Chapter 10: The Human Eye and the Colourful World
Structure of the human eye
The cornea does most of the refraction of light. The iris controls the amount of light by changing the size of the pupil. The eye lens forms the image on the retina. The cells of the retina send signals to the brain through the optic nerve. The ciliary muscles change the focal length of the lens (accommodation). A normal eye has a near point of 25 cm and a far point at infinity.
Defects of vision and their correction
Myopia: distant objects are not seen clearly, corrected with a concave lens. Hypermetropia: near objects are not seen clearly, corrected with a convex lens. Presbyopia: occurs with age, corrected with bifocal lenses. Cataract: the lens becomes cloudy, treated by surgery.
Dispersion and the rainbow
A prism splits white light into seven colours (violet, indigo, blue, green, yellow, orange, red). Violet bends the most and red the least. A rainbow is formed by dispersion, refraction and internal reflection in water droplets.
Atmospheric refraction and scattering
The twinkling of stars is due to atmospheric refraction. Planets do not twinkle. The Tyndall effect is the scattering of light by colloidal particles. The sky looks blue because blue light is scattered more. The Sun looks red at sunrise and sunset because by then most blue light has been scattered away. Danger signals are red because red is scattered the least.
Important questions
Q1. Which part controls the amount of light entering the eye?
The iris, which adjusts the size of the pupil.
Q2. What is the role of the cornea?
Most of the refraction of light entering the eye takes place at the outer surface of the cornea.
Q3. What is the function of the retina?
It is the screen on which the image forms. Its light-sensitive cells produce electrical signals and send them to the brain through the optic nerve.
Q4. What is the power of accommodation?
The ability of the eye lens to change its focal length, with the help of ciliary muscles, to see both distant and near objects clearly.
Q5. What are the near point and far point of a normal eye?
The near point is about 25 cm and the far point is infinity.
Q6. What is myopia and how is it corrected?
In myopia distant objects are not seen clearly because the image forms in front of the retina. It is corrected with a concave lens.
Q7. What is hypermetropia and how is it corrected?
In hypermetropia near objects are not seen clearly because the image forms behind the retina. It is corrected with a convex lens.
Q8. What is presbyopia?
With age the ciliary muscles weaken and the lens becomes less flexible, so the power of accommodation falls. It is corrected with bifocal lenses.
Q9. What is a cataract?
The eye lens becomes milky and cloudy, so vision is reduced. Surgery can restore the sight.
Q10. Why do we have two eyes?
It gives a wider field of view, helps to detect dim objects better and gives a sense of depth.
Q11. What is dispersion of light?
The splitting of white light into its seven colours when it passes through a glass prism.
Q12. Name the seven colours of the spectrum in order.
Violet, indigo, blue, green, yellow, orange, red (VIBGYOR).
Q13. Which colour bends the most and which bends the least in a prism?
Violet bends the most and red bends the least.
Q14. Who explained dispersion? What experiment did he do?
Isaac Newton. He used a second prism to combine the seven colours back into white light.
Q15. How is a rainbow formed?
By dispersion, refraction and internal reflection of sunlight by tiny water droplets in the air. The Sun must be behind the observer.
Q16. Why do stars twinkle?
Starlight is refracted again and again as the density of air in the atmosphere changes, so the apparent position and brightness of the star keep changing.
Q17. Why do planets not twinkle?
Planets are much nearer and act like a collection of many point sources, so the variations in brightness average out.
Q18. What is the Tyndall effect?
The path of light becomes visible because colloidal particles scatter it, such as sunlight seen coming through the trees in a dense forest.
Q19. Why does the sky appear blue?
Air molecules scatter the shorter-wavelength blue light much more than red light.
Q20. Why does the Sun look red at sunrise and sunset?
The light passes through a thicker layer of the atmosphere, the blue and shorter wavelengths are scattered away, and the red light reaches us.
Quiz
1. Which part controls the amount of light entering the eye?
The iris changes the size of the pupil.
2. Which lens corrects myopia?
A concave lens corrects myopia.
3. What is the near point of a normal eye?
The near point is about 25 cm.
4. Which of these does not twinkle?
Planets do not twinkle because they are nearby.
5. Why does the sky look blue?
Air molecules scatter blue light more.