appears bigger than the object
Diminished Image appears smaller than the object
Image appears to be in the same direction as the object
Direction Upright
Inverted
Image appears upside down compared to object
Real
Real images are images you can capture on a screen.
Type
Mirrors: Images are formed on the same side of the mirror as the object
Lenses: Images are formed on the opposite side of the lens from the object
Virtual
Virtual images are images you can see but cannot capture on a screen.
Mirrors: Images are formed on the opposite side of the mirror from the object
Lenses: Images are formed on the same side of the lens as the object
5.1.2 Plane mirrors
i
Incident ray
r
normal
Reflected ray
Law of light reflection:
• The reflected angle is always the same as the incident angle.
• The incident ray, reflected ray, and normal line are in the same plane.
Characteristics of an image formed by a plane mirror:
Size
Same
Direction
Upright, laterally inverted
Type
Virtual
Distance
Distance of an image from the plane mirror is the same as the distance of the object from the
mirror
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Chapter 5: Light and Vision
SPM 2015
5.1.3 Curved Mirrors vs Lenses
Concave mirror
Also known as
Focal lengths
Converging mirrors
Positive
E.g. f = +20cm.
Convex mirror
Diverging mirror
Negative
E.g. f = -20cm.
For both concave and convex mirrors, the focal length is half the radius; i.e. CF = FP.
Convex lens
Also known as
Focal lengths
Concave lens
Converging lens
Positive
E.g. f = +20cm.
Diverging lens
Negative
E.g. f = -20cm.
Determining the Position and Characteristics of an Image with a Ray Diagram
Concave mirror
 A ray parallel to the principal axis
is reflected to pass through F
 A ray through F is reflected
parallel to the principal axis
Convex mirror
 A ray through C is reflected back
along its own path
 A ray parallel to the principal axis
is reflected as if it came from F
 A ray towards F is reflected
parallel to the principal axis
 A ray towards C is reflected back
along its own path
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╞╡§¥ Physics
Chapter 5: Light and Vision
SPM 2015
Convex lens
 A ray parallel to the principal axis
is refracted to pass through F
 A ray through F is refracted
parallel to the principal axis
Concave lens
 A ray through C travels straight
along its own path
 A ray parallel to the principal axis
is refracted as if it came from F
 A ray towards F is refracted
parallel to the principal axis
 A ray towards C travels straight
along its own path
To determine the position and characteristics of an image using a ray diagram:
1. Draw two rays emanating from the top of the object to the mirror or lens, and using the guide in the table above, draw their
reflected/refracted paths.
2. The image is produced at the intersection of the two reflected/refracted rays.
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Position of
object
Between F and
the mirror /
lens
SPM 2015
Chapter 5: Light and Vision
Images formed by a Concave Mirror / Convex Lens
Ray diagram of concave mirrors
Ray diagram of convex
lenses
Characteristics of
image
 Virtual
 Upright
 Magnified





At F

Virtual
Upright
Magnified
At infinity
 Real
 Inverted
 Magnified
Between F and
C/ 2F

 Real
 Inverted
 Same size
At C / 2F

 Real
 Inverted
 Diminished
Greater than C
/ 2F

At infinity
 Real
 Inverted
 Diminished

Position of
object
Anywhere in
front of the
mirror
or
lens
Images formed by a Convex Mirror / Concave lens
Ray diagram of convex mirror
Ray diagram of concave lens
Characteristics of
image
 Virtual
 Upright
 Diminished

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Chapter 5: Light and Vision
╞╡§¥ Physics SPM 2015
SUMMARY OF COMPARISON OF IMAGE CHARACTERISTICS
Characteristics of concave mirrors are the same as convex lenses:
Lens / Mirror
2f
Real, Inverted
Diminished
f
Virtual, Upright
Magnified
Same size
Object distance Image characteristics
u=∞
Real Inverted Diminished
u > 2f
Real Inverted Diminished
u = 2f
Real Inverted Same Size
f < u < 2f
Real Inverted Magnified
u=f
Virtual Upright Magnified
u<f
Virtual Upright Magnified
Characteristics of convex mirrors are the same as concave lenses:
Virtual, Upright, Diminished
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╞╡§¥ Physics
Chapter 5: Light and Vision
SPM 2015
5.1.4 Lens Equation
Focal length, f
Convex lens: positive
Concave lens: negative
1 1 1
 
u v f
where u = object distance [cm]
v = image distance [cm]
f = focal length of lens [cm]
Object distance, u
Always positive
5.1.5 Lens Power
P
1
f
where P = lens power [D]
f = focal length [m]
P
OR
Image distance, v
If positive: real image
If negative: virtual image
100
f
where P = lens power [D]
f = focal length [cm]
5.1.6 Linear Magnification
Linear magnification is the ratio of the image size to the object size.
m
|m| > 1: magnified
|m| = 1: same size
|m| < 1: diminished
hi v

ho u
where m = linear magnification
hi = height of image
ho = height of object
5.1.7
If m is negative, take
the modulus value
Application of Lenses
Complex Microscope
fo < fe
Astronomical Telescope
fo > f e
Magnification =
fo
fe
Normal setting:
Length between lenses = fo + fe
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5.2
Chapter 5: Light and Vision
SPM 2015
Refraction and Total Internal Reflection
Light refraction is a phenomenon where the direction of light is changed when it crosses the boundary
between two materials of different optical densities. It occurs as a result of a change in the speed of light as
it passes from one medium to another.
When a light ray travels from medium A to When a light ray travels from medium C to
medium B which is optically denser than A
medium D which is optically denser than C
The ray of light will refract towards normal; r < i
The ray of light will refract away from normal; r > i
When a light ray crosses the boundary between two different mediums at a right angle
i = 0°, r = 0°
5.2.1 Snell’s Law
Snell’s Law states that the ratio of sin i to sin r is a constant.
sin i
= constant
sin r
5.2.2 Refractive Index
The refractive index or index of refraction of a medium is equivalent to the optical density of a medium.
Note: A material with greater density may not necessarily have greater optical density.
The refractive index / index of refraction of a medium, n can be calculated as:
n =
sin i
sin r
speed of light in air, c
speed of light in the medium, v
actual depth, D
=
apparent depth, d
1
=
sin c
=
(where c is the critical angle)
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╞╡§¥ Physics
Chapter 5: Light and Vision
SPM 2015
5.2.3 Total Internal Reflection
Critical angle, c is the value of the incident angle when the
refracted angle is 90°.
•
•
When i is increased to be greater than c, the light will be
complete reflected back into the material. No light will be
refracted.
This phenomenon is known as total internal reflection.
Conditions for total internal reflection:
1. Light must be traveling from an optically denser medium to a less dense medium.
2. The incident angle must be greater than the critical angle.
   END OF CHAPTER   
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