Interference Of Light Waves And Young S Experiment

Step 1: Working of an LED.
An LED (Light Emitting Diode) emits light when current flows through it, and electrons recombine with holes in the semiconductor material, releasing energy in the form of photons (light). This process occurs in the forward bias condition.
Step 2: Analyzing the options.

- (1) junction is reverse biased: Incorrect. LEDs emit light when forward biased, not when reverse biased.
- (2) depletion region widens: Incorrect. The depletion region would widen under reverse bias, but this does not cause light emission.
- (3) holes and electrons recombine: Correct. This is the process in which light is emitted in an LED.
- (4) junction becomes hot: Incorrect. While current can cause heating, it’s not the primary reason LEDs emit light.

Step 3: Conclusion.
The LED emits visible light when holes and electrons recombine, corresponding to option (3).

Step 1: Properties of light.
When light travels from one medium to another, its frequency remains unchanged, but its velocity and wavelength may change depending on the properties of the mediums.
Step 2: Analyzing the options.

- (1) velocity: Incorrect. Velocity changes when light moves from one medium to another.
- (2) wavelength: Incorrect. Wavelength changes depending on the refractive index of the medium.
- (3) amplitude: Incorrect. The amplitude can change due to the intensity of the light.
- (4) frequency: Correct. The frequency of light remains constant when it transitions between different media.

Step 3: Conclusion.
The property of light that remains unchanged when it travels from one medium to another is the frequency, corresponding to option (4).

Step 1: Huygens' Principle.
According to Huygens' principle, every point on a wavefront acts as a source of secondary wavelets that spread out in all directions. The new wavefront is the surface tangent to all the secondary wavelets.
Step 2: Reflection of Light.
Consider a wavefront of light incident on a reflecting surface. Let be the point on the wavefront that hits the surface at an angle . At the point of incidence, secondary wavelets are generated, and the reflected wavefront is formed by the tangent to these wavelets.
Step 3: Geometry of Reflection.
By applying the principle of reflection at the point of incidence, the angle of incidence is equal to the angle of reflection :
Step 4: Laws of Reflection.

- The incident ray, the reflected ray, and the normal to the surface all lie in the same plane.
- The angle of incidence is equal to the angle of reflection.
Step 5: Conclusion.
Huygens' principle explains the laws of reflection by showing that each point on the incident wavefront acts as a secondary source, leading to the reflection of light with the same angle of incidence and reflection.

Step 1: Formula for fringe width.
In the biprism experiment, the fringe width is given by: where is the wavelength of light, is the distance between the screen and the source, and is the separation between the slits.
Step 2: Given Data.
From the given data:
- The distance between the central bright band and the 20th bright band is 0.4 cm, which corresponds to 20 fringe widths. Thus, the fringe width is:
- The distance between the two virtual magnified images is 0.9 cm. Using the formula for magnification: where is the image distance and is the object distance, the magnification is .
Step 3: Calculate Wavelength.
Using the relation for fringe width and magnification, we can calculate the wavelength: Substituting the values: