Understanding the True Color of Sunlight in Physics
The sun is our major source of light and heat for our planet. It spreads a variety of distinct colors across the sky every day. It is fascinating to see the sky change its hues from light red to bright golden shimmer and then to a pink glow during the sunset. Just by looking at the sunlight, one often thinks that sunlight is yellow and then, it gradually changes to orange. However, upon closer scientific inspection, the facts point to something else entirely - that sunlight is actually white in color. But on deeper inspection, the white light of the sun is divided further into seven colors of the rainbow. So, let us understand what color sunlight exactly is and why it is so.
The Colors Present in Sunlight
Let us visualize the colors present in sunlight with the help of a simple experiment:
Take a glass prism and set it up in a dark room near the window such that direct sunlight falls on the prism through a small aperture in the window.
Fix a whiteboard to capture the rays passing out of the prism.
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The observation shows that the whiteboard has a spectrum of seven distinct colors, which are the colors of the rainbow, i.e., violet, indigo, blue, green, yellow, orange, and red.
Thus, we can conclude that the white light of the sun’s rays is actually made of a combination of seven constituent colors.
Why are there 7 Colors in Sunlight?
The sun’s rays are actually white in color and form a mixture of the seven colors we see in a rainbow, i.e., Violet, Indigo, Blue, Green, Yellow, Orange, and Red, commonly called VIBGYOR. The sun appears to have different colors during the course of a day because of a process called dispersion. Dispersion of light is a phenomenon by which white light splits up into its seven constituent colors, due to the refractive index of the surface of incidence, and the different speeds the different constituent colors have in a medium. The refractive index of a material is defined as a dimensionless number that can describe how fast light travels through a particular material. Different lights have different wavelengths, and hence, different refractive indices in a given material. This causes them to split apart from the original white light, and form a spectrum of the seven colors.
The Different Colors of the Sun at Different Times in the Day
The earth’s atmosphere is made up of various different gasses like oxygen, nitrogen, and carbon dioxide. In addition, there are other impurities like dust, smoke particles, and polluting gasses like methane and CFCs. When the sun's rays strike the earth’s atmosphere to pass through, they are distorted by the earth’s atmosphere due to the presence of all these materials in the atmosphere. As demonstrated earlier, different colors present in the spectrum have different wavelengths.
The wavelength of a material can be defined as the distance between two successive crests(highs) or two successive troughs(lows) of a wave. So, the longer the wavelength, the lower the frequency. Thus, blue and violet are scattered more because of their short wavelengths. Conversely, colors of the other end of the spectrum do not get scattered as much, because they have longer wavelengths. Thus, there are different colors of the sun at sunrise and sunset.
When the sun is directly over us during late morning and noon, the sun’s rays are subject to the least amount of interference because the distance traversed by the rays is the least at that time. Therefore, during this period, blue light gets scattered, and the sky appears to be blue, while the sun’s rays appear to be yellow.
During sunrise, the sun is at its farthest, seemingly rising from the horizon. As a result of this, the light rays have to travel a much longer distance through the atmosphere. Therefore, they are obviously subject to more interference, resulting in an increased amount of scattering. Consequently, out of all the colors, the red light is least scattered. The same phenomenon occurs during sunset when the sun is seemingly moving towards the horizon, and as a result, the sunlight color appears to be of varying shades of red and orange during the dawn and dusk.
Sunlight
Sunlight can be defined as the energy and light which comes from the Sun. It is called insolation when the sun's energy reaches the earth’ surface. What the dwellers of the earth experience are the solar radiation of the sunlight. The heat and radiation from the Sunlight come in the form of electromagnetic waves. When the solar radiation enters the earth, then the atmosphere absorbs about 16% of the solar radiation and some 6% is scattered to space. 28% of the solar radiation is reflected by the clouds and about 47% reaches the earth’s surface. Without the presence of sunlight, no life could survive on the planet. The plants require sunlight for making food by the process of photosynthesis. Here, the plants use solar energy, water and carbon dioxide to form carbohydrates and oxygen. Thus, without sunlight, no life will be able to survive on earth. Solar energy can be both beneficial and harmful for us. The human body requires sunlight for the synthesis of vitamin D in their body. But if there is an excess of sunlight then the radiation can lead to sunburn and skin cancers.
