Light - Particles vs Waves

Question 1

Newton’s Corpuscular Theory

Sum it up

Answer 1

• Newton suggested that light is made up of tiny particles called ‘corpuscles’.
• One of his major arguments was that light travelled in a straight line, this was becuase the experiments at the time were not accurate enough to detect the diffraction of the light. (Light was only know to reflect and refract)
• Newton believed reflection was due to a force acting on particles away from a surface and he believed refraction occured due to corpuscles travelling faster in denser mediums.

Question 2

Huygens Wave theory

Sum it up

Answer 2

Huygens Principle: Every point on a wavefront may be considered to be a point source of secondary wavelets. The new wavefront is the surface tangential to all the secondary wavefronts.

• Huygen predicted light should slow down in denser mediums, rather than speed up.
• Huygen also predicted that light should diffract around tiny objects and that two coherent light sources should interfere with each other.
• Scientists of the period still preffered newtons theory as it fit more with their existing understanding of physics.

Question 3

Youngs Double slit experiment

Sum it up

Answer 3

• One point source of light ( a light source behind a narrow slit )
• In front of this was a screen with two narrow slits in it ( light spreading out by diffraction was equivalent to two coherant point sources )
• Area on screen where light from two slits overlapped created bright and dark ‘fringes’.
• This experiment proved that light could diffract and that light could interfere ( to form interference pattern on screen )

Huygens did propose however that light was a longitudinal wave like sound, but this could not explain double refraction. A decade later him and Fresnel (a french scientist realised that transverse waves could explain light behaviour)

Scientist - Thomas Young

Question 4

Fizeau’s experiment

Sum it up

(Speed of light measurement)

Answer 4

• Beam of light shone between two cog teeth to a reflector about 9km away.
• Cog was rotated at right speed so that reflected beam was blocked by next cog tooth.
• Using freq of rotation and Number of gaps he calculated time taken from reflector back to the cog.
• Then used time taken and distance travelled to calc speed of light.

Question 5

Maxwell predicted…

Answer 5

…EM waves and their speed.
- His mathmatical model said that a change to electric and magnetic fields would create an EM wave.
- Maxwell predicted that there would be a range of EM waves with the same speed at different frequencies.
- He calculated speed of EM waves in a vaccum using : 𝑐 = 1 / √ 𝜇0𝜀0 to get the value of = 2.998… x 10⁸ms^-1

Question 6

Heinrich Hertz

Discovered …. and measure their … .

Answer 6

Heinrich Hertz discovered Radio waves and measured their speed.
- Hertz showed that radio waves were produced when a high voltage from an induction coil caused sparks to jump across a gap of air.
- He detected the radio waves by looking out for the sparks between the gap in the loop of wire.
- The fact a potential difference was induced in loop showed that waves had a magnetic component. (as a changing magnetic field is needed to induce a potential difference)
- He also went on to show radio waves could be reflected, refracted polarised, and show interference.

For speed:
Set up Stationary radio wave at fixed resonant frequency (to measure speed)
- Found λ by measuring distance between nodes
- Using frequency and v = fλ he was able to find speed.
- Experiment conducted in vacuum to show speed of radio wave = speed of light
- Helped confirm Radio waves, like light, are EM waves.