## Chapter Objectives and Summaries

### CHAPTER 15 Light - A wave?

LEARNING OBJECTIVES
Knowledge of subject matter
• Explain the transmission and absorption of light
• Know the speed of light, its measurement and value.
• Know about illumination and intensity and associated formulae.
• Describe the spectrum of visible light.
• Describe the wave nature of light and various properties (a) interference in thin films (b) polarisation of light
• Explain the dual nature of light
• Know the relationship between wavelength and colour
• Know the meaning of the following terms: interference, diffraction path difference, nodal lines, reinforcement, coherence, phase, monochromatic.
• Describe Young's double slit interference and its measurement.
• Explain single slit diffraction
• Describe the diffraction grating
• Know the effects of interference with thin films
• Describe interference with wedge shaped films
• Explain the intensity patterns of interference patterns
• Use the following formulae in simple situations:
(a)For points on a maxima: d sinø = dx/ÿ = nÿ
(b)For points on a minima: d sinø = dx/ÿ = (n- ˝ )ÿ
(c)For single slit: n = xd/ÿ for nodals
(d)Path difference = dx/L

SCIENTIFIC PROCESSES

• Predict and deduce the direction of light through a block of glass and other materials.
• Identify and explain physical situations involving interference.
• Infer why diffraction of light is difficult to see.

COMPLEX REASONING PROCESSES

• Deduce the wavelength of light from data obtained from observating the interference pattern using single and double slits.
• Analyse data obtained from path differences in interference patterns from coherent sources.
• Analyse the nature of the interference pattern produced by thin films and wedges.

### CHAPTER 15 SUMMARY

• Light is a form of electromagnetic energy that does not require a medium for its propagation.
• Light can be shown to exhibit those properties that are characteristic of waves - in particular diffraction and interference.
• Diffraction fringes can be produced by shining light through a single slit onto a screen.
• Thomas Young demonstrated the wave properties of light by his double slit interference experiment. He produced light and dark interference bands on a screen when light was incident on a pair of slits.
• Monochromatic light is light of one colour or wavelength.
• Polychromatic light is light of more than one colour or wavelength.
• For double slits the relationship between the position of the fringes on a screen, the distance between the slits and the wavelength of the light is given by: Sin q = n.l/d = x/L ....for constructive interference, and
Sin q = (n-1/2).l/d = x/L .... for destructive interference.
• Franciso Grimaldi demonstrated diffraction of light in the 17th century.
• Augustine Fresnel explained the diffraction of light by the use of `Fresnel Zones'.
• For single slit diffraction the relationship between the position of the fringes on a screen, the width of the slit, and the wavelength of the light used is given by:
Sin q = (n+1/2)l/w = y/L ... for light bands,
and Sin q = (n)l/w = y/L ...for dark bands.
• The resolving power of an optical instrument is its ability to separate two closely positioned objects when they are viewed through a lens.
• Diffraction gratings consist of block of glass consisting of many very closely spaced opaque lines. They therefore act as many double slits.
• The colours produced by light reflecting from thin films can be explained by using interference principles. In particular; path difference between two rays, and phase change on reflection.
• The wavelength of light in a medium can be found by the formula
lfilm = lair / nfilm
Where: lfilm = the wavelength of the light in the film, lair = the wavelength of the light in air, and nfilm = the absolute refractive index of light going from air to the film.
• Light reflected from wedges produces interference patterns that consist of equally spaced dark and light bands.
• Light is an electromagnetic wave consisting of propagating electric and magnetic fields. This was suggested by James Clerk Maxwell and demonstrated by Heinrich Hertz.
• The electromagnetic spectrum consists of: radio waves, microwaves, infra-red waves, visible light, ultra violet waves, X rays, and gamma rays.
• Radio waves are used for broadcasting and communication; microwaves are used for communication, radar and in microwave ovens; Infra-red light have medical, military, and many commercial uses; ultraviolet light can be used to detect flaws etc. in matter; X rays have many medical and commercial uses; Gamma rays are used in nuclear medicine.
• Polarisation is the ability to block some of the components of the electric and magnetic fields of electromagnetic radiations in particular - visible light. A polariser is a device that is able to do this. Return to Objectives-Summary Menu Page page.