Light - Reflection and Refraction

We see objects because light reflected from them reaches our eyes. Mirrors and lenses explain the images in a shaving mirror, a rear-view mirror and a magnifying glass. In this chapter you will revise the laws of reflection and study images formed by spherical mirrors, concave and convex, using ray diagrams for different object positions. You will use the New Cartesian Sign Convention with the mirror formula 1/v + 1/u = 1/f, the relation R = 2f, and magnification m = h'/h = -v/u, and learn the uses of each mirror. You will then study refraction, the bending of light as it passes from one medium to another because its speed changes. You will state the laws of refraction, including Snell's law, define refractive index n = c/v, and trace light through a rectangular glass slab. Finally, you will study image formation by convex and concave lenses with ray diagrams, the lens formula 1/v - 1/u = 1/f, magnification m = v/u, and the power of a lens, P = 1/f, measured in dioptres.

Detailed Worksheet: Light - Reflection and Refraction Section A - Definitions (10 marks) 1. State the two laws of reflection of light. (2 marks) 2. Define the principal focus and focal length of a concave mirror. What is the relation between focal length and radius of curvature? (2 marks) 3. State the laws of refraction of light. What is Snell's law? (2 marks) 4. Define the absolute refractive index of a medium. What does a refractive index of 1.5 for glass mean? (2 marks) 5. Define the power of a lens and its SI unit. Is the power of a concave lens positive or negative? (2 marks) Section B - Calculations and Applications (15 marks) 6. An object 4.0 cm in size is placed 25.0 cm in front of a concave mirror of focal length 15.0 cm. At what distance from the mirror should a screen be placed to obtain a sharp image? Find the nature and size of the image. (3 marks) 7. A convex mirror used as a rear-view mirror in a bus has a radius of curvature of 3.00 m. If a bus is located 5.00 m from this mirror, find the position, nature and magnification of the image. (3 marks) 8. A concave lens has a focal length of 15 cm. At what distance should the object be placed from the lens so that it forms an image 10 cm from the lens? Also find the magnification. (3 marks) 9. The speed of light in vacuum is 3 x 10^8 m/s. Find the speed of light in glass (refractive index 1.5) and in water (refractive index 1.33). In which medium does light travel faster? (3 marks) 10. A doctor has prescribed a corrective lens of power +1.5 D. Find the focal length of the lens and state whether it is converging or diverging. What is the focal length of a lens of power -2.0 D? (3 marks) Section C - Diagrams (10 marks) 11. Draw ray diagrams to show the formation of images by a concave mirror when the object is placed (i) between F and C (ii) between P and F. State the position, nature and size of the image in each case. (4 marks) 12. Draw a labelled diagram to show refraction of light through a rectangular glass slab, marking the incident ray, refracted ray, emergent ray, angle of incidence, angle of refraction, angle of emergence and lateral displacement. (3 marks) 13. Draw a ray diagram to show the formation of an image by a convex lens when the object is placed between F1 and 2F1. State the nature and relative size of the image. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. A student places an object at different distances in front of a concave mirror and records the image. She finds that an image is formed on the screen only for some positions. Explain, using the mirror formula and ray diagrams, for which object positions she will get a real image and for which she will not. Why are concave mirrors used as shaving mirrors and in torches? (5 marks) 15. Light enters from air to glass having refractive index 1.50. A pencil partly immersed in water appears bent at the water surface. Explain why it appears bent, and why a coin at the bottom of a bucket of water appears raised. Why does a ray of light passing obliquely through a glass slab emerge parallel to its original direction? (5 marks) 16. An object 5 cm high is placed at a distance of 25 cm from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and nature of the image. If the lower half of the lens is covered with black paper, will the lens still produce a complete image? Explain. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Follow the New Cartesian Sign Convention in all numericals. Draw ray diagrams with a sharp pencil and ruler, and show the direction of rays with arrows.