Introduction to
Light
as a wave
Dr Lisa Jardine-Wright
Cavendish Laboratory
What is a Wave?
‹
‹
A wave is a disturbance which travels
through a medium from one place to
another.
As it travels, the wave transfers energy
from one place to another but NOT material.
Types of Wave
‹
Transverse
Direction of the wave
‹
Longitudinal
Direction of the wave
Types of Wave?
Examples of Waves
‹
Sound
• Longitudinal
• Mechanical
Examples of Waves
‹
Ripples on a pond
• Transverse
• Mechanical
Examples of Waves
‹
Light
• Transverse
• Electromagnetic
‹
Electromagnetic waves can travel through a
vacuum
• Mechanical waves need a medium to travel through.
• E.g. Light waves from distant astronomical objects!
Describing Waves
‹
Wavelength, λ
• Distance before the wave repeats.
• Units are metres, m.
λ
• E.g distance between two peaks or troughs of the
wave.
λ
λ
Describing Waves
‹
Frequency, f
• The number of whole waves that pass a fixed point
each second.
• Units are Hertz, Hz.
0.5 Hz
Describing Waves
‹
Amplitude, a
• Maximum displacement of the wave.
Describing Waves
‹
Speed of the wave, v
• Speed = frequency x wavelength
v=fxλ
‹
The speed of a wave is only altered by
changing the medium through which it
travels.
• Speed of sound in air = 340 m/s
• Speed of sound in water ≈ 1500 m/s
Reflection & Refraction
‹
Reflection
• angle of incidence = angle of reflection
θi
θr
Reflection & Refraction
Reflection & Refraction
‹
Refraction
Air
Glass
Air
θt
θi
Interference & Diffraction
‹
Interference
• Wave interference is what happens when two waves
meet while travelling along in the same medium.
Interference & Diffraction
Interference & Diffraction
Interference & Diffraction
‹
Diffraction
• Is the spreading out of waves beyond openings and…
Interference & Diffraction
‹
Diffraction
• and the bending of waves around obstacles
Interference & Diffraction
‹
Interference & Diffraction
Light waves
Screen