Chapter #9 :Nature of Light ( Physical Optics) : Short Q/A / C.R.Q's

Q.Why the central spot in the Newton’s rings is dark?

Ans. The Newton’s rings are formed due to the phenomenon of thin film interference. here, the condition for constructive interference(the ring appearing bright) is that the optical path difference between interfering waves should be an integral multiple of the wavelength. the optical path difference is given by 2t-(l/2) if t is the thickness of the air film at that point and l is the wavelength of light. At the central point, the lens touches the surface so thickness t=0. thus the optical path difference is simply l/2, which is the condition for destructive interference, not constructive interference. so the central spot has to always be dark.

Q. Why the dark and bright fringes of Newton’s experiment are circular?

Ans. The Newton’s rings are formed due to the phenomenon of thin film interference. here, the condition for constructive interference(the ring appearing bright) is that the optical path difference between interfering waves should be an integral multiple of the wavelength. As the Plano convex lens is used in Newton’s rings so the thickness of the film is increasing and then decreasing that’s why the fringes are circular.

Q.24 Why the conditions of constructive and destructive interference are reversed in thin films?

Ans. Reflected light will experience a 180 degree phase change when it reflects from a medium of higher index of refraction and no phase change when it reflects from a medium of smaller index. This phase change is important in the interference which occurs in thin films, the design of anti-reflection coatings, interference filters, and thin film mirrors. So in thing film the phase change of 180 degrees occur that’s why the crests converts into troughs and troughs are converted into crests. therefore the conditions are reversed.

Q. An oil film over a wet footpath shows colors? Explain how does it happen?

Ans.This is known as thin-film interference, because it is the interference of light waves reflecting off the top surface of a film with the waves reflecting from the bottom surface. To obtain a nice colored pattern, the thickness of the film has to be on the order of the wavelength of light. Consider the case of a thin film of oil floating on water. Thin-film interference can take place if these two light waves interfere constructively:

1. the light from the air reflecting off the top surface
2. the light traveling from the air, through the oil, reflecting off the bottom surface, traveling back through the oil and out into the air again.

Q. Why Polaroid sun glasses are better than the ordinary sun glasses?

Ans. There are four things that a good pair of Polaroid sunglasses should do for you:


Sunglasses provide protection from ultraviolet rays in sunlight. Ultraviolet (UV) light damages the cornea and the retina. Good sunglasses can eliminate UV rays completely.

Sunglasses provide protection from intense light. When the eye receives too much light, it naturally closes the iris. Once it has closed the iris as far as it can, the next step is squinting. If there is still too much light, as there can be when sunlight is reflecting off of snow, the result is damage to the retina. Good sunglasses can block light entering the eyes by as much as 97 percent to avoid damage.

Sunglasses provide protection from glare. Certain surfaces, such as water, can reflect a great deal of light, and the bright spots can be distracting or can hide objects. Good sunglasses can completely eliminate this kind of glare using polarization (we'll discuss polarization later).

Sunglasses eliminate specific frequencies of light. Certain frequencies of light can blur vision, and others can enhance contrast. Choosing the right color for your sunglasses lets them work better in specific situations.

Q. Can interference be without diffraction or vise versa?

Ans. You can  have diffraction without interference. Interference occurs when coherent light waves coming from two different sources interact. In single-slit diffraction, the two sides of the slit act as these two sources. If you make the slit much smaller than the wavelength of whatever you're diffracting, though, it effectively becomes a single point source, and no appreciable interference occurs and interference in the thin films is without diffraction.