Light

We see objects when light coming from them enters our eyes. In this chapter you will study the reflection of light by a plane mirror and verify the two laws of reflection: the angle of incidence equals the angle of reflection, and the incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane. You will distinguish regular reflection from diffused reflection. You will see how a plane mirror forms an erect, laterally inverted image of the same size, as far behind the mirror as the object is in front, and how two mirrors at an angle give multiple images, the principle of the kaleidoscope. You will learn that sunlight is made of seven colours, which a prism disperses into a spectrum, and study the structure of the human eye: cornea, iris, pupil, lens, retina and optic nerve, together with the blind spot and persistence of vision. You will also learn about eye care and the Braille system.

Detailed Worksheet: Light Section A - Definitions (10 marks) 1. State the two laws of reflection of light. (2 marks) 2. Define incident ray, reflected ray, normal, angle of incidence and angle of reflection, using a labelled sketch. (2 marks) 3. Distinguish between regular reflection and diffused reflection, giving one example of each. Does diffused reflection break the laws of reflection? (2 marks) 4. What is dispersion of light? Name the seven colours obtained when white light is dispersed by a prism, in order. (2 marks) 5. What is persistence of vision? About how long does the impression of an image last on the retina? (2 marks) Section B - Calculations and Applications (15 marks) 6. A ray of light falls on a plane mirror making an angle of 30 degrees with the surface of the mirror. Find (i) the angle of incidence (ii) the angle of reflection (iii) the angle between the incident ray and the reflected ray. (3 marks) 7. A girl stands 2 m in front of a plane mirror. (i) How far is her image from the mirror? (ii) How far is her image from her? (iii) If she walks 0.5 m towards the mirror, what is the new distance between her and her image? (3 marks) 8. Two plane mirrors are placed at an angle to each other and a candle is placed between them. Using the relation number of images = (360 / angle) - 1, find the number of images when the angle is (i) 90 degrees (ii) 60 degrees (iii) 45 degrees. (3 marks) 9. A ray of light is incident normally on a plane mirror. What are the angles of incidence and reflection? Along which path does the reflected ray travel? What happens to the angle between the incident and reflected rays if the angle of incidence increases from 20 degrees to 35 degrees? (3 marks) 10. Write the word AMBULANCE as it would appear in a plane mirror and explain why it is written in this way on the front of ambulances. Which letters of the English alphabet look the same in a plane mirror? Name any four. (3 marks) Section C - Diagrams (10 marks) 11. Draw a ray diagram showing the reflection of a ray from a plane mirror. Label the incident ray, reflected ray, normal, point of incidence, angle of incidence and angle of reflection, for an angle of incidence of 40 degrees. (4 marks) 12. Draw a neat labelled diagram of the human eye showing the cornea, iris, pupil, lens, retina, optic nerve and blind spot. (3 marks) 13. Draw a labelled diagram showing white light passing through a glass prism and splitting into a spectrum of seven colours on a screen. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain how a kaleidoscope is made using three mirror strips and pieces of coloured glass, and why it produces beautiful repeating patterns. Explain why no two patterns are ever exactly alike, and name one use of kaleidoscopes in design. (5 marks) 15. Explain the functions of the iris, pupil, lens and retina. Why do we find it difficult to see things when we enter a dark cinema hall from bright sunlight, and why does our vision improve after a while? What are rods and cones? (5 marks) 16. Explain how the Braille system works, who developed it and how many dot patterns it has. Suggest three ways in which our society can make public places more accessible to visually challenged people. Give three tips for taking care of our eyes. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw all ray diagrams with a sharp pencil and ruler, show the direction of rays with arrowheads and measure angles with a protractor.