What is a Rainbow?, |What is mean spectacles?, |Why did human see colours?

How is rainbow formed?

The light coming from the sun is mainly white light.In the white light,a lot of different colours are mixed.These colours are seen in a rainbow.A rainbow is formed in rainy seasons.Sunlight falling on the rain is deflected twice by the water droplets.When it enters the water droplets it is deflected first.The light is split into its component colours because the blue light with shorter wavelength bends more than the red light with longer wavelength.The split rays travel to the rear wall of the droplet,where they are refracted again when they come out.Thus,the rays form a colour full rainbow!

Why can people see better with spectacles?

Normally,the light rays entering the eyes are bundled and refracted by the eye lens in such a way that they converge exactly on one point on the retina at the back of the eyeball.If the refractive power of the lens does not match the length of eyeball,this'focus'is shifted either in front of or behind the retina,and the vision becomes blurred.Spectacle lenses refract the light to the degree necessary to compensate for the refractive errors,and thus the eye lens can converge it exactly on the retina.To do this,the spectacle lenses must be shaped very precisely.This task is performed by opticians.

Why do humans see colours,but dogs do not?

There are two types of sensory cells present on our retina:rods and cones.At twilight more than 100 million rods permit us to distinguish between the colourless bright light and the dark.During the day around 6 million cones help us see a variety of colours.The eyes of goes possess very few cones,which are sensitive only to blue and yellow.For this reason,dogs see the world differently from us.Even there are many people who are blind to red and green and cannot correctly distinguish 
between these two colours.