(PHY) Chapter 13 - Light
(PHY) Chapter 13 - Light
(PHY) Chapter 13 - Light
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TOPIC 13:
LIGHT
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THE ABOUT
TIME
CHAPTER
ANALYSIS EXAM
WEIGHTAGE
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KEY CONCEPT
REFLECTION
LAW OF REFLECTION
RAY DIAGRAM & PLANE MIRROR IMAGE
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1) The incident ray, the reflected ray, and the normal at the
point of incidence all lie on the same plane.
STEP 1
Locate the image. It will be the same
distance from the mirror as the object’s
distance from mirror.
RAY DIAGRAM
Image M Object
STEP 2
Draw light rays from image to the eyes.
(Dotted lines in virtual plane and solid lines in
for outside mirror.)
Image M Object
STEP 3
Draw light ray from object to mirror, meeting
at the reflected rays.
Image M Object
STEP 4
Add in the arrows if you haven’t & draw the
normal at the point of reflection.
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RAY DIAGRAM
CHARACTERISTICS OF PLANE MIRROR IMAGE
- Image is virtual.
- Image is upright.
- Image is same size as object.
- Image is laterally inverted.
- Image will be same distance from the mirror as the object is
from the mirror.
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KEY CONCEPT
REFRACTION
LAW OF REFRACTION
REFRACTIVE INDEX
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Light rays bend due to the difference in speed of light in different optical
What path will allow you to reach point B in the shortest amount of time?
water
land
B
You will not just simply travel in a straight line because that means spending
an equal amount of time in water and on land when you travel faster on land.
Given that you walk faster on land, you would cut short the distance you swim
in water and attempt to get on land as soon as possible. You will probably
travel in a path as shown above.
Light rays bend due to the difference in speed of light in different mediums.
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1) The incident ray, the refracted ray and the normal at the
point of incidence all lie in the same plane.
Less dense to denser medium Denser to less dense medium 2) For light passing through any two mediums, the ratio of
Speed of Decreases Increases sin i / sin r is a constant (refractive index).
light
Light ray Towards normal Away from normal
BENDING OF LIGHT RAYS
Diagram
When light travel from a less dense medium to a denser
medium, the refracted ray will bend towards the normal.
Optically less dense medium Optically denser medium When light travel from a denser medium to a less dense
Optically dense medium Optically less dense medium
medium, the refracted ray will bend away from the
Bends towards Bends away
normal.
normal from normal
*See if you are able to visualise the example from the previous page to the 2 refraction
diagrams shown here. (Strongly suggest you understand this instead of memorizing.)
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REFRACTIVE INDEX
n = sin i / sin r, where i is angle of incidence & r is angle of refraction.
Vacuum 1.00
Air 1.003
Water 1.33
Glass 1.50
Diamond 2.42
PRINCIPLE OF REVERSIBILITY
Given that,
PRINCIPLE OF REVERSIBILITY
n = sin i / sin r
n = sin r / sin i
Please note that this for ‘O’ Levels without using Snell’s Law (as it is not within syllabus). n = sin (angle in air) / sin (angle in medium)
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KEY CONCEPT
FORMULA:
sin c = 1 / n
where n is refractive index.
Optic fibres are made of glass and transmit light from one
point to another.
The light ray entering the pipe does not exit but is constantly
undergoing total internal reflection until it reaches the other
end of the fibre.
This is how our internet speed and Wifi has great improved
over the years!
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KEY CONCEPT
LENS
CONVERGING & DIVERGING LENS
RAY DIAGRAMS
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LENS
LENSES
Optical Centre, C
Principal Axis
Focal Length, f
Focal Plane
The spot where 2 light rays intersect is where the image will
be formed.
You will only need 2 out of 3 light rays to locate the image.
Ray 1:
Travel parallel to principal axis hits the lens cuts through
focal point, F
Ray 2:
Straight line that cuts through optical centre, C
Ray 3:
Passes through principal focus F hit the lens travel
parallel to principle axis
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TIPS:
If you shift the object further to the left, you will get u > 2f.
Notice how the entire ray diagram has shifted to the left.
The image is now formed closer to the lens and is diminished.
If we shift the object to the right instead, we get f < u < 2f.
Notice how the entire ray diagram has shifted to the right.
The image formed is now further from the lens and magnified.
If the image is far away and enters the lens as parallel rays, u = ∞.
Parallel rays easily converges at focal point F to form an image.
*I’ve rearrange the sequence of the ray diagrams, original textbook version is on the next page!
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