Simplified notes for SPM students

1. Light and Optics
Form 4 Physics (SPM) – Chapter 5

2. Light


Electromagnetic radiation that is perceived by the human eye
Has two forms




Waveform
Particleform
Light travels in the form of rays in a straight line
Light rays tend to spread away from the source

3. 
Wave properties of light







Reflection
Refraction
Diffraction
Interference
Light is a form of energy
Visible light is made up of 7 colours of various intensities
Light, like any electromagnetic radiation travels through vacuum at a speed of
approximately 3X108 ms-1

4. Optics


Study of interaction of light with objects that reflect or scatter light
Light is able to



Completely penetrate transparent objects
Partially penetrate translucent objects
Not penetrate opaque objects

5. Light rays

6. Key notes



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Object – material that emits light rays
Image – representation of object after light has reflected or refracted
Ray diagram – diagram showing path of light through reflection or refraction
Line of sight – order or position of the eye to view the image formed
Solid lines – real light rays
Dotted lines – virtual light rays
Convergence – meeting of light rays at a point
Divergence – spreading of light rays from a point
Focus point – point where light rays are converged or diverged

7. Reflection






Occurs when the direction of light changes when it strikes an opaque material
Light ray before reflection – incident ray
Light ray after reflection – reflected ray
Imaginary line that is perpendicular to surface – normal line
Angle of incidence, i – angle between incident ray and normal line
Angled of reflection, r – angle between reflected ray and normal line

8. Normal
Incident
ray
i
r
Reflected ray
Plane

9. 
Two types of reflection

Specular reflection


Occurs when light strikes a smooth and shiny surface
Diffused reflection

Occurs when light strikes an uneven surface

10. 
Law of reflection



Incident, reflected ray and normal line lies on the same plane
i=r
Path of light reflection is represented in a ray diagram

11. Key terms in a ray diagrams

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C – centre of curvature of mirrors or optical centre of lenses
F – focus point of curved mirrors or lenses
f – focal length (distance of F from C)
u – distance of object to the surface of reflection or refraction
v – distance of image to the surface of reflection or refraction
Principle axis, P – light ray that is perpendicular to the surface of reflection or
refraction and crosses through C and F
* In concave and convex mirrors, C = 2F

12. Reflection on a plane mirror
Object
u
Plane mirror
v
Image
Line of sight

13. Properties of image formed



Same size as object
v=u
Laterally inverted



Flipped horizontally
Left of Object becomes right of Image
Virtual


Image exists within the mirror, a.k.a. another plane
The image cannot be formed on a screen

14. Reflection on a concave mirror
. .
F
C
P

15. Reflection on a convex mirror
P
. .
F
C

16. 



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A concave mirror is also called a converging mirror
A convex mirror is also called a diverging mirror
The curvier the mirror, the smaller the focal length
When incident rays are parallel to P, then reflected rays will pass through F
When incident rays passes through F, then reflected rays will be parallel to P
When incident rays passes through C, then reflected rays will pass through C
in the opposite direction, parallel to the incident ray

17. Properties of image in concave and convex mirrors
Location of object
Properties of concave image
u>C
Real, Diminished, Inverted
u=C
Real, Same size as object, Inverted
C<u<F
Real, Magnified, Inverted
u=F
Formed at infinity
u<F
Virtual, Magnified, Upright
Properties of convex image
Virtual, Diminished, Upright

18. Applications of reflection
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Parallax mirror in measuring instruments
Wing and rear view mirrors
Periscope
Vanity mirror
Satellite dish
Headlights and torchlight reflectors

19. Refraction




Phenomenon where the speed of light changes as it propagates from one
medium to another
The change of speed causes a change in the direction of propagation
When light propagates from a medium of low density to a medium of high
density, its speed decreases, causing the direction of propagation to approach
the normal
The opposite is true when light passes from a medium of high density to a
medium of low density

20. 


Incident ray – i
Refracted ray – r
When light travels from a medium of low density to a medium of high density:

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Its speed decreases
Its direction changes
i>r
When light travels from a medium of high density to a medium of low density:

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
Its speed increases
Its direction changes
i<r

21. Low density
High density
r
i
r
i>r
Low density
High density
i
i<r

22. Refractive Law


The incident ray, refracted ray and the normal line all lie on the same plane
The ratio of sin i to sin r yields a constant known as the refractive index
sin i
= n, where n = refractive index
sin r

23. Refractive Index, n





Has no units
Indicates the light bending ability of a medium
Value equals to the ratio of sin i to sin r
Value also equals to ratio of speed of light in vacuum to speed of light in
medium
Value also equals to ratio of real depth to apparent depth

24. Total internal reflection



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
Is a form of light refraction.
Occurs when light travels from a medium of high density to a medium of low
density, where i > r.
Occurs when the i is very large causing the r to be more than 90˚.
Critical angle, c is the value of i that results in r = 90˚.
When i > c, total internal reflection occurs and the reflected ray is present in
the same medium as the incident ray.

25. Low density
Low density
r = 90˚
i=c
Critical angle, c
r > 90˚
High density
i>c
Total internal reflection
High density

26. Observations and applications of refraction and total
internal reflection
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Sunset below the horizon
Rainbow formation
Mirages
Fish’s eye view
Fibre optics
Prism periscope
Prism binoculars
Perfectly cut diamond

27. Lenses
Bi
Bi

28. Refraction through a biconvex lens
F = Focal point
C = Optical centre
P = Principle axis
f = focal length
.
.
F
C
f
.
F
f
.
P

29. Refraction through a biconcave lens
F = Focal point
C = Optical centre
P = Principle axis
f = focal length
.
.
F
C
f
.
F
f
.
P

30. 
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A concave lens is also called a diverging lens.
A convex lens is also called a converging lens.
The larger the lens, the larger the f value.
The thicker the lens, the smaller the f value.
When incident rays are parallel to P, then refracted rays will pass through F
When incident rays passes through F, then refracted rays will be parallel to P
No refraction occurs when incident rays passes through C. The rays simply
pass through the lens in a straight line

31. Properties of image in convex and concave lenses
Location of object
Properties of convex image
u=∞
Real, Diminished, Inverted
u > 2f
Real, Diminished, Inverted
u = 2f
Properties of concave image
Real, Same size as object, Inverted
Virtual, Diminished, Upright
2f < u < f
Real, Magnified, Inverted
u=f
Image is formed at infinity
u<f
Virtual, Magnified, Upright

32. 
Value of f



Value of u and v



Positive – convex lens
Negative – concave lens
Positive – real image
Negative – virtual image
Lens law

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1/f = 1/u + 1/v
f = focal length
u = object length
v = image length

33. 
Linear magnification, m

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
m = v/u
m = hi/ho, where hi = height of image and ho = height of object
Power of lens, P


P = 1/f
Unit = m-1 or Diopter (D)

34. Magnifying glass
.
.
F
C
.
F
.
P

35. Compound Microscope
Construction line
Eyepiece lens
Objective lens
Fo
.
Fo
Fe
. .
Fe
.
Image is:
•Magnified
•Inverted
•Virtual

36. Telescope
Eyepiece lens
Construction line
Objective lens
Fo
.
Fo / Fe
.
Fe
.
Image is formed at infinity