EM & Light

What is EM radiation

A form of energy that exhibits wave-like behaviour as it travels through space

How does EM radiation behave

It has wave-particle duality - it can behave as both wave and particle

What does EM radiation do

It is emitted and absorbed in photons

What is ionising radiation

Radiation that has enough energy to remove an electron from an atom, creating an ion

2 examples of high energy (ionising) radiation

• X-rays

• Gamma rays

What does white light consist of

Photons with a range of wavelengths across the visible spectrum - not a single wavelength

Why do objects appear different colours + example

They absorb and reflect different wavelengths, e.g. a red balloon reflects red wavelengths while absorbing others

What is incandescence

Light produced by heating

What is luminescence

Light produced by something other than heating

What are the two types of luminescence

• Photoluminescence

• Chemoluminescence

What are the two types of photoluminescence

• Fluorescence

• Phosphorescence

What is fluorescence + example

Material absorbs EM radiation. Its electrons:

1. Absorb energy

2. Move to higher energy level

3. Fall back to lower energy level

4. Release energy as light

E.g. UV light on white t-shirt

What is phosphorescence + example

Similar to fluorescence but material does not immediately re-emit the absorbed radiation. Delay can last up to hours, e.g. glow-in-the-dark stars

What is the one type of chemiluminescence + example

Bioluminescence - Living organisms produce light through chemical reactions, e.g. jellyfish

Speed of EM radiation

c = 3×10⁸m/s

Wave equation

v = λf

Wave equation for EM radiation in a vacuum (plus its variants)

c = λf

therefore:

λ = c/f and f = c/λ

What is reflection

The change in direction of a wavefront at an interface so that it returns into its original medium

What is diffuse reflection + example

Rough surface → light reflects in many directions (e.g. wall)

What is specular reflection + example

Smooth surface → organised reflection (e.g. mirror/water)

What is an incident ray

Ray travelling towards surface

What is a reflected ray

Ray travelling away after reflection

What is the normal

Line perpendicular to surface at point of incidence

What is the rule of reflection

i = r

What is refraction

The change in direction of a wave due to a change in its speed as it passes from one medium to another

What is angle of incidence (i)

Angle between incident ray and the normal

What is angle of reflection (r )

Angle between reflected ray and normal

Example of refraction

Straw in water

What is snell’s law

sin i / sin r = n, where n is a constant depending on the two media and direction of travel

How does refraction behave when going from dense to less dense

Moves away from normal

How does refraction behave when going from less dense to dense

Moves towards normal