Why can't you see a rainbow in the middle of the day?

Rainbows are typically visible in the morning or late afternoon because the sun needs to be less than 42 degrees above the horizon. When the sun is too high in the sky, the top of the rainbow arc falls below the horizon, making it invisible from the ground.

How do polarizing sunglasses affect rainbows?

Polarized sunglasses can make rainbows disappear or become brighter depending on their orientation. Rainbow light is polarized because it reflects off the back of raindrops at Brewster's angle. Polarizing filters block light with a specific polarization, thus affecting the visibility of the rainbow.

What causes the darker area above a rainbow?

The area above a primary rainbow is darker, known as Alexander's dark band, because raindrops in that region are not reflecting light towards the observer at the necessary angles (between 42 and 50 degrees). Most of the light from these drops is either transmitted or reflected outside this range.

What are supernumerary rainbows?

Supernumerary rainbows are the extra, fainter bands of color sometimes seen inside the primary rainbow. They occur due to interference effects from very small raindrops, where light rays travel slightly different distances, causing constructive and destructive interference.

How did the observation of 'glories' lead to a Nobel Prize?

The observation of colored rings around shadows in mist, known as glories, greatly interested scientist CTR Wilson, inspiring him to invent the cloud chamber. Although he initially aimed to study glories, he discovered the cloud chamber could visualize particle tracks, leading to his Nobel Prize.

Why is understanding rainbows important?

Understanding rainbows, rather than just memorizing facts, leads to true mastery and deepens appreciation for the natural world. It highlights the satisfaction of solving nature's persistent challenges and encourages a proactive approach to learning.

Key Moments

What They (Probably) Don't Teach You About Rainbows At School

Veritasium

Education4 min read28 min video

Dec 1, 2024|9,436,458 views|246,781|12,110

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Key Moments

TL;DR

Rainbows are optical illusions caused by light's interaction with raindrops, involving refraction, reflection, and interference.

Key Insights

Rainbows form due to the specific angles of refraction and reflection of sunlight within raindrops, creating a cone of light.

The visible rainbow is an illusion; each raindrop projects a cone of colors, and an observer sees a specific color from a specific raindrop based on angle.

Rainbows are circular because the 42-degree angle of light reflection and refraction is constant relative to the observer and the sun's position.

Polarization explains why sunglasses can make rainbows disappear and why they are brighter underneath, relating to the orientation of light waves.

Secondary rainbows and fainter arcs result from multiple internal reflections within raindrops, leading to inverted colors and Alexander's dark band.

Supernumerary rainbows and glories (or Brocken bows) are caused by light wave interference in very small water droplets, creating colorful rings.

THE FUNDAMENTAL MECHANICS OF RAINBOW FORMATION

Rainbows are not simple phenomena as often explained; they require sunlight, raindrops, and an observer. When sunlight strikes a raindrop, it refracts upon entering, reflects off the back surface, and refracts again upon exiting. While some light reflects off the front surface and some is transmitted, the crucial part for rainbows involves the back-surface reflection and subsequent refraction. This process, governed by Snell's Law, dictates how light bends due to changes in medium and its inherent properties.

THE ROLE OF CAUSTICS AND MAXIMAL SCATTERING

The formation of a rainbow is deeply tied to the concept of caustics. As light rays enter and reflect within a spherical raindrop, they concentrate along specific paths. A critical aspect is the maximum scattering angle, where a range of incoming light rays converge at a similar outgoing angle. This occurs due to the geometry of reflection within the sphere, leading to a concentration of light. For red light, this maximum scattering angle is approximately 42 degrees below the horizontal.

COLOR DISPERSION AND THE SPECTRUM

Different colors of light, corresponding to different frequencies, interact slightly differently with the charges within water molecules. Higher frequency light, like blue, causes charges to vibrate with greater amplitude, resulting in a larger 'phase kick' and thus a greater degree of refraction. This means blue light bends more than red light. The maximum scattering angle is therefore slightly smaller for blue light (around 40 degrees) than for red light (around 42 degrees), causing the colors to separate and spread out.

THE RAINBOW AS AN OPTICAL ILLUSION AND PERSPECTIVE

A rainbow is perceived as a single arch, but it's a collective effect from billions of raindrops, each projecting a cone of light of specific colors. An observer sees red light from raindrops at a 42-degree angle relative to the sun and their eye, and violet light from raindrops at a 40-degree angle. The center of the rainbow's arch always aligns with the line from the sun through the observer's head, meaning the observer's shadow marks the center of their unique rainbow. No two people can see the exact same rainbow.

SECONDARY RAINBOWS AND ALEXANDER'S DARK BAND

Secondary rainbows, which appear fainter and above the primary bow with inverted colors, are formed by light undergoing two internal reflections within a raindrop. This additional reflection results in a larger angle of light exiting the drop, leading to a wider scattering angle (around 50 degrees for red). The region between the primary (42 degrees) and secondary (50 degrees) bows appears darker than the sky outside, known as Alexander's dark band, because raindrops do not reflect light towards the observer within this angular range.

INTERFERENCE PHENOMENA: SUPERNUMERARY RAINBOWS AND GLORIES

Supernumerary rainbows, observed as fainter bands of color inside the primary bow, occur when raindrops are very small. Light rays traveling slightly different paths within these small drops can interfere constructively and destructively, creating patterns of bright and dark bands. Glories, or Brocken bows, are similar phenomena seen around an observer's shadow in fog or clouds. They are caused by interference in tiny droplets, creating a 'bullseye' pattern of colored rings due to different wavelengths interfering at different angles.

POLARIZATION AND OBSERVATIONAL EFFECTS

The light forming a rainbow is polarized, particularly when reflected off the back of a raindrop near Brewster's angle. This means the light waves oscillate preferentially in one direction. Polarized sunglasses can therefore block this light, making the rainbow disappear or appear brighter depending on the filter's orientation. The brightness beneath a rainbow is due to raindrops reflecting light towards the observer, while the area above is darker because those drops are outside the optimal scattering angles for visible light reaching the eye.

Mentioned in This Episode

●Concepts

Common Questions

Rainbows are curved because they are formed by raindrops, each projecting a cone of light towards the observer. You see a specific color when the angle between the sun, raindrop, and your eye is approximately 42 degrees, creating a circular arch.

Topics

Rainbows Light Refraction Optics Brewster's Angle Supernumerary Rainbows Glories Cloud Chamber

Mentioned in this video

Concepts

supernumerary rainbow

Additional rainbow-like bands that appear under a primary rainbow, caused by interference effects in very small raindrops.

Brewster's angle

A special angle at which light reflects off a surface, such that light polarized parallel to the plane of reflection is transmitted, and only light polarized perpendicular to it is reflected, which is relevant to the polarization of rainbow light.

glories

Circles of color seen around the shadow cast on mist or clouds, caused by interference effects in tiny water droplets, and which inspired CTR Wilson's invention of the cloud chamber.

Snell's law

A mathematical expression that relates the angles of incidence and refraction to the indices of refraction of two media, governing how light bends when passing between them.

Alexander's dark band

The dark region observed between the primary and secondary rainbows, caused by the absence of light reflected once or twice within raindrops at those specific angles.