The Human Eye and the Colourful World

The human eye works like a camera: light enters through the cornea, the iris controls the size of the pupil, and the eye lens forms a real, inverted image on the retina, whose light-sensitive cells send signals through the optic nerve to the brain. In this chapter you will learn how the ciliary muscles change the focal length of the lens, the power of accommodation, and why the least distance of distinct vision for a normal eye is about 25 cm. You will study the defects of vision, myopia, hypermetropia and presbyopia, and their correction using concave, convex and bifocal lenses. You will then study refraction of light through a glass prism, the dispersion of white light into the spectrum VIBGYOR and the formation of a rainbow. Finally, you will learn how atmospheric refraction causes stars to twinkle and makes sunrise earlier and sunset later, and how scattering of light explains the Tyndall effect, the blue sky and the reddish Sun at sunrise and sunset.

Detailed Worksheet: The Human Eye and the Colourful World Section A - Definitions (10 marks) 1. What is meant by the power of accommodation of the eye? Which part of the eye makes it possible? (2 marks) 2. Define the near point and the far point of a normal human eye and give their values. (2 marks) 3. What is myopia? State two possible causes of this defect. (2 marks) 4. What is dispersion of white light? Name the sequence of colours obtained. (2 marks) 5. What is the Tyndall effect? Give one example from daily life. (2 marks) Section B - Calculations and Applications (15 marks) 6. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of corrective lens used to restore proper vision? Find its focal length and power. (3 marks) 7. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect, so that the person can read at 25 cm? Assume that the near point of the normal eye is 25 cm. (3 marks) 8. A student has difficulty reading the blackboard while sitting in the last row, but can read a book clearly. If her far point is 5 m, find the type, focal length and power of the lens she needs. (3 marks) 9. The near point of a person is 50 cm. Find the power of the lens needed for him to read a book held at 25 cm. Name the defect. (3 marks) 10. Due to atmospheric refraction, the Sun is visible about 2 minutes before actual sunrise and about 2 minutes after actual sunset. Calculate the total extra time of daylight in a day and in a 30-day month. Explain why the apparent flattening of the Sun's disc occurs at sunrise and sunset. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of the human eye showing the cornea, iris, pupil, eye lens, ciliary muscles, retina and optic nerve. (4 marks) 12. Draw ray diagrams showing (i) a myopic eye (ii) the correction of myopia with a concave lens. (3 marks) 13. Draw a labelled ray diagram showing the dispersion of white light by a glass prism, marking the angle of deviation and the order of colours of the spectrum. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Why is a normal eye not able to see clearly the objects placed closer than 25 cm? Explain what happens to the focal length of the eye lens when we look at distant objects and at nearby objects. What is presbyopia, why does it occur, and how is it corrected? (5 marks) 15. Explain why the sky appears blue on a clear day, why it would appear dark to an astronaut in space, and why the Sun appears reddish at sunrise and sunset. Why are danger signal lights red in colour? (5 marks) 16. Explain the twinkling of stars and why planets do not twinkle. Describe the formation of a rainbow with a labelled diagram showing how the water droplet acts as a small prism. Why is a rainbow always seen in a direction opposite to that of the Sun? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Follow the sign convention for lenses in numericals. Draw ray diagrams neatly with a ruler and pencil, showing directions of rays with arrows.