1分钟英语大运演讲短文

A scientific explanation of rainbow
The rainbow's appearance is caused by dispersion of sunlight as it goes through raindrops. The light is first refracted as it enters the surface of the raindrop, reflected off the back of the drop, and again refracted as it leaves the drop. The overall effect is that the incoming light is reflected back over a wide range of angles, with the most intense light at an angle of 40°–42°. The angle is independent of the size of the drop, but does depend on its refractive index. Seawater has a higher refractive index than rain water, so the radius of a 'rain'bow in sea spray is smaller than a true rainbow. This is visible to the naked eye by a misalignment of these bows.[2]
The amount by which light is refracted depends upon its wavelength, and hence its colour. Blue light (shorter wavelength) is refracted at a greater angle than red light, but because the area of the back of the droplet has a focal point inside the droplet, the spectrum crosses itself, and therefore the red light appears higher in the sky, and forms the outer colour of the rainbow. Contrary to popular belief, the light at the back of the raindrop does not undergo total internal reflection and some light does emerge from the back. However, light coming out the back of the raindrop does not create a rainbow between the observer and the sun because spectra emitted from the back of the raindrop do not have a maximum of intensity, as the other visible rainbows do, and thus the colours blend together rather than forming a rainbow.
A rainbow does not actually exist at a particular location in the sky. It is an optical illusion whose apparent position depends on the observer's location and the position of the sun. All raindrops refract and reflect the sunlight in the same way, but only the light from some raindrops reaches the observer's eye. This light is what constitutes the rainbow for that observer. The position of a rainbow in the sky is always in the opposite direction of the Sun with respect to the observer, and the interior is always slightly brighter than the exterior. The bow is centred on the shadow of the observer's head, or more exactly at the antisolar point (which is below the horizon during the daytime), appearing at an angle of 40°–42° to the line between the observer's head and its shadow. As a result, if the Sun is higher than 42°, then the rainbow is below the horizon and cannot be seen as there are not usually sufficient raindrops between the horizon (that is: eye height) and the ground, to contribute. Exceptions occur when the observer is high above the ground, for example in an aeroplane (see above), on top of a mountain, or above a waterfall. A rainbow can be generated using a garden sprinkler but to get sufficient drops they must be very small.
彩虹的出现是由于分散的阳光,因为它经过雨滴.根据首先是因为它折射进入表面的雨滴,反映了后面的下降,并再次折射,因为它使辍学.的总体影响是,即将上任的光反射回了广泛的角度,以最强烈的光的角度40 ° -42 ° .角是独立的规模下降,但并不取决于它的折射率.海水中有较高的折射率比雨水,所以半径的一个rain'bow的浪花小于一个真正的彩虹.这是肉眼可见的错位这些弓箭. [ 2 ]
该数额光折射取决于它的波长,因此它的颜色.蓝光（波长较短）是在一个更大的折射角度比红光,但由于该地区的背面滴了一个联络点内的雾滴,频谱跨越本身,因此,红灯似乎更高的天空,和各种形式的外部颜色的彩虹.大家普遍认为的相反,鉴于在后面的雨滴不接受全内反射和一些轻没有摆脱回来.然而,鉴于出来后面的雨滴不会建立观察员之间的彩虹和太阳光谱排放,因为后面的雨滴没有最高的强度,因为其他可见彩虹做,因此,颜色混合一起,而不是形成了彩虹.
彩虹实际上并不存在某一地点在天空.这是一种错觉,其明显的立场取决于观察的位置和地位的阳光.所有雨滴折射和反映了阳光以同样的方式,但只能根据一些雨滴达到观察员的眼睛.鉴于这一点是什么构成的彩虹为观察员.的立场彩虹天空中总是在相反方向的太阳与尊重观察员,以及内部总是稍微亮度比外表.船头是集中在阴影中的观察员的头,或更确切地说在antisolar点（这是地平线在白天） ,出现在角40 ° -42 °之间的界线,以观察员的头部和自己的影子.因此,如果太阳高于42 ° ,那么彩虹的地平线下,不能被看作是通常没有足够的雨水之间的地平线（即：眼高）和地面,作出贡献.例外情况发生时,观测高空,例如在飞机（见上文） ,在一个山头,或以上的瀑布.彩虹可以生成使用花园洒水,但获得足够的下降,他们必须非常小.