Class Notes (810,421)
Canada (494,121)
Physics (299)
PHYS 101 (41)
Lecture

# Chapter 24 - Light.docx

4 Pages
103 Views

School
McGill University
Department
Physics
Course
PHYS 101
Professor
Kenneth Ragan
Semester
Fall

Description
Chapter 24 – Light and its Phenomena 24.1 – Waves Vs. Particles; Huygens’ principle and Diffraction: Light clearly carries energy, two ways this is done: particles (that is, physically carrying mass) and waves (the medium doesn’t carry, but energy is carried by the medium) Newton favored “particle/corpuscular” theory of light, taking as evidence the sharpness of shadows (as compared to the way sound waves bend around corners).  But experiments, concerning interference seemed to indicate that light was a wave.  It is said to be an electromagnetic wave, therefore it does not require a medium at all  In the 1900s, it became apparent that it had particle properties too, photos as “particles of light” Huygen’s Principle: a wave theory, which states that each point on a wave front can be considered as a new source of wavelets (starting point of new waves). The new wave front is the envelope of all the wavelets.  Useful for analyzing what happens when waves impinge an obstacle and the wave front is interrupted, also predicts that waves bend in behind the obstacle  Diffraction: the bending of waves around obstacles (occurs for waves, but not particles) The law of diffraction says that an “edge” the (angular) bending of the wave will be: 𝜃𝑑𝑖𝑓𝑓 = 𝜆/𝐷 This means a wave only casts a “shadow” if its size D is greater than the wavelength of the wave.  Diffraction, like refraction is a wave phenomenon. o It does occur for light, but the angles are so small that we do not normally perceive it: shadows appear sharp unless we look very closely o So Newton was wrong, and light appears to be a wave o The wave model of light accounts for diffraction, how ray models do not When light travels from one medium to another, its frequency does not change, but the wavelength does. 𝜆 𝜆𝑛 = 𝑛 Chapter 24 – Light and its Phenomena 24.3 – Interference: Young’s Double-Slit Experiment 1. Light from a single source falls on a screen containing two closely space slits 1 and S2 2. If light consisted of particles, one would see two bring lines on the screen behind the slit. 3. But instead a series of lines are observed 4. This is due to diffraction, the waves leaving the two small slits spread out  Constructive Interference: (bright area at the center of the screen) the waves from the two slits travel the same distance, so they are in phase: a crest of one wave arrives at the same time as a crest of the other wave. Hence the amplitudes of the two waves add to form a larger amplitude o Also occurs when the paths of the two rays differ by one wavelength (or any whole number of wavelengths) 𝑑sin𝜃 = 𝑚𝜆  Destructive Interference: (screen is dark) if one ray travels and extra distance of ½ wavelength, the two waves are exactly out of phase when they reach the screen: the crests of one wave arrive at the same time as the troughs of the other wave and so they add to produce a zero amplitude 1 𝑑sin𝜃 = 𝑚 + 𝜆 2 The bright fringes are beaks or maxima of light intensity, while the dark fringes are minima. The intensity of the bright fringes is greatest for the central fringe (m=0) and decreases for higher orders. Fringe spacing depends on wavelength, with longer wavelengths having larger spacing. So what happens if you put a mix of wavelengths onto the slit?  The zeroth order fringe (n=0) has no wavelength dependence, so it will be white light  But all higher order fringes (n>0) will be
More Less

Related notes for PHYS 101

Log In

OR

Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view

OR

By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.

Request Course
Submit