NEWTON’S RINGS - Q20

20) How does the sodium source, which you are using in your experiment work?

Ans: A sodium vapor lamp is a gas discharge lamp that uses sodium in an excited state to produce light.

NEWTON’S RINGS - Q19

19) What do you understand by (a) fringes of equal thickness (b) fringes of equal inclination and (c) fringes of equal chromatic order?

Ans: 

• Fringes of equal thickness: fringes which are separated by equal distances are called fringes of equal thickness.
• Fringes of equal inclination: a pattern of alternating light and dark bands observed when a transparent layer of constant thickness (a plane-parallel plate) is illuminated with a diverging or converging beam of monochromatic light. Each fringe passes through those points of the layer on which the light rays are incident at the same angle φ—hence the term “fringes of equal inclination.”
• Fringes of equal chromatic order: fringes which have same chromatic properties are called fringes of equal chromatic order.

NEWTON’S RINGS - Q18

18) Is it desirable to measure the radius of curvature of the given lens by a spherometer in the usual way?

Ans: No. Measuring the radius of curvature with a spherometer may lead to some errors. But radius of curvature from newton’s rings experiment is quite accurate.

NEWTON’S RINGS - Q17

17) Why should a lens of large radius of curvature be used in this experiment?

Ans: Lens of large radius of curvature is used so that fringes can be clearly distinguished as diameter of ring increases with increase in radius of curvature.

NEWTON’S RINGS - Q16

16) What would happen if a glass plate is replaced by a plane mirror?

Ans: If glass plate is replaced by a plane mirror, the transmitted rays before will be reflected now. Due to the superposition between reflected and transmitted (now reflected) rays, uniform illumination will result.

NEWTON’S RINGS - Q15

15) On which factors does the diameter of a ring depend?

Ans: Diameter of a ring depends on the wavelength of light used, refractive index of the medium between lens and glass plate, order of the ring and radius of curvature of Planocovex lens.

NEWTON’S RINGS - Q14

14) What is the difference between biprism fringes and Newton’s ring fringes?

Ans: Biprism produces straight fringe systems while Newton’s rings are circular fringe patterns. Two coherent sources are used in biprism setup, but only one source is used in newton’s rings. The distance between successive dark fringes is same in biprism fringes but it is not same in newton’s rings.

NEWTON’S RINGS - Q13

13) What do you expect to see in the microscope if you use a white light source?

Ans: If a white light (polychromatic source) is used, for each wavelength a pattern will be formed. These diameters will depend on the wavelengths. Few rings are clear at centre and blurring occurs at distance away from centre due to overlapping of different patterns.

NEWTON’S RINGS - Q12

12) What would happen to the ring if the space between lens and the plate is filled with a liquid of refractive index μ?

Ans: Diameter of rings will be smaller by factor √μ. Rings will be closer.

NEWTON’S RINGS - Q11

11) Is it possible to determine the refractive index of the liquid by this experiment?

Ans: Yes. From suitable calculations, we can derive a formula for calculating the value of refractive index (μ) of the liquid from radii of rings from air film and from liquid film.

NEWTON’S RINGS - Q10

10) What happens with the central spot when a liquid of refractive index μ greater than that of the lens and less than that of the glass plate is introduced between the lens and the glass plate?

Ans: Since the wavelength of light in a medium is reduced by a factor equal to its refractive index μ, the radius of each ring is reduced by a factor equal to square root of μ. Thus central spot’s radius decreases. And the central spot becomes bright

NEWTON’S RINGS - Q9

9) Instead of reflected rays, if you look at transmitted rays, what do you expect to observe?

Ans: Newton’s rings can also be formed in transmitted light, as shown in figure. In this case there is no phase change because the reflection is not considered. So the two rays are in phase and the central spot is bright. The conditions for the bright and dark rings in the transmitted system are opposite to those in reflected system. But the visibility of fringes is much higher in reflected system than in transmitted system.

NEWTON’S RINGS - Q8

8) How is the central spot in your experiment, bright or dark? Why?

Ans: Central spot is dark. Although at centre, the thickness of air-film is zero, at the point of contact the two interfering rays are opposite in phase and produce zero intensity. This forms a dark fringe at centre.

NEWTON’S RINGS - Q7

7) In place of lens, if a wedge shaped film formed by two glass plates is supplied to you, will you be able to observe Newton’s ring? Why?

Ans: Straight fringes will be formed. The glass plates may be arranged as shown. The two glass plates are inclined at a small angle (This can be achieved by simply separating them by a thin strand of wire at one end as shown). It can be shown that the reflected rays are coherent. And thus they form localized straight fringes are formed in the air film between the two glass plates.

NEWTON’S RINGS - Q6

6) What will happen if a point source or an illuminated slit is used instead of the extended source?

Ans: Only a portion of fringe pattern will be observed.

NEWTON’S RINGS - Q5

5) Why is an extended source used in this experiment?

Ans: To view the whole air film, an extended source is needed.

NEWTON’S RINGS - Q4

4) Are all rings equispaced?

Ans: No. Rings get closer as the order increases (m increases) since the diameter does not increase in the same proportion.

NEWTON’S RINGS - Q3

3) Why are the fringes circular?

Ans: Newton’s rings are loci of constant thickness of the air film and these loci being concentric circles, the fringes are circular.

NEWTON’S RINGS - Q2

2) Where have the fringes formed?

Ans: Fringes are formed between the lens and the plate as the center.

NEWTON’S RINGS - Q1

1) In the Newton’s ring experiment, how does interference occur?
Ans: When a parallel beam of monochromatic light is incident normally on a combination of a Plano-convex lens L and a glass plate G, as shown in below Figure, a part of each incident ray is reflected from the lower surface of the lens, and a part, after refraction through the air film between the lens and the plate, is reflected back from the plate surface. These two reflected rays are coherent. Hence they will interfere and produce a system of alternate dark and bright rings with the point of contact between the lens and the plate as the center.