Light
• Light travels in straight lines called rays
• A light ray arriving at a surface (for example, mirror) is
called an incident ray
• Incident ray: ray coming onto a surface
• A line perpendicular (90 degrees) to the mirror
surface is called normal
• Angle of incident is the angle between incident ray
and the normal
• Angle of reflection = angle of incident
Ray Diagram
Angle of Incidence Angle of Reflection
Mirrored Surface
Law of Reflection
Angle of Incidence = Angle of Reflection
• The reflection in a plane
mirror is the same size as the
object.
• The reflected image is as far
behind the mirror surface as
the object is in front.
• Everything in the reflected
image is laterally inverted.
Examples
Side mirror of a car Metal spoon
Vanity mirror Security mirror
Scattered and Reflected Light
Completely smooth surfaces like Rougher surfaces scatter light in
a mirror will reflect light in a many different directions – this is
specific direction. called a diffuse reflection.
Mirror Paper
Example
How to draw ray diagrams
1. Draw the mirror and the incident ray. 4. Use a protractor to measure the
angle of incidence (i ).
2. Use a ruler to make the incident ray meet 5. Measure an angle equal to angle of
the mirror. incidence on the other side of the
normal and draw reflected ray.
3. Use a protractor to draw the normal
(dashed line perpendicular to the mirror).
Label the angle of reflection ( r ).
Let’s Try!
same
straight
different
reflection
same
Let’s Try!
Normal Incident ray Angle of reflection, r
Angle of incidence, i Plane mirror Reflected ray
Let’s Try!
45°
Textbook pg 193
Light Refraction
Refraction = the bending of light rays
Refraction happens as the rays travel at a slightly different speed
For Example:
• When they enter a more dense
medium e.g. water - the ray
slows down.
• When they enter a less dense
medium e.g. air - the rays
speed up.
Refraction
• Refraction happens when light travels
through different medium
• Medium is the material that light passes
through (air, glass, water)
• The change of speed can cause light to
change direction
Light passing from air into water or
glass
• Light slows down when it passes from air into glass
or water
• The light is bent towards the normal
• Angle of incidence is greater than angle of refraction
Light passing from water or glass into
air
• Light speeds up when it passes from glass or water
into air
• The light is bent away from the normal
• Angle of refraction is greater than angle of incidence
Light Refraction Through Glass
and Water
Glass Water
60° 60°
34.5° 40.6°
60° 60°
Refraction in Everyday Life
• Can stop people from clearly seeing
through wet windows
• Each drop of water on the window
refracts light in different direction,
making it difficult to see
• This is why vehicles have windscreen
wipers
• The windscreen wipers remove the
water droplets so it is easier to see
through the windscreen
• Lenses are used to cause refraction of
light
• For example, in our eyes, in cameras
and in glasses that people wear.
• Lens: a curved piece of glass that is
designed to refract light in a known
way
Let’s Try!
(Textbook pg 200)
Let’s Try!
Let’s Try!
refraction of light
angle of incident
towards
increasing
away from
decreasin
dense g
r less
dense
angle of refraction
smaller
Let’s Try!
Reflection or Refraction?
