# 2.13 Quarks and photons

The following slide show describes elementary particles including quarks and photons. While quarks are tiny – smaller than neutrons – they do not have a defined size. However, they are placed representing the smallest size in the size–time explorer [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] . Photons are packets of energy, which have properties of both waves and particles. They have wavelengths and frequencies, some of which you can see as different coloured light. The activities for this section focus on photons.

While working through the slides, record the following information to in the questions that follow:

- the size, or description of the size, of an elementary particle
- the speed of light in a vacuum
- the wavelength of red photons absorbed by plants (you may need information from multiple slides to work this out).

## Erratum

Please note that there is a small error in the voice over for Slide 3 of this slide show. The final sentence states that ‘Photons with a frequency of around 400–700 nm are visible to humans...’, but this should be ‘Photons with a *wavelength* of around 400–700 nm are visible to humans...’.

The value of the speed of light quoted in the slides gives a value in m s^{−1}. In this question you will calculate how many seconds it takes a photon to travel a metre, which can be expressed in units of s m^{−1}.

To do this you need to invert the relationship, so *n* m s^{−1} (where *n* could be any number) becomes:

As an example, if a ball travels 5 m s^{−1}, you can write this as:

The inverse of this is therefore:

Which means the ball takes 0.2 seconds to travel 1 m.

## Question 1

## Question 2

The Earth is about 1.5 × 10^{11} metres from the Sun, our main source of light. Using the answer from Question 1, which of the following correctly calculates how long it takes light from the Sun to reach the Earth.

## Question 3

Using the answer to Question 2, how many minutes does it take photons from the Sun to reach Earth? Round your answer to one significant figure.

Figures in the slides showed the visible light spectrum of photons with different wavelengths, and the proportion of these absorbed by chlorophyll and proteins in photosynthetic plants. There were two main peaks in the absorption graph.

In addition, the equation describing the relationship between photon wavelength and frequency was given as:

where *λ* represents the wavelength of light, *f* is the frequency (in cycles per second) and *c* is the speed of light in a vacuum. This equation can be rearranged to calculate the frequency of waves, such that:

## Question 4

Use values for the speed of light and the wavelength of red light corresponding to a peak in absorption to determine the frequency of the photons absorbed by plants.

## Question 5

Use the time it takes photons to travel from the Sun to the Earth, and the size of the wavelength of red light to add the bar ‘Red light travelling from the Sun to the Earth’ to the size–time explorer.

## Summary

From your calculations, you now know how long it takes photon from the Sun to reach the Earth, and which photons are absorbed in photosynthesis. The slide show finished with the comment that the efficiency of photosynthesis is related to the way elementary particles move through leaves, which forms a nice link to other levels in the size–time explorer.

## Study note

Don’t forget to answer the questions and fill in the entries associated with this level in the scales data table that you downloaded or printed in Activity 1.

**Next**: That’s as small as you can go. It is recommended that you now go to ‘The oak woodland’ section and start working towards the bigger size scales, or you can return to the size–time explorer and choose your own level.