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PHY101(PHYSICS) Graded Discussion Board No. 1 Opening Date: Thursday 13th February 2014.

Graded discussion board (GDB)

Graded Discussion Board for the course PHY101 (PHYSICS) will start on Thursday 13th February 2014.

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This discussion board will be closed on Friday 14th February 2014.

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The topic for the discussion is


Have you ever seen a spectrum of colors produced by a soap bubble or by the oily film on a water pond in a parking lot? Keep in mind these colors are not the result of separation of white light by a prism or of absorption of colors in a pigment. “Explain that under what principle this spectrum of colors is formed. What is the name of this phenomenon?”

 

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Light Refraction

Light refraction is the reverse process of color mixing. It shows that white light is a combination of all the colors of the visible light spectrum. To demonstrate refraction a prism is used, which is a piece of glass that is polished in a triangular shape. A light beam travelling through a prism is broken twice in the same direction, causing the light beam to change its original course.

Beams with a long wavelength (the red beams) are refracted less strongly than beams with a short wavelength (the violet beams), causing the colors to fan out. The first fan out is enlarged by the second fan out, resulting in a color band coming out, consisting of the spectrum colors red, orange, yellow, green, blue, indigo, and violet. There are no clear boundaries between the various colors, but thousands of transitional areas. A rainbow is a perfect example of the principle of light refraction in nature.

forming of soap bubble:

The principle of interference is responsible for the brilliant hues of certain butterflies and beetles, and is also seen in soap bubbles. In bubbles, thin film interference occurs.

Why are soap bubbles colored?

The thickness of the film - or rather, its thinness - determines whether iridescence is apparent. Light is reflected from both the inner and outer surface of the soap bubble.

The light rays that are reflected off the inner surface of the bubble travel further than the light rays that are reflected off the outer surface. Some wavelengths will interfere destructively and others constructively, depending on the extra distance traveled by a transmitted-and-reflected ray. Whether the reflected rays are in or out of phase with each other depends on the extra distance (through the film and back) that the second ray must travel before rejoining the first ray. This distance depends on the angle of the incident light and the thickness of the film.


White light is made up of different colors, corresponding to specific wavelengths. As the film thickness changes, the extra distance the ray must travel changes. Interference is constructive when the total extra distance matches a specific wavelength of light, and is destructive when it is half a wavelength. So if white light shines on a bubble, the film reflects light of a specific hue, and this hue changes with the film’s thickness.

How does a soap bubble make colors?

To figure out why a soap bubble does funny things to light, we need to know a bit more about bubbles themselves. What are they anyway? Soap is a kind of detergent and the bubbles it makes are a bit like balloons filled with air, but with one important difference. Where a balloon is made of fairly sturdy latex (thin rubber, in other words), the edge of a soap bubble is made from a thin film of soap and water. You make a soap bubble a bit like you make a sandwich. You need an ultra-thin layer of soap (like one piece of bread), then a layer of water (your filling, in the middle), and then another layer of soap (the top layer of bread). Wrap your sandwich into a perfect sphere and—hey presto—there's your soap bubble. How do you make a soap and water sandwich wrap into a sphere? Easy. Blow on soapy water! You'll find the soap-sandwich film wraps up all by itself, trapping the air inside. And what you get—if you're really lucky—is a perfectly spherical soap bubble held together by surface tension. It forms a sphere because that just happens to be the smallest, most stable structure it can have.

When white light shines on a bubble, strange things happen. Remember that light can behave like a wave. When light waves hit a bubble, some of them bounce straight back off the outer part of the soap film. Others carry on through but then bounce off the inner part of the film. So one set of light rays shine into a soap bubble, but two sets of rays come back out again. When they emerge, the waves that bounce off the inner film have traveled a tiny bit further than the waves that bounced off the outer film. So we have two sets of light waves that are now slightly out of step. Like two sets of ripples on a pond, these waves start merging. Just like on a pond, some add together and some cancel out. The overall effect is that some of the colors in the original white light disappear altogether, leaving other colors behind. These are the colors you see in soap bubbles.

Photo: In this soap-bubble closeup, you can see how the thickness of the soap film varies from place to place. This is very like what you see when gasoline (petrol) or diesel sits on water in the street.

