2 nd & 3 th N.U.T.S. Workshops Gulu University Naples FEDERICO II University 8 – Weather Optics

8- Weather Optics 2nd & 3th NUTS Workshop ( Jan 2010) 2 Dispersion by Prisms In a prism, the amount of bending depends on the wavelength or colour because the speed of light depends on the colour In other words, the index of refraction depends on the colour Blue light has a higher index of refraction than red light, and thus is bent (refracted) more

8- Weather Optics 2nd & 3th NUTS Workshop ( Jan 2010) 3 Dispersion and Rainbows What is a rainbow? The traditional rainbow is sunlight spread out into its spectrum of colours and diverted to the eye of the observer by water droplets. The "bow" part of the word describes the fact that the rainbow is a group of nearly circular arcs of colour all having a common centre.

8- Weather Optics 2nd & 3th NUTS Workshop ( Jan 2010) 4 Sun – Rainbow Direction Most people have never noticed that the Sun is always behind you when you face a rainbow, and that the centre of the circular arc of the rainbow is in the direction opposite to that of the Sun. The rain, of course, is in the direction of the rainbow.

8- Weather Optics 2nd & 3th NUTS Workshop ( Jan 2010) 5 How Rainbows Work Light is first refracted by the raindrops, then reflected once, and then refracted once again Sunlight Dispersion causes the angle of refraction to be different for blue light than for red light. This is because the blue light travels more slowly in the raindrop than red light.

8- Weather Optics 2nd & 3th NUTS Workshop ( Jan 2010) 6 Rainbow Size The rainbow has a width of almost 2° (about four times larger than the angular size of the full moon). Note that even though blue light is refracted more than red light in a single drop, we see the blue light on the inner part of the arc because we are looking along a different line of sight that has a smaller angle (40°) for the blue.

8- Weather Optics 2nd & 3th NUTS Workshop ( Jan 2010) 7 Inner and Outer Colour of Rainbow Sun light raindrops Red is the outmost colour, violet the inner one

8- Weather Optics 2nd & 3th NUTS Workshop ( Jan 2010) 8 How do We See Rainbows?

8- Weather Optics 2nd & 3th NUTS Workshop ( Jan 2010) 9 Why are Rainbows Bowed? The rainbow ray for red light makes an angle of 42° between the direction of the incident sunlight and the line of sight. Therefore, as long as the raindrop is viewed along a line of sight that makes this angle with the direction of incident light, we will see a brightening. The rainbow is thus a circle of angular radius 42°, centred on the antisolar point. Why don't we see a full circle?

8- Weather Optics 2nd & 3th NUTS Workshop ( Jan 2010) 10 Why Don't we See a Full Circle? We don't see a full circle because the earth gets in the way. The lower the sun is to the horizon, the more of the circle we see -right at sunset, we would see a full semicircle of the rainbow with the top of the arch 42°above the horizon.. The higher the sun is in the sky, the smaller is the arch of the rainbow above the horizon.

8- Weather Optics 2nd & 3th NUTS Workshop ( Jan 2010) 11 What makes a Double Rainbow? Sometimes we see two rainbows at once. A part of the ray is reflected again and travels along inside the drop to emerge from the drop at a different angle. The rainbow we normally see is called the primary rainbow and is produced by one internal reflection; the secondary rainbow arises from two internal reflections

8- Weather Optics 2nd & 3th NUTS Workshop ( Jan 2010) 12 Double Reflection in Waterdrops The secondary rainbow arises from two internal reflections and the rays exit the drop at an angle of 50° rather than the 42° for the red primary bow. Blue light emerges at an even larger angle of 53°. This effect produces a secondary rainbow that appears higher in the sky and has its colours reversed compared to the primary. The secondary (upper) rainbow always appears fainter: why?

8- Weather Optics 2nd & 3th NUTS Workshop ( Jan 2010) 13 Double Rainbow the secondary (upper) rainbow always appears fainter

8- Weather Optics 2nd & 3th NUTS Workshop ( Jan 2010) 14 Why the Sky is Blue and the Clouds …White?

8- Weather Optics 2nd & 3th NUTS Workshop ( Jan 2010) 15 Blue Sky The air molecules diffuse more the blue-violet light than other colours (in all directions) The Sun light interacts with the air molecules

8- Weather Optics 2nd & 3th NUTS Workshop ( Jan 2010) 16 Why the Sun is red at Sunset? The blue light diffusion is so high that only red-orange light will pass When the Sun is low on the horizon its beams have to pass through a larger path in the atmosphere atmosphere Sun at sunset Long path Middle day sun Short path