The Human Eye and the Colourful World
The Human Eye and the Colourful World covers accommodation, defects and their correction, dispersion through a prism, atmospheric refraction and scattering of light.
The eye
Light enters through the cornea, the iris controls pupil size, and the crystalline lens focuses an image on the retina, where it is real and inverted. Accommodation is the ciliary muscle changing the lens curvature; the near point is 25 cm and the far point infinity for a normal eye. Rods work in dim light, cones give colour vision.
Defects
Myopia (short sight): the image forms before the retina, corrected by a concave lens of negative power. Hypermetropia (long sight): the image forms behind the retina, corrected by a convex lens. Presbyopia is loss of accommodation with age; astigmatism needs a cylindrical lens.
Dispersion and scattering
White light through a prism splits into VIBGYOR because each colour has a different refractive index. A rainbow involves refraction, dispersion and internal reflection in raindrops. Scattering varies as 1/lambda to the fourth power, so blue scatters most, making the sky blue; at sunrise light travels far and blue is scattered out, so the sun looks red.
Worked example
A myopic eye has a far point of 80 cm. The correcting lens must image an object at infinity at 80 cm, so f = -80 cm = -0.8 m and P = 1/f = -1.25 D. The negative sign confirms a diverging lens.
Traps
Correcting myopia with a convex lens; saying the retinal image is erect; forgetting the prism bends violet most and red least.