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Brighter and darker light sources

Teaching Guidance for 14-16 Supporting Physics Teaching

A photon picture of identical bright and dim bulbs

Wrong Track: The brighter light must be giving out bigger photons of light. That's why it's brighter.

Right Lines: Both bulbs are giving out the same colour light and so the photons from each have the same range and mixture of frequencies. Therefore the energy shifted by the average photon from each is the same. The brighter one is simply giving out more photons per unit time.

Not more photons, just higher energy photons

Thinking about the teaching

The description of bright and dim sources of light of different frequencies in terms of photons is absolutely fundamental to this way of thinking and needs to be emphasised in teaching:

Teacher: So, we turn up the supply to the lamp. What happens? Describe what you can see.

Debbie: It's brighter.

Teacher: Yes, it's brighter. So who can tell me what's happening here in terms of photons?

Tash: They're coming out quicker.

Teacher: Careful! More photons leaving the lamp each second.

Teacher: Now then… one of the students in my other class said that there must be bigger photons coming out from the lamp. What do you make of that?

Roger: It's wrong!

Teacher: Yep… but why?

Karim: Because bigger photons means a different frequency and this is just one lamp.

The key teaching and learning point here is to differentiate clearly between intensity and frequency in terms of the photon model.

is exhibited by Photoelectric Effect
can be explained by the Bohr Model
can be described by the relation E=hf

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