Flashcards in 4.5.1 - Photons Deck (10):

1

## What is a quantum?

### A small discrete unit of energy.

2

## What is a photon?

### A quantum associated with electromagnetic radiation.

3

## What is the quantum model?

###
- 1900, Max Planck

- Work on the electromagnetic radiation emitted by hot objects, such as stars or a red-hot glowing lump of metal.

- Made him realise that the radiated energy could not have continuous values.

- Planck hypothesised that the total radiated energy had to be emitted in packets, with each packet or quantum having a definite, fixed amount of energy.

4

## Describe the photon model of electromagnetic radiation.

###
- 1905, Einstein

- Used quantum idea to show that atoms absorb and emit light in individual packets of energy.

- Idea that light sometimes behaves in ways that can only be explained by thinking of it as consisting of particles.

- Louis de Broglie

- Showed that light could behave as both a particle and a wave.

- Quanta of light or other electromagnetic energy became known as photons.

5

## What equation can be used to calculate the energy of a single photon?

###
E = hf

photon energy (J) = Planck's constant (6.626 * 10^-34 Js) * frequency of the radiation (Hz)

6

## What is the alternative equation for calculating the energy of photons?

###
E = hc / λ

photon energy (J) = Planck's constant (6.626 * 10^-34 Js) * speed of light (3 * 10^8 ms^-1) / wavelength (Hz)

7

## Define the term 'electron volt'.

### The kinetic energy gained by an electron when it is accelerated through a potential difference of 1 volt.

8

## Why is the electron volt used?

### Since the energies associated with individual photons are very low, when expressed in joules, these very small numbers require an answer expressed in standard form with a large negative power. So a more convenient unit often used by scientists is called the electron volt.

9

## How do you convert from electron volts (eV) to joules (J)?

### Multiply by 1.602 * 10^-19.

10