Alternative conceptions about light

This article introduces common student alternative conceptions about light based on education research.

It is important that teachers are aware of these during a sequence of lessons so students have a chance to change their thinking as the sequence progresses.

...unless we know what children think and why they think that way, we have little chance of making any impact with our teaching no matter how skilfully we proceed.

Osborne & Freyberg, 1985, p.13

Success in helping students to change their conceptions can occur as students are given a chance to:

• identify their existing conceptions

• reflect on their existing conceptions as they are engaged in practical activities and discussions

• become dissatisfied with their existing conceptions as a valid way to explain their world

• construct new meanings that make sense of their experiences and observations.

It is important that the new conceptions are “intelligent, plausible and fruitful” (Fetherstonhaugh & Treagust, 1992).

Just telling the student the correct answer will not lead to lasting change. Unless they go through the stages of identification of existing conceptions, being challenged on these through activities, discussion and reflection and having the chance to form their new conceptions, they will often revert to what they have always thought.

Student’s view

Scientist’s view

Light travels a limited distance: Many students think light from a dim source such as a candle only travels a short distance from that source and that, for brighter objects like the Sun, light travels further. (This may be partly due to the observation of the glow around a candle in an otherwise dark place.)

Light keeps travelling in a straight line until it hits another object: The evidence for this is that we can see a dim candle even from a distance on a dark night. We can see it because light from that candle has reached our eyes. Some of it has entered our eyes and caused us to see an image.

Light from a candle travels further at night-time: Some think of the distance light from a candle travels as the distance to the edge of the ‘glow’ they see around a flame. In the daytime, there is less glow visible around the flame, so the light must not be travelling as far.

Light travels the same distance during the daytime as at night-time: The ‘glow’ seen around a flame does not represent how far the light travels. Light keeps travelling until it hits something else, such as a wall, the retina at the back of our eyes or another object many kilometres away.

A shadow is a kind of reflection Some think the shadow is the outline of an object such as a person that appears on a wall or on the ground. This is a kind of reflection because it is the same shape as the object.

A shadow is simply an absence of light: A shadow is caused because light travels in straight lines until it hits an object. If none of that light can get past the object, the whole region after that object will have less light reaching it. This shadow region is the whole region between the object and the ground or wall, not just the outline that appears on the ground or wall.

Rough surfaces don’t reflect light: Students have experience with reflectors being shiny things like mirrors, lakes or polished metal. Some do not consider that a rough object like a tree or a person is reflecting light. They think these objects are illuminated, but that the light ‘stays on the surface’ of the object somehow.

Rough surfaces do reflect light: This is called diffuse (or irregular) reflection. The reason we can see objects like a person or a tree is because light is reflecting off that object and travelling in all different directions.

Light does not take time to travel somewhere – it is instantaneous: When a light bulb is switched on in a darkened room, the whole room lights up instantly.

Light takes time to travel from one place to another: Light travels through space at a speed of 300,000 kilometres per second. In a darkened room when a light is switched on, it gives the impression of instantaneous illumination. Light still takes a very small fraction of a second to reach the walls.