Use wavelength and frequency to describe light, and identify what a wave diagram does—and does not—represent.
An extension of the models introduced in the history notes.
A wave plot can show how an electric field varies with position. It is not a picture of a light particle moving up and down along the drawn curve. Measure wavelength horizontally between neighbouring crests or troughs, not from crest to adjacent trough.
For a wave of speed v, v = frequency × wavelength. At a fixed speed, increasing frequency decreases wavelength. In vacuum, all electromagnetic radiation travels at the same speed c. When light crosses into another medium, frequency stays the same while speed and wavelength change.
In a simple monochromatic comparison, frequency is associated with spectral colour; changing field amplitude affects intensity. Real colour perception can involve mixtures of wavelengths. Increasing brightness is not the same as increasing frequency.
Rays explain many directions and image locations. Waves account for interference and diffraction; diffraction becomes important when sizes are comparable to wavelength. Photons describe quantized energy exchange. These are complementary ways to predict observations, not three unrelated types of light.
Describe wavelength, frequency and amplitude and compare ray, wave and photon models.
Veritasium · Experiment
Compare the observed bright and dark pattern with the prediction of two independent light strips. Then try the Slits screen below.
Choose Slits, switch to the light source and turn on the screen. Compare one slit with two.
Change slit separation while keeping wavelength fixed. Then keep the slits fixed and change wavelength. Explain what the ray model alone leaves out.
Sketch one before-and-after comparison. Label what you changed and what stayed the same. Use the lesson’s vocabulary to explain the result.
The curve represents an oscillating field, not a photon’s path or moving matter. Wavelength is measured in a vacuum; the visible range is approximate.
At fixed speed, frequency doubles. State half or double for the new wavelength and explain your reasoning.
Keep the product speed = frequency × wavelength constant.
Doubling one factor halves the other.
When light crosses from air into water, does its frequency change? State yes or no and explain your reasoning.
The source frequency remains continuous.
Its speed and wavelength change; its frequency does not.
Light enters glass. Which stays the same: frequency, speed or wavelength?
Frequency stays the same across a stationary boundary. Speed and wavelength change together according to v = fλ.
The curve represents an oscillating quantity, such as electric field, against position or time. It is not a literal track of a photon.
Commit to an answer, then use the feedback to check your reasoning.
