The interplay of light with things around us gives rise to serveral beautiful phenomenona. The spectacle of colour that we see around us all time is possible only due to sunlight. The blue of the sky, white clouds, the red- hue at sunrise and sunset, the rainbow, the brilliant colours of some pearls, shells, and wings of birds, are just a few of the natural wonders we are used to. We describe some of them here from here from the point of view of physics.
The rainbow
The rainbow is an example of the dispersion of sunlight by the water drops in the atmosphere. This is a phenomenon due to combined effects of dispersion, refraction e reflection of sunlight by spherical water droplets of rain 🌧️. The conditions for observing a rainbow 🌈 are that the sun 🌞 should be shining in one part of the sky (say near western horizon) while it is raining in the opposite part of the sky (say eastern horizon). An observer can therefore see a rainbow only when his back is towards the sun.
In order to understand the formation of rainbows, consider. Sunlight is first retracted as it enters a raindrop, which cause the different wavelengths (colours) of white the separate longer wavelength of light (red) are bent least while the shorter wavelength (violet) are bent the most. Next, these components rays strike the inner surface of the water drops and get internally reflected if the angle between the refracted ray and normal to the drop surface is greater them the critical angle (48°, in this case).
The reflected light is greater is refracted again as it comes out of the drop ad shown in the figure. It is found that the violet light emerges at the angle 40°. For other colors, angle lie in between these two values.
Explains the formation of primary rainbow. We see that red from drop 1 and violet light from drop 2 reach the observers eye. The violet from drop 1 and red light from drop 2 are directed at level above or below the top and violet on the bottom. Thus, the primary rainbow is a result of three - step process, that is, refraction, reflection and refraction.
When light rays undergoes two internal reflections inside a raindrop, 🌈 instead of one as the primary rainbow, a secondary rainbow is formed as shown picture. It is due to four- step process . The intensity of light is reduced at the primary rainbow. Further, the order of the colours is reversed in ad is clear images.
