Newton's Corpuscular Theory
- 1672, Newton published his New Theory about Light and Colors. Which suggested that light was made up of a stream of tiny particles that he called 'corpuscles'.
- One of his major arguements is that light as a particle was known to travel in straight lines while waves were known to bend in the shadow of an obstacle (diffraction).
- Newton's theory was based on his laws of motion - that all particles including corpuscles will travel in straight lines.
- Newton believed that reflection was due to a force that pushed the particle away from the surface - just like a ball bouncing back off of a wall (F = m x a ).
- Refraction worked if the corpuscles travelled faster in a denser medium - light travels faster in an optically denser medium, causing its path to bend when entering glass.
Huygens' Wave Theory
Huygens Principle: Every point on a wavefront may be considered to be a point source of secondary wavelets that spread out in the forward direction at the speed of the wave. The new wavefront is the surface that is tangential to all of these secondary wavelets.
Wavelets: Wave-like oscillations that were thought to originate from every point along a wavefront and spread out in the forward direction, according to Huygens' principle.
- In other words, each point along is considered as a point wave source that then spreads out in front in a hemispherical 'wavelet' to form the new wavefront in front.
- By applying his theory to light, Huygens' found that he could explain reflection and refraction.
- He predicted that light should slow down when entering a denser medium, rather than speed up.
- He also predicted that light should diffract around tiny objects.
- He predicted that two coherent light sources should interfere with each other.
Why Huygens' Theory was not Accepted
- Even after the double-slit experiment supported Huygen strongly, his theory was still not accepted.
- Newton's other theories had been successful and he was so respected that his theory was still accepted.
- There were also problems with Huygens' wave theory. It used longitudinal waves, but light was known to be able to be polarised - a property of transverse waves only. It also failed to explain why sharp shadows were formed by light.
- It wasn't until the speed of light in water (which is more optically dense than air) was measured to be less than its speed in air that many people started to believe Huygens' theory and it became more widely accepted.
Production of Fringes by Young's Double-slit Exper
- Diffraction and interference were both uniquely wave properties, if it could be shown that light showed interference patterns then it would help decide between the corpuscular theory and the wave theory.
- Interference patterns need two coherent light sources, but this was difficult as light is emitted from most sources in random bursts.
- Young solved this by using only one point source of light (a light source passed through a narrow slit). In front of this was a slide with two narrow slits - a double slit. Light spreading out by diffraction from the double slits was equivalent to two coherent point sources.
- Light from the double slit was projected onto a screen and bright and dark 'fringes' were formed where light from the two slits overlapped.
- The dark and bright fringes are formed by constructive and deconstructive interference.
- When the two waves are in phase they will reinforce each other. This is called constructive interference and produces bright fringes on the screen.
- When the waves are exactly out of phase the waves will cancel each other out. This is called destructive interference and produces a dark fringe on the screen.
Significance of Young's Double-slit Experiment
- Young's experiment was proof that light could both diffract and interfere.
- Newton's corpuscular theory predicted that there would only be two fringes, corresponding to the two slits that the corpuscles could pass through.
- Young's experiment showed that this clearly wasn't happening, and Huygens' theory could explain everything.