FAQs on Is Sunlight White or Colored?
1. What is the actual colour of sunlight, and why does it appear white to us?
The actual colour of sunlight is white. While it might appear yellow or orange at different times of the day, sunlight is a composite light made up of all the colours of the visible spectrum. When all these colours—Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR)—are combined, our eyes perceive the resulting light as white. This can be proven by passing sunlight through a prism, which separates it back into its constituent colours.
2. What is the phenomenon of dispersion of light?
Dispersion of light is the phenomenon in which a beam of white light splits into its seven constituent colours when it passes through a transparent medium like a glass prism. This happens because the speed of light in a medium depends on its wavelength, and each colour has a different wavelength. As a result, each colour bends at a slightly different angle upon entering and exiting the prism, causing them to separate and form a colourful spectrum.
3. How does a prism demonstrate that white sunlight is composed of seven colours?
A prism demonstrates the composition of white light through dispersion. When a narrow beam of sunlight is directed at one face of a glass prism, it undergoes refraction (bending) as it enters. Because the refractive index of glass is different for each colour's wavelength, the colours separate.
- Violet light (shortest wavelength) bends the most.
- Red light (longest wavelength) bends the least.
4. Why does the sky appear blue on a clear day?
The sky appears blue due to a phenomenon called Rayleigh scattering. Earth's atmosphere is filled with tiny gas molecules and other particles that are much smaller than the wavelength of visible light. These particles scatter shorter-wavelength light (blue and violet) much more effectively than longer-wavelength light (red and orange). As sunlight passes through the atmosphere, the blue light is scattered in all directions, filling the sky. Our eyes are more sensitive to blue than violet, which is why we perceive the sky as blue.
5. If the sky is blue, why do the sun and clouds appear reddish during sunrise and sunset?
During sunrise and sunset, the sun is very low on the horizon. Sunlight must travel through a much thicker layer of the Earth's atmosphere to reach our eyes. This longer path causes most of the shorter-wavelength light (blue and violet) to be scattered away from our line of sight. Consequently, the longer-wavelength light (red, orange, and yellow) is what predominantly passes through and reaches our eyes, making the sun and the clouds illuminated by it appear reddish or orange.
6. Is it true that the Sun's peak emission is green? If so, why doesn't it look green?
Yes, it is true that the sun's peak intensity is in the green part of the visible spectrum. However, the sun does not appear green because it also emits massive amounts of light in all other visible colours (blue, yellow, orange, red, etc.). Our eyes are not designed to pick out a single peak colour from a broad spectrum. Instead, our brain processes the combination of all these colours together, and this full-spectrum combination is perceived by us as white light. The sun would only appear green if it emitted light exclusively in the green wavelength.
7. What is the main difference between the dispersion and scattering of light?
While both phenomena affect how we see light, dispersion and scattering are different processes:
- Dispersion is the splitting of white light into its constituent colours because each colour travels at a slightly different speed through a medium like a prism or water droplet. It is a separation of colours.
- Scattering is the process where light is absorbed and re-emitted in different directions by particles in its path, like air molecules or dust. It is a redirection of light, with the amount of scattering depending on the light's wavelength and the particle's size.
8. How are rainbows formed in the sky?
Rainbows are optical phenomena created by the interaction of sunlight with water droplets (like rain or mist) in the atmosphere. The process involves three main steps:
- Refraction and Dispersion: As sunlight enters a water droplet, it bends (refracts) and splits into its seven colours (disperses), just like in a prism.
- Total Internal Reflection: The dispersed light travels to the back of the droplet and is reflected off the inner surface.
- Final Refraction: The reflected light then travels back to the front of the droplet and refracts again as it exits, separating the colours even further.
