5/12/09

Post date: May 11, 2009 11:50:41 PM

Bell Ringer: What is a mirage?

  • Total internal reflection

    • For certain angles, no light escapes a high n medium to a lower n medium. The last angle where light escapes (the last time light is refracted) is called the critical angle.

    • critical angle = arcsin(n2/n1)

    • at critical angle, light is refracted along the water's surface. Increasing the angle results in ONLY reflection inside the material, no more light escapes through the surface.

    • This is the principal that fiber optic technology operates on, where light is transmitted through long tubes which have an index of refraction that is higher than that of air.

  • Effects of refraction

    • Mirage

      • Light (just like sound) gets bent on hot days when the air near the ground is warmer than the air right above it

      • Light moves faster in warmer air, so the light gets bent upward

      • The light from some higher point (such as the blue sky) comes down toward the ground, but instead of hitting the ground gets bent up and hits our eye, making us see blue sky on the ground. This looks very similar to the reflection of objects in water.

      • The opposite happens in very cold areas, such as glaciers - light bends back downward, so we see ground where we expect to see sky, or other reflections above an object.

  • Dispersion of light

    • the amount that light refracts depends on its wavelength (or color)

    • some colors get bent more than others

    • sunlight contains all visible colors, which get separated through refraction when there are water droplets in the air (and we see a rainbow)

  • Chapter 18 Notes

    • Begin Mirrors and Lenses Lab