The reasons why Newton’s theory was preferred.

The idea

Newton’s corpuscular theory dominated for over a century for reasons that were as much about authority and evidence as about physics. Newton was held in enormous scientific esteem, and his model fitted the observations then available: light travels in straight lines, casts sharp shadows, reflects predictably and refracts at a boundary. The wave theory of Huygens also predicted refraction, but it appeared to require a medium, the aether, and it seemed to imply that light could bend around obstacles, which was not obvious in everyday observation. Crucially, decisive evidence for interference and diffraction came later, so the particle model was not displaced until those experiments were performed and understood.

What earns the marks

• Explains that Newton’s high scientific reputation made his particle model highly influential.
• States that the corpuscular model accounted for straight-line travel, sharp shadows and reflection, which matched everyday observations.
• Notes that the particle model could also account for refraction at a boundary, so it was not immediately contradicted by simple optical experiments.
• Explains that the wave model appeared to need a medium, the aether, whose properties were uncertain.
• Notes that clear evidence for interference and diffraction, which the wave model explains, was not available until later experiments.
• Recognises that scientific theories are accepted or rejected on the basis of available evidence as well as the standing of their proponents.

Common mistakes and how to put them right

• Saying Newton’s theory was preferred purely because it was correct.

  Put it right: It was preferred because it fitted the evidence then available and because of Newton’s authority.

• Claiming that no one had ever observed diffraction before the nineteenth century.

  Put it right: Relevant effects were observed, but their significance for the nature of light was not established until later.

• Stating that the aether was proved not to exist by Newton.

  Put it right: The aether idea persisted into later physics and was abandoned only after further experimental and theoretical developments.

Exam tips

• Link each reason to a specific piece of evidence or to the state of knowledge at the time.
• Avoid presenting the preference as a simple mistake; explain it as a reasonable judgement given the evidence available.
• Use words such as authority, available evidence and aether to show awareness of the historical context.