Coherent Sources - UNSOLVED PRACTICE SET
Chapter: Wave Optics | Topic: Coherent Sources
COHERENT SOURCES - UNSOLVED PRACTICE SET
Topic: Coherent Sources
Multiple Choice Questions
Q1. Two sources of light are said to be coherent if they emit waves that have:
- The same amplitude
- The same wavelength
- A constant phase difference
- The same intensity
Q2. Two independent light sources cannot produce a sustained interference pattern because:
- They emit light of different wavelengths
- They emit light of different amplitudes
- The phase difference between them changes rapidly and randomly
- They emit light in different directions
Q3. Coherent sources can be produced by:
- Using two different laser sources
- Using two different sodium lamps
- Dividing a single wavefront into two parts
- Using two independent white light sources
Q4. The methods of producing coherent sources include:
- Division of wavefront and division of amplitude
- Only division of wavefront
- Only division of amplitude
- Reflection and refraction
Q5. In Young's double slit experiment, the two slits act as coherent sources because:
- They are illuminated by the same monochromatic source
- They have the same width
- They are placed close to each other
- They emit light of the same intensity
Q6. The coherence time of a light source is the time during which:
- The light is emitted continuously
- The phase of the wave remains predictable
- The intensity remains constant
- The wavelength remains unchanged
Short Answer Questions
Q7. Define coherent sources. Why are coherent sources necessary to observe interference?
Q8. Why cannot two independent sodium lamps act as coherent sources?
Q9. Explain the two methods of producing coherent sources: (a) Division of wavefront, (b) Division of amplitude.
Q10. What is the difference between temporal coherence and spatial coherence?
Q11. Why is a laser considered a highly coherent source of light compared to an ordinary light source?
Q12. In Young's double slit experiment, why is a single slit placed before the double slit?
Long Answer Questions
Q13. Explain why two independent light sources cannot produce a sustained interference pattern. Discuss the conditions necessary for two sources to be coherent.
Q14. Describe the methods of producing coherent sources by:
(a) Division of wavefront (with examples)
(b) Division of amplitude (with examples)
Q15. Explain the concept of coherence length and coherence time. How are they related to the monochromaticity of a light source?
Numerical & Application-Based Problems
Q16. A light source emits waves with a wavelength of 600 nm and a spectral line width of 0.01 nm.
(a) Calculate the coherence length of the light.
(b) Calculate the coherence time.
(c) Determine the number of oscillations in one coherence length.
(d) Explain why interference can be observed only within this coherence length.
Q17. Two coherent sources emit light of wavelength 500 nm. The path difference at a point P is 2.5 ฮผm.
(a) Determine whether the point P corresponds to a bright or dark fringe.
(b) Calculate the phase difference between the two waves at point P.
(c) If the path difference is changed to 3.0 ฮผm, what will be the nature of the fringe at P?
Q18. In your school's physics lab, a student is trying to set up an interference experiment.
(a) She uses two different torch lights to illuminate two slits. Explain why she cannot observe interference fringes and what she should do instead.
(b) She then uses a single sodium lamp and a double slit. The wavelength of sodium light is 589 nm. Calculate the minimum path difference required for constructive interference and destructive interference.
(c) The student places a thin glass sheet of refractive index 1.5 and thickness t in front of one slit. She observes that the central bright fringe shifts by 5 fringe widths. Calculate the thickness t of the glass sheet.
(d) A classmate suggests using a laser pointer instead of a sodium lamp. Discuss the advantages of using a laser in terms of coherence and interference pattern quality.
(e) In holography (used in security features on Indian currency notes and credit cards), coherent light is essential. Explain why ordinary light cannot be used to create holograms.