What is Light?

What is light?

• We can not understand what light is by examining light under microscope.

• We have to make whether it explains the observed a theory and check phenomena related to light.

What does light do?

• Reflection (yansıma)• Refraction (kırılma)• Diffraction (kırınım)• Interference (girişim)• …

Light Reflects

Light Refracts

Light Refracts

Reflection and Refraction Together

Then What is Light?

• Light is a stream of particles (Newton)

• Light is wave phenomena (Huygens)• Both in the 17th century.

What is a wave?

How does the particle theory explain reflection?

How does the wave theory explain reflection?

Both theories are succesful in explaining the

reflection of light.

What about refraction?

How does the particle theory explain the refraction of

light?

How does the wave theory explain the refraction of

light?

Both theories predict refraction, but…

• They predict the sense of refraction to be opposite.

• If light slows down in a dense medium then wave theory predicts the sense of refrection correctly while the particle theory predicts it wrong.

• However people found it easier to think in terms of particles.

Snell’s law

b

a

v

v

b

a

sin

sin

Principal of Least Action

Least time is given by the path obeying Snell’s law

b

a

v

v

b

a

sin

sin

Diffraction of Light

• Diffraction is the slight bending of light as it passes around the edge of an object.

• If observed carefully, the edges of shadows are not solid, but slightly fuzzy.

• Shadow where you do not expect.

One hole diffraction

What is Diffraction?

Interference of Waves(Young 1801)

Double Slit Experiment

Light is an electromagnetic Wave

EM waves

EM wave tops

EM spectrum

Planck (1900)

• Light is emitted and absorbed as packets of energy (particles)

Einstein 1905

• Light is a particle.

• The particle of light is called photon.

Quantum Mechanics

• Light and all fundamental particles are also waves.

• E.g. electrons are waves. p

h