Dispersion is the phenomenon where white light separates into its constituent colors when passing through a prism or other medium. Different colors (w
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Why is a secondary rainbow fainter than a primary rainbow?
In a secondary rainbow, light reflects twice inside water droplets instead of once. Each internal reflection causes some light loss, so less light reaches the observer. Also, the secondary rainbow lies above and outside the primary rainbow with inverted color order.
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Why does a prism separate white light into colors but a plane mirror does not?
A prism causes refraction at non-parallel surfaces, bending different wavelengths by different amounts. A plane mirror just reflects all colors at the same angle, so no separation occurs.
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Why can we see a rainbow only when the sun is behind us?
For dispersion by water droplets, the sun must be behind the observer so that light can enter the droplets, reflect and refract, and return to the observer's eyes at an angle of about 42° for the primary rainbow.
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