Evans SPH 3U1

Unit 3: Light and Optics

Note 6: Applications of Refraction and Total Internal Reflection (Qualitative)

Jigsaw Students do Posters of Applications - Students award the prize for best group poster.

Reference: Chapter 11.4, 11.6

Apparent Depth (qualitative):

Understand apparent depth using ray diagrams (see figures 11.10, 11.11, 11.12 on page 363-364 in text)

Dispersion in Prism:

A prism is a triangular cross section piece of glass which disperses white light into its different colours.

It occurs because refractive indices are wavelength dependent. Therefore different wavelengths are refracted by different amounts.

Longest wavelengths are refracted the least.

Apparent Water Patches on Dry Pavement:

Different layers of air have different temperatures. Temperature affects the density of air, and density affects the index of refraction of light. On a hot day the hot pavement on the road makes a large temperature difference with the air. The apparent water patches are just images of the sky that are made from the sun's rays refracting through air of varying densities.

Apparent Sunsets:

Sunsets actually occur earlier than we see it. The rays of light actually bend due to the different temperature layers of air. The apparent sun position we see is made by extending the rays entering our eyes back to the point where the converging rays appear to cross. (figure 11.17 pg. 366)

Twinkling Stars:

Light from stars travels through changing temperatures in the atmosphere. The small changes in refractive indices of these layers in the atmosphere causes a bending of the light in different directions. (figure 11.18 pg. 367)

Fibre Optics:

Uses total internal reflection (show core and cladding).

A covering called the cladding covers the glass fibre. The cladding has a low index of refraction compared to the glass (recall that total internal reflection can only occur going from a more dense to a less dense medium).

The cladding ensures that there will be no cross-talk between the fibres and prevents losses through leakage of light (since we will have virtually no refraction)

Rainbows:

Rainbows are formed from sunlight rays (white light) separating into its component colours with the rain droplets acting like prisms. Recall that the longest wavelengths refract the least (i.e. red has a longer wavelength than violet).

A primary rainbow has light reflecting ONCE inside each droplet and since red has the longest wavelength it will refract the least and be the top colour in the rainbow.

A secondary rainbow has light reflecting TWICE inside each droplet so the rainbow is much lighter AND the second reflection causes the colours in the rainbow to be reversed so violet is now at the top.

Sparkling Diamonds:

The more faces you can cut on a stone the more times light will reflect off of it and the more it will sparkle. The critical angle for diamond-air is 24 degrees. The critical angle for glass-air is 42 degrees.
Therefore you can make many more faces (cuts of 24 degrees) on a diamond than on glass for the same size piece so the diamond will sparkle more.

Review:

pg. 391 #'s 24, 28ac, 30ac, 31, 35ac, 37ac, 38a, 40, 49, 50ac