Refraction Of Light

or

Sun is visible a little before the actual sunrise and until a little after the actual sunset. This is due to refraction.

Step 1: Understanding the Components of a Light Microscope
In a light microscope, the diaphragm is the component responsible for regulating the amount of light that passes through the specimen and enters the viewing area.
Step 2: Explanation of Other Options
Option (a) is incorrect because the coarse adjustment screw is used for focusing the image, not regulating the light.

Option (b) is incorrect because the fine adjustment screw is used for fine focusing the image, not regulating light.

Option (d) is incorrect because the condenser lens focuses light onto the specimen but does not regulate the light intensity.
Step 3: Conclusion
Thus, the diaphragm is responsible for regulating the amount of light entering the viewing area of the microscope.

Step 1: Condition for total internal reflection.
Total internal reflection occurs when light passes from a denser medium to a rarer medium, and the angle of incidence in the denser medium exceeds the critical angle.

Step 2: Critical angle.
The critical angle is the angle of incidence beyond which the light cannot pass into the rarer medium but is entirely reflected back into the denser medium.