Light: Shadows and Reflections: questions with answers

A switched-on torch gives out its own light, so it is luminous. Man-made objects can be luminous too. The other objects only reflect light.

The Moon gives out no light of its own. Sunlight falls on the Moon, and the Moon reflects it towards us. That reflected sunlight is what we see. So the Moon is a non-luminous object. How bright it looks makes no difference.

Light travels in a straight line. Only branch B continues straight along the horizontal pipe. This gives Rekha a clear, straight path to part of the candle's body. The other two branches meet the horizontal pipe at an angle, so their viewers cannot see the candle's body through the pipe.

Light travels in a straight line. Moving the hole out of the light's path makes the matchbox block the light, so the spot disappears.

Light travels in straight lines. An aligned straight pipe provides a clear path from the candle flame to the eye. A sharp bend blocks this straight path, so the flame cannot be seen through the bent pipe.

A transparent object lets almost all the light through. The small part it still stops is enough to make a faint shadow on the screen. Its colour does not make it block light, so the shadow does not turn dark.

Coloured transparent paper lets light pass through it. With red transparent paper covering the torch, a red patch of light appears on the white screen.

The torch and the wall stay in the same place, so only the ball's position changes the shadow. The nearer the ball is to the torch, the larger its shadow on the wall. The nearer it is to the wall, the smaller and sharper the shadow.

An opaque object blocks light, forming a dark shadow on the screen. The shadow does not take the object's colour.

The shape, size and sharpness of a shadow depend on where the object stands between the light source and the screen. Here the torch and the screen do not move. An opaque object close to the torch gives a larger shadow. The same object close to the screen gives a smaller shadow. 'Closer to the light' feels as though it should mean 'smaller', and it is the other way round.

Activity 11.7 and Activity 11.8 test the properties of the image one at a time. Each answer is what was observed, not what was announced. The image is the same size as the object at every position, and it is erect. It cannot be obtained on a screen, whether the screen is behind the mirror or in front of it. It is also laterally inverted. Standing further back moves the image further away without changing its size. Position and size are different questions.

Activity 11.8 separates two things that get confused. The image DOES move: stand close to the mirror and the image is close, stand far and it appears far. The image does NOT change size: Activity 11.7 moved the pen to several positions and the image matched the object every time. Everyday perspective makes distant things look smaller, which is why the true half of the observation drags the false half along with it.

A plane mirror is needed at each corner, tilted to reflect light along the next part of the tube. A mirror at the entry corner alone cannot direct light around the second corner to the eye.

A pinhole camera forms an image that shows the object's colours. The image of the flame is yellow.

A periscope uses two plane mirrors fixed inside a long box. The first mirror turns the light that comes in and sends it down the box. The second mirror turns that light again, so it reaches the eye. A kaleidoscope uses three mirror strips of equal width joined into a triangle. Because there are three mirrors, reflections of reflections build many patterns.