Stare at any one soap bubble and you'll notice that the colors vary across its surface (from place to place) and they also gradually change with time until the bubble bursts. Why is that? The soap film isn't quite the same thickness all over. Where the soap film is thick, red light is canceled out leaving the bubble looking blue or green. When the film is thinner, green is canceled, leaving the film magenta. If you blow on the film, the soap solution starts to evaporate and the bubble gets thinner. If you blow gently enough, you can make the colors change slowly from blue or green to yellow and violet, in the exact order you see them in a rainbow (red-orange-yellow-green-blue-indigo-violet). Try it next time you're in the bathtub or washing up at the sink! Eventually, as the film grows thinner and thinner, the colors disappear. The bubble goes totally clear. At this point, the film is just a few molecules thick. Then it bursts.

Interference Phenomena in Soap Bubbles

Most of us observe some type of optical interference almost every day, but usually do not realize the events in play behind the often-kaleidoscopic display of color produced when light waves interfere with each other. One of the best examples of interference is demonstrated by the light reflected from a film of oil floating on water. Another example is the thin film of a soap bubble, which reflects a spectrum of beautiful colors when illuminated by natural or artificial light sources. This interactive tutorial explores how the interference phenomenon of light reflected by a soap bubble changes as a function of film thickness.

Thanks for sharing

so confused mujy physics siray sa smj he ne ate yeh book meray grade low kray ge......

"Light is diffracted like a wave, leading to the interference we get when several waves are superposed. 
Beautiful rainbow-like colors are generated because the light reflected on both surfaces of soap bubbles interfere with one another, emphasizing the light. The colors you see depend on the viewing direction and angle, as well as on the film thickness of the soap bubble."

How does a soap bubble make colors?

To figure out why a soap bubble does funny things to light, we need to know a bit more about bubbles themselves. What are they anyway? Soap is a kind of detergent and the bubbles it makes are a bit like balloons filled with air, but with one important difference. Where a balloon is made of fairly sturdy latex (thin rubber, in other words), the edge of a soap bubble is made from a thin film of soap and water. You make a soap bubble a bit like you make a sandwich. You need an ultra-thin layer of soap (like one piece of bread), then a layer of water (your filling, in the middle), and then another layer of soap (the top layer of bread). Wrap your sandwich into a perfect sphere and—hey presto—there's your soap bubble. How do you make a soap and water sandwich wrap into a sphere? Easy. Blow on soapy water! You'll find the soap-sandwich film wraps up all by itself, trapping the air inside. And what you get—if you're really lucky—is a perfectly spherical soap bubble held together by surface tension. It forms a sphere because that just happens to be the smallest, most stable structure it can have.

When white light shines on a bubble, strange things happen. Remember that light can behave like a wave. When light waves hit a bubble, some of them bounce straight back off the outer part of the soap film. Others carry on through but then bounce off the inner part of the film. So one set of light rays shine into a soap bubble, but two sets of rays come back out again. When they emerge, the waves that bounce off the inner film have traveled a tiny bit further than the waves that bounced off the outer film. So we have two sets of light waves that are now slightly out of step. Like two sets of ripples on a pond, these waves start merging. Just like on a pond, some add together and some cancel out. The overall effect is that some of the colors in the original white light disappear altogether, leaving other colors behind. These are the colors you see in soap bubbles.

Photo: In this soap-bubble closeup, you can see how the thickness of the soap film varies from place to place. This is very like what you see when gasoline (petrol) or diesel sits on water in the street.

Stare at any one soap bubble and you'll notice that the colors vary across its surface (from place to place) and they also gradually change with time until the bubble bursts. Why is that? The soap film isn't quite the same thickness all over. Where the soap film is thick, red light is canceled out leaving the bubble looking blue or green. When the film is thinner, green is canceled, leaving the film magenta. If you blow on the film, the soap solution starts to evaporate and the bubble gets thinner. If you blow gently enough, you can make the colors change slowly from blue or green to yellow and violet, in the exact order you see them in a rainbow (red-orange-yellow-green-blue-indigo-violet). Try it next time you're in the bathtub or washing up at the sink! Eventually, as the film grows thinner and thinner, the colors disappear. The bubble goes totally clear. At this point, the film is just a few molecules thick. Then it bursts.

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