Transcript law of reflection - Science with Ms. Tantri

LAW OF REFLECTION
•
The behavior of light as it reflects off a mirror follows the Law of Reflection
•
At the point of incidence where the incident ray strikes the mirror, a line (Normal)
can be drawn perpendicular to the surface of the mirror
•
Angle between the _____________________ and normal is the angle of incidence θ(i)
•
Angle between the ____________________ and normal is the angle of reflection θ(r)
•
The law of reflection states that when a ray of light reflects off a surface:
The angle of incidence is equal to the angle of reflection
=
Eye
DRAWING RAY DIAGRAMS FOR PLANE MIRRORS
For each line of the object:
1.
Draw a reference line from the base of the object perpendicular to the mirror and extend it (use
dashed lines) behind the mirror
2.
Draw a ray from the top of the object perpendicular to the mirror and extend it (use dashed
lines) behind the mirror. It will be parallel to the reference line.
3.
Draw a ray from the top of the object to the point where the reference line meets the mirror
4.
Draw the reflected ray according to the law of reflection
5.
Extend the reflected ray behind the mirror (use dashed lines) until it intersects the ray from the
top of the object. This is the top of the image
O
Object
Mirror
POSITION OF THE IMAGE
• Each sight line can be traced backwards beyond the mirror
• All sight lines (viewers) will intersect at the image location
• The image is therefore formed at the single point on the opposite side of the mirror
from where all the light rays appear to have diverged
Viewer 1
Object
Viewer 2
Viewer 3
PRINCIPLES OF REFRACTION
• MEDIUMS: substances that light can move through (water, glass, air)
• REFRACTION: Light travels at different speeds in different mediums. Light bends
when it moves from one medium to another if they have different densities
• NORMAL: An imaginary line perpendicular to the point of incidence
For mediums of different
densities
Angle of
Incidence
θ(i)
θ(r) ≠ θ(i); for θ(r) ≠ 0 º
Medium 1
Medium 2
Example:
θ(i) [air]
= 56 º
θ(r) [water] = 38.2 º
Angle of
Refraction
θ(r)
REFRACTION OF LIGHT BETWEEN MEDIUMS
How can you predict whether light bends towards or away from the normal ?
Light traveling from a less
dense medium to a more dense
medium, light bends TOWARDS
the normal
Light traveling from a more
dense medium to a less dense
medium, light bends AWAY
from the normal
More Dense
Less Dense
More Dense
Less Dense
INDEX OF REFRACTION (n)
Speed of light in a vacuum (c)
Speed of light in another medium (v)
•
n = number of times slower that light travels in a medium relative to its speed in a
vacuum
•
Light travels slowly in a medium with a high density (v)
•
Mediums with high density have high index of refraction (n)
•
Light will refract more in a high density, slow medium
Vacuum
Air
Ice
Water
Ethyl Alcohol
Plexiglas
Crown Glass
Light Flint Glass
Dense Flint Glass
Zircon
Diamond
Gallium Phosphide
Index of Refraction (n)
1.0000
1.0003
1.31
1.333
1.36
1.51
1.52
1.58
1.66
1.923
2.417
3.50
Increasing
Optical Density
Medium
Applications of Refraction
Inferior Mirages
• Occur when cooler layers of air
lie above warmer air (close to ground!)
– The index of refraction of air decreases from sky to ground
– As light travels downwards, it refracts away from the normal
– Light is eventually totally internally reflected upward from
one of the layers
– We perceive a reflective (shiny) surface, which is actually an
image of the sky reflected upward
“from the road”.
Why does the road look wet in this photograph?
SNELL’S LAW
Snell's law gives the relationship between the angles of
incidence and refraction for light impinging on an interface
between two mediums with different indices of refraction
θ1
n1
n2
θ2
SNELL’s LAW
n1.sin(θ1) = n2.sin(θ2)