Question

A parallel beam of white light is reflected from a thin wedge-shaped film. The colour of the fringe at the edge of the wedge will be 

Solution

Correct option is

Black

Interference in films due to the superposition of waves reflected from the top and the bottom of the film. If the path difference between the two waves satisfies the condition ∆ = mλ, we have bright fringes. The path difference at any point on the film depends upon the film thickness at that point, which is zero at the edge of the wedge. However there is an additional path difference of λ/2 due to a phase change of π at reflection from the upper face of the film. Thus, at the edge of the film, the path difference is λ/2. Hence the fringe at the edge of the film is dark. Thus the fringe colour is Black.

SIMILAR QUESTIONS

Q1

How will the image formed by a convex lens be affected if the central portion of the lens is wrapped in black paper, as shown in fig.

                                                                            

Q2

The equi-convex lens, shown in the figure, has a focal length f. What will be the focal length of each half if the lens it cut along AB? (See fig.)

                                                                   

Q3

A person can see clearly only up to a distance of 30 cm. He wants to read a book placed at a distance of 50 cm from his eyes. What is the power of the lens he requires for his spectacles?   

Q4

A person with defective eyesight is unable to see objects clearly nearer than 1.5 m. He wants to read a book placed at a distance of 30 cm from his eyes. Find the power of the lens he requires for his spectacles.   

Q5

An object, a convex lens of focal length 20 cm and a plane mirror are arranged as shown in fig. How far behind the mirror is the final image formed? 

                                                         

Q6

A small plane mirror is rotating at a constant frequency of n rotations per second. With what linear velocity (in ms–1) will a light spot move along a spherical screen of radius of curvature of R metres if the mirror is at the centre of curvature of the screen?   

Q7

A ray of light incident normally on face AB of an isosceles prism travels as shown in fig. The least value of the refractive index the prism must have should be    

                                                                        

Q8

A beaker is filled with water to a height of 8.00 cm. The apparent depth of a needle fixed at the bottom of the beaker as measured by a travelling microscope is found to be 6.00 cm. If the height of water in the beaker is increased to 12.0 cm, by what distance would the microscope have to be moved to focus on the needle again?       

Q9

When a thin wedge-shaped film is illuminated by a parallel beam of light of wavelength 6000 Ã…, 7 fringes are observed in a certain region of the film. How many fringes will be observed in the same region of the film if light of wavelength 4200 Ã… is used?

Q10

When a ray of light is incident on a glass slab at an angle of 60o, the angle between the reflected and refracted rays is 90o. The refractive index of glass is