Question

why the colour of vens is blue

Answer 1

Skin around the vein looks more red than the vein itself. So, the color contrast enhances the veins bluish hue. Hence, veins may appear blue under the skin due to refraction and involvement of other factors. When oxygen binds to haemoglobin it then achieves bright red colour. In the absence of oxygen it is dark red in colour.

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Answer 2

Actually the real colour of venus is yellow in color But it seems to be blue in colour because of light travels through many layers of atmoshphere and gets reflected. The surface of the venus contains many toxic clouds which reflect the light so, vigorously and we can even see it from earth and its also called evening star.