Refraction, Light, and Sound

1. Refraction, Light, and Sound Sound and light both travel in waves. Refraction is the bending of waves, which occurs when waves travel from one medium to another at an angle of less than 900

2. Refractive Index The refractive index is a ratio that expresses how much light is refracted from one medium to another. The higher the index, the greater the refraction. The refractive index of water changes with changes in temperature, salinity and pressure.

5. Because of the refractive index, a device that measures the refraction of light can be used to determine salinity to a low precision as long as temperature and pressure are controlled for.

6. Light in the Ocean Sunlight does not travel well in the ocean. Scattering and absorption weaken light.

7. Scattering occurs when light is bounced between air and water molecules, dust and other objects such as suspended sand and organisms in the water • Colligative Properties

8. QUESTION Why don’t SCUBA divers usually see shadows even when the sun is shining directly on the ocean at their location?

9. ANSWER The greater density of water (along with the greater number of suspended and dissolved particles) makes scattering more prevalent in water than in air. A few meters below the surface, light in the ocean seems to come from all directions, rather than primarily from one direction.

10. Absorption occurswhen light’s electromagnetic energy is converted to heat in the molecules of seawater. Except for very long radio waves, water rapidly absorbs nearly all electromagnetic radiation

11. 65% of all light is absorbed in the 1st meter • 84% is gone after 10m • Only 1% is left after 100m • No where in the ocean can light be found below 1000m

13. The decrease in intensity of light over distance is called attenuation. This is dependant on turbidity (the clearer the water, the less attenuation) Light meters and secchi disks can measure attenuation

14. Photic Zone • The depth to where light penetrates in the ocean (on average 100 m) • The depth varies depending on conditions • Photosynthesis occurs in this layer • Heat and gases are exchanged from the atmosphere to the ocean and vice versa • Most ocean life is found in this layer

15. Aphotic Zone • No outside light • Some animals in this area produce their own light

16. Color Absorption Within 15 to 20m, red is absorbed Within 100m, yellowis gone Within 200m, green and violet are gone Beyond 200m, there is only blue light

18. Color of the Ocean The oceans appear blue-green because those wavelengths are least absorbed and are most available to reflect and scatter The color of the water is also dependant on what is suspended Ocean water that is clear blue has little life and few suspended particles

20. Sound Transmission in the Ocean • Velocity of sound in the ocean averages 1450m/s (in air, 334 m/s) • Velocity varies with changes in temperature, salinity, and pressure

21. Sound can be absorbed, reflected and scattered by bubbles, animals and the ocean basins Sounds travel further in the ocean than in air

22. QUESTION I can’t tell where sound is coming from when I’m underwater. It seems to be coming from inside my head. Why?

23. ANSWER Our normal sensation of stereo hearing depends in part on the difference in the arrival times of sound from one ear to the other. The speed of sound in water, however, is more than four times greater than its speed in air. Our brains are unable to sense arrival-time differences from sounds originating underwater.

24. QUESTION Why don’t sounds seem to travel easily from air to ocean, or vice versa?

25. Sound waves make the transition from one medium to another with little energy loss only when the densities of the two media are similar. The densities of water and air are too different for an efficient transition to be made. This is why you can’t hear people shouting from the edge of the pool while you are underwater, but the weak sound of a submerged pebble clicking against the side is very sharp and clear. ANSWER

26. ECHO LOCATION • Sound can be used to find objects, but refraction of the sound wave can cause a misinterpretation of the location of an object • Shadow zones can also exist due to a high velocity layer at around 80m , just above the pycnocline that splits and/or refracts the sound wave

28. SOFAR Channel • At around 1200m in the North Atlantic and 600m in the North Pacific, sound waves travel at a minimum velocity

29. As the sound waves travel out of this layer of minimum velocity and speed up, they refract back into the layer due to the sudden change in velocity, effectively trapping the waves – the sound waves cannot diffuse and get trapped in this SOFAR Channel (Deep Sound Channel) • Biologists speculate that whales use this channel to communicate – sounds travel great distances with little loss

30. Problems • Marine mammals use sound to “see” • Experiments be harmful to decibel level • Whale migration might change • Other such experiments exist throughout the world, some for military purposes

31. ATOC • Acoustic Thermography of Ocean Climate • 1995 – Loud, low frequency sounds broadcast in South Pacific • Purpose – To refine models of ocean circulation and investigate global warming

32. ACTIVE SONAR • Active Sonar is used to locate objects under water • Low-frequency active sonar is used by the Navy to find very quiet enemy submarines • Problem – it appears to be killing whales and dolphins