What is the function of the rheostat on a microscope?

• A. Controls light source intensity
• B. Adjusts the stage position
• C. Focusing knob
• D. Changes objective magnification

Answer: (A)

Controlling how bright the illumination is a microscope used to rely on for getting a clear, high-contrast image. The rheostat works as a variable resistor in the lamp circuit, adjusting the current to the light source. Turning it up increases brightness so more light reaches the slide, while turning it down dims the illumination. This directly affects visibility and contrast without changing where the stage is, how sharp the image is, or which objective lens is used. The stage position is moved with the stage controls, the focusing knob changes the distance between the lens and the slide to bring the specimen into sharp focus, and magnification is altered by swapping objective lenses.