Power

Solution:
To find the value of , we calculate the power gain using the relation: Step 1: Calculate Current Gain ( )
From the given graph,
For and : Step 2: Calculate Power Gain
Substitute , , and into the power gain formula: Step 3: Find
Since the power gain is :

The question requires us to find the percentage decrease in illumination of a lamp when the current decreases by 20%. The key concept to use here is that illumination (intensity of light) is proportional to the square of the current flowing through the lamp. This is based on the relationship given by the formula for power in terms of electric current:

Here, represents power, and represents current. Since illumination is related to the power of the lamp, we use this concept to find the change in illumination.

1. If the original current is , the initial illumination can be represented as proportional to .
2. Given that the current decreases by 20%, the new current is:
3. The new illumination is proportional to:
4. The percentage change in illumination can be calculated as:

Thus, the illumination of the lamp decreases by 36%.

Therefore, the correct answer is .