Questions from diffraction chapter (optics questions) 

Q1. Why diffraction can’t be observed in our life?

Ans. For a well defined diffraction, the size of the obstacle or aperture should be of the order of the wavelength used. For light, we require very small objects and apertures, which can be obtained in our daily life.

Q2. What is the cause of light streaks one sees while looking at a strong source of light with half shut eyes?

Ans. One can see light streaks while looking at a strong source of light with half shut due to the diffraction of the light at the edges of eyelids.

Q3. If we look at sun through a piece of fine cloth, we observe colored spectra at the sight of holes in the cloth, why?

Ans. If we look at the sun through a piece of cloth, the diffraction of light crossing through the holes in the cloth takes place as the size of the holes is comparable to the wavelength of visible light. Hence we see the colored spectra at the sight of the holes in the cloth.

Q4. What is a half period zone? Why is it so called?

Ans. To explain the phenomenon of diffraction, the exposed part of the wave front is divided symmetrically into zones such that at the observation point, the light waves reaching from the consecutive zones differ in the path by λ/2 (or phase by π). These are called Hpz. These are so called because difference of the phases between two consecutive zones is half of one period (or half of one wavelength). 

Q5. What are the factors on which the amplitude of light waves from hpz at the observation point depends? 

Ans. The amplitude of light waves from a half period zone at the observation point depends upon—
1. Area of the hpz. 
2. Distance of the hpz from observation point.
3. The obliquity (θ) that is the angle between the line joining the hpz at the observation point and the normal to the wave front.

Q6. What are the main classes of diffraction?

Ans. These are two main classes of diffraction—
Fresnel diffraction- 
In this, both the source and the screen are placed at the finite distance from aperture or obstacle and it is general case of diffraction. 

Fraunhoffer diffraction-
In this, the source or the screen or both are at infinite distance from the aperture or obstacle and it is the limiting case of diffraction. 

Q7. What is a zone plate?

Ans. A zone plate is a specially designed screen that obstructs light from the alternate hpzs. This is achieved by cutting off the light either from the odd numbered or even number hpzs. If has multiple foci unlike a convex lens. 

Q8. In case of single slit experiment, what happens when
       1. λ increase  2. The slit width increase 

Ans. when λ increase, the width of the central maximum also increase and if slit width of central ‘b’ increases, the width of the central maximum decrease. 

Q9. Can diffraction occur for virtual images, why?

Ans. No, diffraction can’t occur for virtual images, because for diffraction pattern, we need the actual superposition of light waves.

Q10. Compare interference and diffraction as the superposition of waves?

Ans. In case of interference, superposition of waves occurs for the waves originating from two different coherent source of light. While in case diffraction, the superposition of light waves occur for the waves originating from the different parts of the same wave front. 

Q11. Differentiate between interference and diffraction?

Ans. Interference 
1). It is caused by the superposition of light waves coming from two different coherent sources.
2).The intensity of the light at the minimum is zero.
3).All the interference fringes are of equal width.
4).The intensity of maximum is same.

Diffraction 
1).It is caused by the superposition of light waves coming from the different parts of the same wave front. 
2).The intensity of light at the minimum is not zero.
3).The diffraction fringes are not of equal width.
4).The intensity of the maximum decrease as its order increase.