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Chapter 5: Light and Vision
Physics SPM 2017
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: Same Image is exactly the same size as the object Size Magnified Image appears bigger than the object object Diminished Image appears smaller than the object Image appears to be in the same direction direction as the object Direction Upright Inverted Image appears upside down compared to object Real Real images are images you can capture capture on a screen. Type same side of the mirror as the object Mirrors: Images are formed on the sam 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 fr om the object same side of the lens as the object Lenses: Images are formed on the sam
5.1.2 Plane mirrors
Incident ray
normal
Reflected ray
Law of light l ight 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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Page 1 of 8
Chapter 5: Light and Vision
Physics SPM 2017 5.1.3 Curved Mirrors vs Lenses Concave mirror
Also known as Focal lengths
Converging mirrors Positive E.g. f E.g. f = +20cm.
Convex mirror
Diverging mirror Negative E.g. f 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 E.g. f = +20cm.
Diverging lens Negative E.g. f E.g. f = = -20cm.
Determining the Position and Characteristics Characteristics of an Image with a Ray Diagram Concave mirror
A
ray parallel to the principal axis is reflected to pass through F through F
A
ray parallel to the principal axis is reflected as if it came from F from F
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A
ray through C is is reflected back along its own path
A
A
ray through F through F is is reflected parallel to the principal principal axis Convex mirror
ray towards F towards F is is reflected parallel to the principal principal axis
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A
ray towards C is is reflected back along its own path Page 2 of 8
Chapter 5: Light and Vision
Physics SPM 2017 Convex lens
A
ray parallel to the principal axis is refracted to pass through F through F
A
ray parallel to the principal axis is refracted as if it came from F from F
A
ray through C travels travels straight along its own path
A
A
ray through F through F is is refracted parallel to the principal principal axis Concave lens
ray towards F towards F is is refracted parallel to the principal principal axis
A
ray towards C travels travels 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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Page 3 of 8
Chapter 5: Light and Vision
Physics SPM 2017
Images formed by a Concave Mirror / Convex Lens
P osition si tion of of object
R ay di agra agr am of conca concave ve mi r r or s
R ay di ag r am of conve convexx lenses
Between F and the mirror / lens
C har har acte cter i sti sti cs of image
At F
Between F and C/ 2F
At C / 2F
Greater than C / 2F
At infinity
Virtual Upright Magnified
Virtual Upright Magnified At infinity
Real Inverted Magnified
Real Inverted Same size
Real Inverted Diminished
Real Inverted Diminished
Images formed by a Convex Mirror / Concave lens
Positio Positi on of object
R ay di agra agr am of conve convexx mi r r or
R ay di ag r am of conca concave ve lens lens
Anywhere in front of the mirror or lens
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C haracte haracter i stics of image
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Virtual Upright Diminished
Page 4 of 8
Chapter 5: Light and Vision
Physics SPM 2017
SUMMARY OF COMPARISON OF IMAGE CHARACTERISTICS
Characteristics of concave mirrors are the same as convex lenses : Lens / Mirror
2 f Real, Inverted Diminished
f Virtual, Upright Magnified
Same size
Object distance Image characteristics characteristics u = ∞ Real Inverted Diminished u >2 >2 f Real Inverted Diminished u = 2 f Real Inverted Same Size f < u <2 <2 f Real Inverted Magnified u=f Virtual Upright Magnified u
Characteristics of convex mirrors are the same as concave lenses: Virtual, Upright, Diminished
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Page 5 of 8
Chapter 5: Light and Vision
Physics SPM 2017 5.1.4 Lens Equation 1 u
1 v
Focal length, f length, f Convex lens: positive Concave lens: negative
1 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
Image distance, v If positive: real image If negative: virtual image
100
OR
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.
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
f o< f e
Astronomical Astronomical Telescope f o> f e Magnification Magnification =
f o f e
Normal setting: setting: Length between lenses = f = f o + f e
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Chapter 5: Light and Vision
Physics SPM 2017 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 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 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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Chapter 5: Light and Vision
Physics SPM 2017 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 criti cal angle.
