Waves, Sound and
Electromagnetism
2.3 Light
Kick start
What do you think?
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Why are there shadows?
How are rainbows made?
Why is your shirt white and your blazer green?
How do mirrors work?
https://www.youtube.com/watch?v=eCVPhjHh57
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• https://www.youtube.com/watch?v=I024MxO3A
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Properties of Light
• Light travels in straight lines:
Laser
• Light is a form of energy
• Shadows
Shadows are places where light is “blocked”:
Rays of light
When light is blocked
• Transparent objects let light
pass through in straight
lines eg. glass
• Translucent objects let
some light pass, but scatter
light in all directions so you
can’t see clearly through
them eg. tissue paper. Faint
shadows are made.
• Opaque objects block light
from passing eg. a tree.
Darker shadows are made.
Questions:
1. Think of 2 more examples of
transparent, translucent and opaque
objects
2. If energy is always conserved, what
happens to light energy blocked by an
opaque object?
• Light travels VERY FAST – around
300,000 kilometres per second.
At this speed it can
go around the world 8
times in one second.
•
Light travels much faster than sound. For example:
1) Thunder and lightning
start at the same time,
but we will see the
lightning first.
2) When a starting pistol
is fired we see the
smoke first and then
hear the bang.
• We see most things because they reflect light
into our eyes,
however…
• A few types of objects actually emit their own
light (eg. Sun, light bulbs, fireflies)
Homework
Sound and Light Compared
Properties of Light summary
1)
2)
3)
4)
Light is a form of energy
Light travels in straight lines
Light travels much faster than sound
We see things because they reflect light
into our eyes
5) Shadows are formed when light is
blocked by an object
Reflection
• Reflection is when a wave bounces
off an object.
• Reflected sound is called an echo.
• We see objects only if light is
emitted by them or reflected from
them.
Reflection
• Reflection from a mirror:
Normal
Reflected ray
Incident ray
Angle of
incidence
Angle of
reflection
Mirror
• The Law of Reflection
Angle of incidence = Angle of reflection
In other words, light gets reflected from a surface at
____ _____ angle it hits it.
The
same !!!
Mirror
Clear vs. Diffuse Reflection
• Smooth, shiny surfaces have a
clear reflection:
Rough, dull surfaces have a
diffuse reflection.
Diffuse reflection is when
light is scattered in different
directions
Using mirrors
2) A car headlight
1) A periscope
Curved Mirrors
Convex lens
• Also called converging lens as it
makes rays of light meet at a point
called the focal point or focus
• Often used to magnify images
• Uses?
Focal
point
Concave lens
• Also called diverging lens.
• Makes images seem smaller
and closer.
Refraction
• Refraction is the bending of light because it speeds up
or slows down as it passes from one medium into
another
• Bends towards normal if going from fast, less dense
medium (air) to slow, more dense (Perspex) medium
• Bends away from normal if going from slow to fast
medium
• Doesn’t bend if it enters the second medium
perpendicular to boundary
Refraction ray diagram
Refraction