Definitions – Chapters 2-5
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Reflection is defined as the bouncing of light off an object
Laws of Reflection of Light*-The incident ray, the normal at the point of incidence and the
reflected ray all lie on the same plane. The angle of incidence is equal to the angle of
reflection ( = ).
A Real Image is an image formed by the actual intersection of light rays.
A virtual image is formed by the apparent intersection of light rays.
Uses of Concave Mirrors – Searchlights, Floodlights, Headlights in a car, Shaving and
make-up mirrors, dentist’s mirror.
Uses of Convex Mirrors, Door mirror of a car, In shops to deter shoplifters, At concealed
entrances to give a view of oncoming traffic
Relationship between focal length (f), object distance (u) and image distance (v) for
spherical mirrors
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f

1 1

u v
Convention:
For a concave mirror f is positive
For a convex mirror f is negative
For a real image v is positive
For a virtual image v is negative
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The last two lines are what is referred to as the ‘Real is Positive’ convention (RiP).
Magnification
𝐻𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑖𝑚𝑎𝑔𝑒
𝑀𝑎𝑔𝑛𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛 =
𝐻𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑜𝑏𝑗𝑒𝑐𝑡
Refraction is the bending of a wave as it passes from one medium to another
The two Laws of Refraction of Light
1. The incident ray, the normal and the refracted ray all lie on the same plane.
(not to be confused with the reflected ray, from the laws of reflection of light).
2. The ratio of the sin of the sin of the angle of incidence to the sin of the angle of
refraction is a constant called the Refractive Index.
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𝐬𝐢𝐧 𝐢
𝐬𝐢𝐧 𝐫
The second law of refraction is also known as Snell’s Law*.
The Refractive Index of a Medium is the ratio of the sine of the angle of incidence to the
sine of the angle of refraction when light travels from a vacuum into that medium*.
Real Depth and Apparent Depth
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𝐑. 𝐈. =
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R. I. =
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Real Depth
Apparent Depth
Refractive Index in Terms of Relative Speeds
R. I. =
speed of light in air
speed of light in medium
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Total internal reflection occurs when the angle of incidence in the denser of the two media
is greater than the critical angle and light is reflected back into the denser medium.
The critical angle corresponds to the angle of incidence in the denser of two media which
causes the angle of refraction to be 900.
Relationship between Critical Angle and Refractive Index*
 𝑹. 𝑰. =
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𝟏
𝒔𝒊𝒏 𝑪
Applications of Total Internal Reflection - Reflective road signs, Optical Fibres,
Endoscopes, Periscopes
Relationship between focal length (f), object distance (u) and image distance (v)
1 1 1
 
f u v
Convention:
For a convex lens f is positive
For a concave lens f is negative
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For a real image v is positive
For a virtual image v is negative
Power of a Lens = 1/focal length
(The unit of power is m-1.)
𝑃=
1
𝑓
If two lenses of power P1 and P2 are placed in contact, the power P of the combination is
given by
PTotal = P1 + P2
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It follows from this that if two lenses of focal length f 1 and f2 are placed in contact, the focal
length f of the combination is given by
1
𝑓𝑇𝑜𝑡𝑎𝑙
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=
1
1
+
𝑓1 𝑓2
Short Sight
A Short-sighted person can see nearby objects clearly but cannot bring distant objects into
focus.
Long Sight
A Long-Sighted person can see distant objects clearly but cannot bring nearby objects into
focus.