How to Make Teaching Come Alive - Walter Lewin - June 24, 1997

Professor Walter Leen is introduced as a renowned lecturer at MIT, particularly for his engaging teaching style in physics. The speaker emphasizes the beauty of understanding physics beyond its surface-level appeal, like the aesthetics of a rainbow or sunset. The presentation aims to explore the 'hidden beauty' of physics, which is the profound knowledge behind everyday phenomena. The lecture's goal is to inspire teachers to convey this beauty to their students, enhancing their appreciation for both art and science.

The script delves into the physics of colors, explaining how sunlight is decomposed into its constituent colors using a prism, illustrating the concept of additive color mixing with red, green, and blue light. It discusses how different colors are created by mixing these primary colors in various proportions, a principle utilized in color TVs and computer monitors. The lecture also touches on the philosophical aspects of color perception and the complexities of color experiences.

A famous demonstration by Edwin Land is described, where black and white slides, one filtered through a red lens, are projected to create an unexpected color experience. Despite the slides being devoid of color, when projected with a red filter, they produce a palette of colors, challenging conventional understanding of color perception. This phenomenon, not typically covered in physics textbooks, is attributed to the brain's unique response to visual stimuli.

The lecture introduces Benham's top, a 19th-century demonstration showing that colors can be perceived through black and white stimuli when in motion. When the top is spun, viewers perceive colors that are not physically present, suggesting a neurophysiological explanation for color perception beyond the physics of light and wavelength.

The script presents a detailed exploration of rainbows, discussing the necessary conditions for their formation and the physics behind their appearance. It covers the concepts of refraction, reflection, and the dispersion of light within water droplets. The lecture aims to answer specific questions about rainbows, such as their radius, color sequence, and polarization, using principles like Snell's law.

This paragraph provides a deeper quantitative analysis of light refraction within a water droplet, using mathematical expressions to describe the angles involved in the formation of a rainbow. It explains how light enters a droplet, is reflected and refracted, and exits at specific angles that contribute to the maximum angle for light dispersion, which is crucial for the visibility of a rainbow.

The script describes the geometry of a rainbow, explaining how each raindrop acts as a prism, reflecting light back in a cone-shaped pattern. It details how the colors of the rainbow are distributed within these cones, with red on the outside and other colors, including white light, within the cone. The explanation includes the use of a special triangle to visualize the angles at which different colors are maximized.

This paragraph discusses the relationship between the position of the sun and the visibility of rainbows. It explains how the sun's position in the sky affects the angle at which light is refracted and reflected by raindrops, and how this in turn influences the colors and brightness of the rainbow. The explanation includes the concept of the rainbow's axial symmetry and how it appears to an observer.

The script presents a historical and artistic perspective on rainbows, including a painting from 8th century Turkey and a humorous advertisement for a 'rainbow maker.' It also discusses a personal account of creating a rainbow in the backyard, highlighting the scientific principles involved in the process.

The lecture delves into more complex rainbow phenomena, such as secondary bows and supernumerary bands. It explains the formation of secondary bows, which are higher in the sky and have reversed color sequences, and the appearance of supernumerary bands, which occur when water droplets are very fine and diffraction effects become significant.

This paragraph recounts a personal anecdote about observing rainbows at Plum Island and the role of polarization in their visibility. It discusses the Brewster angle and how polarization affects the perception of rainbows, particularly when wearing polarized sunglasses.

The script explores the scattering of light in the atmosphere, explaining why the sky appears blue due to the preferential scattering of shorter wavelength light, such as blue. It contrasts this with sunsets, which appear red due to the scattering of blue and green light out of the direct line of sight from the sun, leaving primarily red wavelengths to reach the observer.

The lecture includes a demonstration using cigarette smoke to illustrate the principles of Rayleigh scattering, which causes the sky to appear blue. It explains how fine particles in the smoke scatter light, creating a blue hue, and how the polarization of this scattered light can be observed with a polarimeter.

This paragraph describes a laboratory demonstration intended to replicate the blue sky and red sunset effect. It involves the use of sodium thiosulfate and sulfuric acid to create particles of varying sizes, which scatter light differently, thus changing the perceived color from blue to red, simulating the colors of the sky at different times of the day.

The script concludes with a final demonstration of a sunset, using the chemical reaction from the previous paragraph to gradually change the color of the light from blue to red, symbolizing the setting sun. The lecture emphasizes the educational joy of understanding and sharing knowledge about natural phenomena.