Exercise Short Question of Physical Optics, Chapter 9 of 1st Year Physics

Q # 1: Under what conditions two or more sources of light behave as coherent sources?

Ans. Two or more sources are said to be coherent sources if they emit identical waves (same frequency and same wavelength) having constant phase difference. For example:

When a single source is splitted into two sources, then these two sources will behave as coherent sources.

Q # 2: How is the distance between interference fringes affected by the separation between the slits of Young's experiment? Can fringes disappear?

Ans. According to the formula of fringe spacing \(\mathrm{\Delta}y = \frac{\lambda L}{d}\), increasing the distance between the slits, the distance between the fringes decreases and vice versa.

If we increase the separation between the slits beyond the limit, then there might be a chance that the interference phenomenon stops occurring and fringes disappear.

Q # 3: Can visible light produce interference fringes? Explain.

Ans.  Visible light consist of seven colors of different wavelengths. Each color forms its own fringes but collectively they overlapped to form no clear fringes pattern.

Q # 4: In the 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?

Ans.  Due to different wavelength, the interference will not take place and we get a mixture of two colors on the screen with almost constant intensity.

Q # 5: Explain whether the young's experiment is an experiment for studying interference or diffraction effects of light?

Ans. In young's double slit experiment, in addition to interference the phenomenon of diffraction also takes place. Due to which the central fringe is always bright. But actually the young's experiment was carried out to study the interference effects of light.

Q # 6: An oil film over a wet foot path shows color. Explain how does it happen?

Ans. The colors are appeared due to interference of light in thin of oil on the water surface.

Q # 7: Could you obtain Newton rings with transmitted light? If yes, would the pattern be different from that obtained with reflected light?

Ans. Yes, it is possible to obtain Newton rings with transmitted light. In this case, we will get a central bright fringe instead of dark.

Q # 8: In the white light spectrum obtained with the diffraction grating, the third order image of a wavelength coincides with forth order image of a second wavelength. Calculate the ratio of two wavelengths.

Ans.  Using the formula, \(d\sin\theta = m\lambda\).

For case 1:  \(d\sin\theta = 3\lambda_{1}\)

For case 2: \(d\sin\theta = 4\lambda_{2}\)

Dividing the two equations, we get:

\[1 = \frac{3\lambda_{1}}{4\lambda_{2}} \Longrightarrow \frac{\lambda_{1}}{\lambda_{2}} = \frac{4}{3}\]

Q # 9: How would you manage to get more orders of spectra using a diffraction grating?

Ans.  Using the formula

\[d\sin\theta = m\lambda\]

\[m = \frac{d\sin\theta}{\lambda}\]

This shows that by increasing the value of grating element ``\(d\)'' or by decreasing the value of wavelength ``\(\lambda\)'' used, we can get more order of spectra.

Q # 10: Why the Polaroid sun-glasses are better than ordinary sun-glasses?

Ans. The Polaroid sunglasses reduce the intensity of light passing through them, due to which the glare of light is decreased. That is why, they are better than ordinary sunglasses.

Q # 11: How would you distinguish between un-polarized and plane-polarized light?

Ans. A Polaroid is placed in the path of light and slowly rotated. If the light become dimmer and dimmer and then vanishes then the incident light was plane polarized. And if the light keeps on coming on the other side then it was un-polarized.