Chapter 13 – Light
Consider the following statements and click to reveal the answer.
1. Think of some examples of where we see things almost instantaneously but hear the sound that they make a little later.
We usually see lightning before hearing the thunder. Loud sounds made across a sports field, such as the click of a ball on a cricket bat, can be heard slightly after seeing what makes the sound. Jet aircraft often appear ‘ahead’ of the sound they make.
2. What kind of energy is changed to light when a battery lights a bulb?
Some of the chemical energy in the battery will have changed to electrical energy flowing in the circuit. Some of this, in turn, will change to heat and light energy in the bulb.
3. What happens when you cover one eye for a while and then look at both pupils in a mirror?
The pupil which was covered will have adjusted to the lack of light by opening up. When you look in the mirror this pupil will appear larger than the other!
4. How could you make a totally black shadow?
Make a shadow with a single light source in a place where there can be no reflected or scattered light. A room with blackened walls or a flat area of ground outside at night might be the nearest we can get to this. A shadow formed by moonlight in the desert would be an example, but starlight would certainly spoil a perfect shadow.
5. Which colour is mostly absorbed by orange paint?
The brain detects orange when mostly red and green light strikes the retina, so blue light must be absent to get the sensation of orange. Blue light is mostly absorbed by orange things.
6. What light would you get on an overhead projector screen if you overlapped red and blue filters on it?
The red filter would block the blue and green light. The blue would block the red and green light so no light would get through. The screen would be black if the filters were perfect.
7. What are the three primary colours of light?
Red, blue and green are the primary colours of light.
8. Describe the ways in which you might show children that light travels in straight lines.
Light from a projector in a darkened room. Sunlight shines through clouds or tree tops. Shadows often have sharp edges. Laser beam seen as a thin straight line. Look through a thin straight tube. A light source can be viewed through holes made in three cards.
9. Draw a picture to show the kind of shadow formed by a person standing between a neon striplight and a wall. How would this shadow be different if the light was a small point source like the Sun?
The neon striplight will give a fuzzy-edged shadow (with a penumbra). The point source (such as the Sun) will give a sharp-edged shadow.
10. Explain why a blue ball looks blue.
The ball will absorb most of the red and green light from the white light falling on it and reflect or scatter most of the blue into the eye.