Refraction
• when light passes from one material (aka medium)
to another, it bends because the speed of light
travels at different speeds in different mediums
• Light bends toward the
normal when it slows
down, and away from the
normal as it speeds up.
- vacuum: 3.0 x 108 m/s
- water: 2.26 x 108 m/s
- acrylic: 1.76 x 108 m/s
Light travels thru water
(why we see blue best)
This bending of light can play tricks on our
minds. Ie. Broken/Bend pencil/spoon.
This is why a fish in the water looks just below the
surface, and may be deeper and further away.
Another Trick
Light travels faster at
higher altitudes because
the air is less dense.
Another example of refraction of
light is the twinkling of a star
in the night sky
As starlight travels from space
into the Earth’s atmosphere,
the rays are refracted.
Since the atmosphere is
constantly changing, the
amount of refraction also
changes.
• Light can undergo partial reflection and refraction
at the same time at a surface.
Ex. Sunglasses, two way mirrors (buildings = less air
conditioning)
Refraction Lab
Purpose: To determine relative speeds of light through different
mediums
Observations:
1. Trace a petri dish, label the normal and draw
in an incident light ray for you to follow.
2. Fill the petri dish with water
3. Shoot the light ray down the incident ray.
4. Trace the light ray coming out of the petri
dish
5. Remove petri dish and connect the lines and
label ‘water’
6. Repeat steps 2 – 5 with glycerin in petri dish.
7. Use a protractor to label the incident angle
and both of the refracted angles.
Analysis:
1.
Which ray bends most toward the normal; air, water,
glycerin? Which ray bends the least?
2.
Which substance is the light travelling the fastest in and
why?
3.
Which substance is the light travelling the slowest in
and why?
Dispersion
- The refraction of white light into different
colours. (Pink Floyd – dark side of the moon)
- Since each colour travels at a different
speeds, it bends at different angles
• Ex. Rainbows – in order to see a complete
rainbow, you need to stand so that its raining in
front of you, and the sun is directly behind you.
Total Internal Reflection
Sometimes light bends (refracts) so much that
it hits the inside wall of a substance at 90 degrees,
and totally reflects off (no refraction occurs). The
light gets trapped inside the
object.
Ex. Fiber Optics
– used to send signals quickly over long distances
– glasses, phone cables, surgery
Demo – dollar store fibre optics
Demo –laser follows stream of water (2L water bottle)
Ex. Diamonds
– jewellers cut angles surfaces (facets)
to make the light become trapped in
diamond, and reflect out the top, making it sparkle.
Prisms
• Pg. 503 # 1 – 3, 7