Optics

Step 1: Understanding diffraction.
Diffraction is the bending of light around the edges of an obstacle or aperture. When light passes through a small slit or around the edge of an object, it spreads out and forms a pattern. The amount of diffraction increases when the wavelength of the light is comparable to the size of the obstacle or aperture.
Step 2: Phenomenon of diffraction.
This bending around the corners or edges is characteristic of all wave phenomena, including light waves. The diffraction effect is observed when light encounters an obstruction or passes through an opening, and it demonstrates the wave nature of light.
Step 3: Conclusion.
Thus, diffraction is indeed the bending of light around corners or edges of an aperture, making this statement true.

Step 1: Understanding Huygens' wave theory.
Huygens' wave theory explains the propagation of waves through wavefronts. According to this theory, every point on a wavefront can be considered as a source of secondary spherical wavelets, and the new position of the wavefront is the envelope of these secondary waves.
Step 2: Conclusion.
Thus, Huygens' theory primarily explains the propagation of wavefronts, corresponding to option (D).

Step 1: Understanding the behavior of light in different mediums.
The speed of light depends on the medium through which it travels. In a vacuum, the speed of light is the fastest, traveling at approximately . Light travels slower in any medium with a refractive index greater than 1, such as water, glass, or diamond. The refractive index of a medium is a measure of how much light slows down in that medium compared to its speed in a vacuum.
Step 2: Comparing the refractive indices of different substances.

- The refractive index of a vacuum is , meaning light travels at its maximum speed.
- The refractive index of water is approximately .
- For glass, the refractive index is around .
- For diamond, the refractive index is even higher, approximately .
Step 3: Conclusion.
Since light travels fastest in a vacuum, the velocity of light is maximum in a vacuum, which corresponds to option (A).

Step 1: What is diffraction of light?
Diffraction of light refers to the bending or spreading of light waves around obstacles or through small openings. This phenomenon occurs when the wavelength of the light is comparable to the size of the obstacle or aperture. Diffraction causes the light to spread out and form patterns of dark and light regions, called interference patterns.
Step 2: Diffraction at a single slit.
When a monochromatic light source passes through a single narrow slit, the light diffracts and forms a pattern on a screen placed behind the slit. This diffraction pattern consists of a central bright fringe (central maxima) and a series of dimmer fringes on either side (secondary maxima and minima).
Step 3: Condition for minima.
The angular positions of the minima in the diffraction pattern are given by the condition: where:
- is the width of the slit,
- is the angle of diffraction,
- is the wavelength of the light,
- is the order of the minima (for the first minima, , for the second minima, , and so on).
Step 4: Expression for the width of the central maxima.
The central maxima is the region between the first minima on both sides. The angular width of the central maxima is given by: where is the angle of the first minima. Using the condition for minima, we have: For small angles, , so: Thus, the angular width of the central maxima is: Step 5: Conclusion.
The angular width of the central maxima in a single-slit diffraction pattern is , where is the wavelength of the light and is the width of the slit.

Step 1: Formula for focal length of lens combination.
When two thin lenses are in contact, the focal length of the combination is given by the formula: where and are the focal lengths of the individual lenses. Step 2: Determining the focal length.
For two lenses in contact, the total power is the sum of the individual powers: where and .
Step 3: Conclusion.
Thus, the focal length of the combination can be determined from the above formulas. If you know the powers or individual focal lengths of the lenses, you can calculate the total focal length of the combination.

Step 1: Formula for power of combination.
The total power of two lenses in contact is the sum of the individual powers: where (diverging) and (converging). Step 2: Calculating the total power.
The negative sign indicates that the combination is a diverging lens. Step 3: Calculating the focal length.
The focal length is related to the power by: Substituting the value of : Step 4: Conclusion.
Thus, the focal length of the combination is , and the power of the combination is .