For diffraction to be observed, the main condition is

Options

A. the size of an obstacle should be of the same order as the wavelength
B. the size of an obstacle should be very large as compared to the wavelength of light used
C. the size of an obstacle should be exactly `lambda//2`
D. there should be no relation between the wavelength and the size of the obstacle

Answer

To determine the main condition for diffraction to be observed, we can analyze the relationship between the wavelength of light and the size of the obstacle. Here’s a step-by-step solution:

### Step 1: Understand the Concept of Diffraction
Diffraction refers to the bending of waves around obstacles or the spreading of waves when they pass through small openings. It is a phenomenon that occurs with all types of waves, including light waves.

### Step 2: Identify the Key Factors
For diffraction to be noticeable, the size of the obstacle or aperture must be comparable to the wavelength of the light being used. This means that the wavelength (λ) should be of the same order of magnitude as the size of the obstacle (d).

### Step 3: Analyze the Options
1. **Size of the obstacle should be of the same order as the wavelength**: This is a strong candidate because if the size of the obstacle is similar to the wavelength, diffraction effects will be significant.
2. **Size of the obstacle should be very large compared to the wavelength**: This would not lead to noticeable diffraction, as the light would mostly travel in straight lines.
3. **Size of the object should be exactly λ/2**: While this may allow for some diffraction effects, it is not a general condition for diffraction to occur.
4. **There should be no relationship between the wavelength**: This is incorrect as diffraction is fundamentally related to the wavelength of the light.

### Step 4: Conclusion
The correct condition for diffraction to be observed is that the size of the obstacle should be of the same order as the wavelength of light. Therefore, the answer is:

**The size of the obstacle should be of the same order as the wavelength of light.**