12.7 Natural Phenomena'sA Phenomena an observable event or fact; an object or aspect known through the senses rather than by thought or intuition.

Nature has many interesting phenomena involving light. We can use our understanding of geometric optics to explain many of these phenomena

Apparent Depth Apparent depth is the depth that an object appears to be

at due to the refraction of light in a transparent medium.

Examples: when looking at an object in water, it is apparent depth that gives us the optical illusion that the object is closer to the surface then it truly is.

A Mirage A mirage can appear when light is travelling

from cool air into warmer air. The index of refraction for air decreases as the air gets warmer. This results in the light bending farther away from the normal as the air temperature continues to increase.

Eventually, total internal reflection occurs in the lowest(hottest) air layer. This light ray now travels up from the hottest layer to the cooler layer above and gradually refracted toward the normal as the air temperature decreases. Then the light ray eventually enters your eye.

You see a false image of water on the road as it seems to show a reflection of the surrounding sky line.

Mirages of the sky look like patches of water but even when trees or distant hills are being miraged, the impression of a water reflection can be very striking.

MIRAGE : water on the road

The “Flattened” SunWhen the sun is close to the horizon, light from the bottom of the sun is refracted more than the light at the top of the sun.

The air near the earth’ s surface is more dense than the air higher up in the atmosphere. So this change in density causes the light ray’s to bend.

Also, the light at the bottom of the sun have a greater angle of incidence than the rays coming off the top of the sun. These two things combined give the appearance that the sun is flat rather than it’s normal round shape.

Rays from the setting Sun (lower) are refracted by the atmosphere and make it appear higher in the sky.   The lower limb is lifted more than the top, making its image oval, and flattened.

The RainbowIn the previous section, we learned that white

light can be separated into a continuous sequence of colours called the visible spectrum.

Each colour of the visible spectrum travels at a different speed. This is evident when separated by a glass prism.

Example- Violet light slows down more then red light. That is usually why we usually see more violet light as part of a rainbow.

A rainbow is produced by water droplets in the atmosphere.

Refraction occurs when the light enters the water droplet from the air, resulting in the dispersion of the visible light.

Partial internal reflection occurs when light hits the back of the rainbow.

The light then refracts again as it goes from the water back into the air.

You can only see a rainbow when the sun is behind you because your brain must project the sun’s rays back to your eyes to create a virtual image of the visible spectrum.

HOMEWORK

Read pages 535 -539ANSWER page 539 # 1, 2, 3, 4, 6