Why does or doesn't a rainbow form?

In summary: For a magnifying glass, the light is split pretty evenly in all directions, so you don't see a rainbow. But if you put a prism in front of the light, some of the light gets split into a bunch of different colors.
  • #1
gauss44
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When light enters some pieces of glass from the air, such as a magnifying glass or window, rainbows usually don't form. But when light enters a prism, rainbows form.

Why do rainbows form in the prism, but not in the magnifying glass or window?(This is my own personal curiosity and because I tutor physics. I am NOT a student and do NOT have homework so please don't tag this post.)
 
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  • #2
You can split light into its components with a magnifying glass, but they are designed to reduce that effect.
Different colors are refracted to different angles - in a window, the effects of "air->glass" and "glass->air" cancel exactly because those surfaces are parallel to each other. This is different for prisms.
 
  • #3
gauss44 said:
Why do rainbows form in the prism, but not in the magnifying glass or window?

A window's surfaces are parallel to each other, so the light that enters the window will leave at the same angle as it entered and you won't get a noticeable rainbow effect. A cheap magnifying glass can make a rainbow, but you may need to help it a bit: Take a piece of paper, cardboard, or other thin, opaque object and cut it into a circle that's just a bit smaller than the magnifying glass's lens. Now place it on the lens. You want only the outside edge of the lens to be visible. Shine a bright light source through it and you should see circular bands of colors around the focal point. The lens is acting like a round prism. (Note that this may be easily visible or barely visible depending on the quality of the magnifying glass, the power of the lens, and other factors.)

The paper blocks out all the light that would otherwise mix with and obscure the light. You want to block the center and shine light through the outer edges because the chromatic aberration is greatest near the edges of the lens than in the center.
 
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  • #4
To get a good spectrum display, you need everything to be just right. A 60oprism of good, lead glass does this (see this link) pretty well when you shine light from a vertical slit through it. Any old piece of glass of any shape will tend to split a beam of light into is constituent wavelengths a bit (even the most expensive camera lenses have a finite amount of 'Chromatic Aberration') You can think of a lens as a set of extremely small prisms at different angles and in different planes so that's not too surprising.It's all a matter of degree, how much the light is actually split.
 
  • #5


The formation of a rainbow is dependent on the refraction and dispersion of light. When light enters a prism, it undergoes refraction and is dispersed into its component colors due to the varying indices of refraction for different colors. This results in the formation of a rainbow.

On the other hand, when light enters a magnifying glass or window, the refraction and dispersion of light is not significant enough to create a visible rainbow. This is because the shape and material of these objects do not allow for the same level of refraction and dispersion as a prism.

Additionally, the angle at which light enters and exits a prism is crucial for the formation of a rainbow. If the angle is too small or too large, the light will not undergo enough refraction and dispersion to form a rainbow. In contrast, the angle of light entering a magnifying glass or window is not as precise, making it less likely for a rainbow to form.

In conclusion, the formation of a rainbow is dependent on the properties of the material and the angle at which light enters and exits an object. A prism is specifically designed to cause a high level of refraction and dispersion, making it the ideal object for creating a rainbow.
 

1. Why does a rainbow form?

A rainbow forms when sunlight passes through water droplets in the atmosphere. The light is refracted, or bent, as it enters the water droplets and is then reflected inside the droplets. The light is then refracted again as it exits the droplets and is spread out into its component colors, creating the colors of the rainbow.

2. How does the angle of the sun affect the formation of a rainbow?

The angle of the sun is crucial in the formation of a rainbow. The sunlight must be coming from behind the observer in order for a rainbow to be seen. This is because the light needs to enter the water droplets at a specific angle to be refracted and reflected in the correct way to create a rainbow.

3. Why do we only see rainbows after it rains?

Rainbows are created by the interaction of sunlight and water droplets, so they can only be seen under specific conditions. After it rains, there are often still water droplets in the air, and when the sun comes out, the light can pass through these droplets to create a rainbow.

4. Can a rainbow be seen from a different angle or perspective?

Yes, a rainbow can be seen from a different angle or perspective, although the angle of the sun must still be correct. This is why if you move or change your position, the rainbow will appear to move or even disappear.

5. Why do rainbows appear as a semi-circle?

Rainbows appear as a semi-circle because of the way the light is refracted and reflected inside the water droplets. The light that is reflected back to the observer forms a cone shape, with the tip of the cone at the observer's eye. This creates the appearance of a semi-circle when multiple droplets are reflecting the light at different angles.

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