Visible light is the small range of electromagnetic radiation detected by human eyes. Its wavelengths appear as different colors, and its interaction with matter explains familiar effects from a sharp shadow to a red sunset.
Shadows, reflection and refraction
A shadow forms where an opaque object blocks light from a source. Its darkest region is the umbra. With an extended source such as the Sun, a partially illuminated penumbra softens the edge. The object’s distance from the light and the receiving surface changes the shadow’s size and sharpness.
Reflection redirects light at a surface. A smooth surface preserves enough directional order to form an image, while a rough surface scatters reflected light in many directions.
Refraction occurs when light changes speed and direction as it crosses between materials. Lenses use refraction to focus light, and the same principle makes submerged objects appear displaced.
The geometry of a rainbow
Sunlight entering a spherical raindrop is refracted and separated by wavelength, reflected inside the drop and refracted again as it exits. An observer sees a primary rainbow when the Sun is behind them and rain is ahead. Each visible color comes from a different set of droplets at a particular angle. Red appears on the outside of the primary arc and violet on the inside.
A rainbow is therefore not a fixed object at a distant location. Move the observer and the contributing droplets change.
Why sunsets turn red
Air molecules scatter shorter visible wavelengths more efficiently than longer ones. During the day, scattered blue light reaches our eyes from across the sky. Near sunrise or sunset, direct sunlight travels through a longer atmospheric path. More blue and violet light is scattered out of that path, leaving a larger proportion of orange and red light.
Aerosols and clouds can change the brightness and color, but more particles do not always produce a better sunset; thick pollution can simply obscure the light.
These effects share one idea: what we see depends on wavelength, material and geometry. Light reveals the world while its path explains the colors and shapes we observe.
Sources
- NOAA, What Causes a Rainbow?: https://www.nesdis.noaa.gov/about/k-12-education/optical-phenomena/what-causes-rainbow
- NOAA, Why Is the Sky Blue?: https://www.nesdis.noaa.gov/about/k-12-education/atmosphere/why-the-sky-blue