Ch 9 Physics Class 11 Notes

• Define wave front and a ray of light: Ans: Wavefront: The surface on which all the points of waves have same phase of vibration is known as wavefront. It has two types: • Spherical wavefront • Plane wavefront Ray of light: The line normal to the wavefront which shows the direction of propagation of light is called a ray of light.
• State Huygens’s Principle: Ans: Huygens’s Principle: If the location of the wavefront at any instant t is known then Huygen’s principle enables us to determine shape and location of the new wavefront at a later time t + Δt. It has two parts: • Every point of wavefront may be considered as a source of secondary wavelets which spread out in forward direction with a speed equal to the speed of propagation of the wave. • The new position of the wavefront after a certain interval of time can be found by constructing a surface that touches all the secondary wavelets.
• What are the condition for detectable interference?
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• What is meant by coherent source? Give one example.
• Why central spot of Newton’s rings is dark?: Ans: Dark Central Spot: At the point of contact of the lens and the glass plate, the thickness of the film is effectively zero but due to reflection at the lower surface of air film from denser medium, an additional path difference of λ/2 is (phase change of 1800 ) introduced. Consequently, the centre of Newton’s rings is dark due to destructive interference.
• Define Michelson’s experiment. Write its uses.
• What is difference between interference and diffraction?
• What is meant by optical rotation?
• Under what conditions, the two sources of light behave as coherent sources?
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• How is the distance between interference fringes affected by the separation between the slits of Young’s experiment? Can fringe disappear? 
• Can the visible light produce interference fringes? Explain.
• In Young’s experiment, one of the slits is covered with blue filter and other with red filter. What would be the pattern of light intensity on the screen?
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• Explain whether Young’s experiment is an experiment for studying interference or diffraction effects of light.
• An oil film spreading over a wet footpath shows colours? Explain how does it happen?
• Could you obtain Newton’s rings with transmitted light? If yes, would the pattern be different from that obtained with reflected light?
• In white light spectrum obtained with diffraction grating, the third order image of wavelength coincides with fourth order image of second wavelength. Calculate the ratio of the two wavelengths?
• How would you manage to get more orders of spectra using grating?
• Why Polaroid sunglasses are better than ordinary sun glasses?: Ans: Polaroid sunglasses are better than sunglasses. Because: • They reduce the glare of light. • Snow and rough road is partially polarized and produce glare, therefore the glare is reduced by the polaroid sunglasses. • They protect the eyes from harmful and bright rays of sun light.
• How would you distinguish between unpolarized and plane polarized light? 
• Fill in the blanks. Ans: (i) According to Huygens’s principle, each point on a wave front act as a source of secondary wavelets. (ii) In Young’s experiment, the distance between two adjacent bright fringes for violet light smaller than that for green light.