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Physics SPM 2017 Chapter 5: Light and Vision
Hoo Sze Yen www.physicsrox.com Page 1 of 8
CHAPTER 5: LIGHT AND VISION
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:
Size Same Image is exactly the same size as the object
Magnified Image appears bigger than the object
Diminished Image appears smaller than the object
Direction Upright Image appears to be in the same direction as the object
Inverted Image appears upside down compared to object
Type Real Real images are images you can capture on a screen.
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
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
i r
normal Incident ray Reflected ray
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5.1.3 Curved Mirrors vs Lenses
Concave mirror Convex mirror
Also known as Converging mirrors Diverging mirror
Focal lengths Positive
E.g. f = +20cm.
Negative
E.g. f = -20cm.
For both concave and convex mirrors, the focal length is half the radius; i.e. CF = FP.
Convex lens Concave lens
Also known as Converging lens Diverging lens
Focal lengths Positive
E.g. f = +20cm.
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
A ray through C is reflected back
along its own path
Convex mirror
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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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
A ray through Ctravels straight
along its own path
Concave lens
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 Ctravels straight
along its own path
To determine the position and characteristics of an image using a ray diagram:
1. Draw tworays 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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Images formed by a Concave Mirror / Convex Lens
Position of
object
Ray diagram of concave mirrors Ray diagram of convex
lenses
Characteristics of
image
Between F and
the mirror /
lens
Virtual
Upright
Magnified
At F
Virtual
Upright
Magnified
At infinity
Between F and
C/ 2F
Real
Inverted
Magnified
At C / 2F
Real
Inverted
Same size
Greater than C
/ 2F
Real
Inverted
Diminished
At infinity
Real
Inverted
Diminished
Images formed by a Convex Mirror / Concave lens
Position of
object
Ray diagram of convex mirror Ray diagram of concave lens Characteristics of
image
Anywhere in
front of the
mirror or
lens
Virtual
Upright
Diminished
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SUMMARY OF COMPARISON OF IMAGE CHARACTERISTICS
Characteristics of concave mirrors are the same as convex lenses:
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
Lens / Mirror
f 2f
Real, Inverted Virtual, Upright
Same size
Magnified Diminished
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5.1.4 Lens Equation
fvu
111
where u = object distance [cm]
v = image distance [cm]
f = focal length of lens [cm]
5.1.5 Lens Power
fP
1
OR f
P100
where P = lens power [D]
f = focal length [m]
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.
u
v
h
hm
o
i
where m = linear magnification
hi = height of image
ho = height of object
5.1.7 Application of Lenses
Complex Microscope
Astronomical Telescope
Focal length, f
Convex lens: positive
Concave lens: negative
Object distance, u
Always positive
Image distance, v
If positive: real image
If negative: virtual image
fo< fe
|m|> 1: magnified
|m| = 1: same size
|m|< 1: diminished
If m is negative, take
the modulus value
fo> fe
Magnification =
e
o
f
f
Normal setting:
Length between lenses = fo + fe
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5.2 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
medium B which is optically denser than A
When a light ray travels from medium C to
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.
r
i
sin
sin = constant
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 =r
i
sin
sin
=
v
c
medium, in thelight of speed
air,in light of speed
=
d
D
depth,apparent
depth, actual
=
csin
1
(where c is the critical angle)
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5.2.3 Total Internal Reflection
Critical angle, c is the value of the incident angle when the
refracted angle is 90°.
• When iis 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