Light, Shadows and Reflections

We can see objects only when light from them reaches our eyes. In this chapter you will learn that objects like the Sun, a candle flame and an electric bulb give out their own light and are called luminous, while most objects, such as a book or the Moon, do not and are non-luminous. Materials are grouped by how they let light pass: transparent objects like clear glass let light pass completely, translucent objects like butter paper let it pass partly, and opaque objects like wood or a stone do not let light pass at all. When an opaque object blocks light, a dark shadow forms on a screen behind it. A shadow needs a source of light, an opaque object and a screen, and it is always dark whatever the colour of the object. You will make a pinhole camera, which forms an inverted image, and look through straight and bent pipes to show that light travels in a straight line. Finally, you will see that a mirror changes the direction of light by reflection.

Detailed Worksheet: Light, Shadows and Reflections Section A - Definitions (10 marks) 1. Distinguish between luminous and non-luminous objects with two examples of each. (2 marks) 2. Define transparent, translucent and opaque objects with one example of each. (2 marks) 3. What is a shadow? What three things are needed to form a shadow? (2 marks) 4. What is a pinhole camera? What kind of image does it form? (2 marks) 5. What is reflection of light? Name an object that reflects light well. (2 marks) Section B - Calculations and Applications (15 marks) 6. Classify as transparent, translucent or opaque: clear glass, wooden board, butter paper, frosted glass, steel plate, clean water, cardboard, thin plastic sheet, oily paper. (3 marks) 7. A stick 1 m tall casts a shadow 1.5 m long at 4 p.m., and at the same time a tree casts a shadow 9 m long. (i) How many times longer is the tree's shadow than the stick's shadow? (ii) Find the height of the tree. (iii) Why do shadows become shorter around noon? (3 marks) 8. Explain with reasons: (i) a red ball and a blue ball give shadows of the same dark colour (ii) shadows form on the side opposite to the light source (iii) we see the Moon even though it is non-luminous. (3 marks) 9. In a pinhole camera, the image of a tree is upside down. (i) Explain why. (ii) What happens to the image if the pinhole is made larger? (iii) What will you see if you look at a person walking from left to right through the camera? (3 marks) 10. Light travels at about 3,00,000 km per second. (i) How far does light travel in 5 seconds? (ii) The Sun is about 15,00,00,000 km away; about how long does sunlight take to reach the Earth, in seconds and in minutes? (iii) Why do we see lightning before we hear thunder? (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram showing how a shadow is formed, with a torch, an opaque object and a screen. Show the rays of light and the shadow region. (4 marks) 12. Draw a labelled diagram of a pinhole camera forming an image of a candle flame. Show the rays passing through the pinhole and the inverted image. (3 marks) 13. Draw a diagram to show a beam of light falling on a plane mirror and being reflected. Label the incident beam, mirror and reflected beam. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Paheli looked at a lit candle through a straight pipe and then through a bent pipe. (i) In which case could she see the flame? (ii) What does this show about light? (iii) How does this explain the formation of shadows? (iv) Give one more everyday observation that shows light travels in straight lines. (5 marks) 15. A student says, "The shadow of an object always has the same shape as the object." (i) Test this using a cylinder (a bottle) held in different positions in front of a torch. (ii) What shapes of shadow can a bottle give? (iii) Can two different objects give the same shadow? (iv) What changes the size of a shadow? (5 marks) 16. Boojho made a periscope-like toy using two mirrors. (i) How does a mirror change the path of light? (ii) How can he see objects over a wall using two mirrors? (iii) Why is the image in a mirror laterally inverted? (iv) Give two uses of mirrors in daily life. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw rays of light with a ruler and show their direction with an arrowhead on the line.