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Q.- 1. Explanation of the Power of a Lens
The ability of a lens to bend (converge or diverge) light rays is referred to as its power or capacity.
- Definition: The power of a lens is equal to the inverse of its focal length ($f$).
- Formula:
$$P = \frac{1}{f \text{ (in meters)}}$$ - Unit: Its SI unit is the Dioptre, which is denoted by the symbol ‘D’.
- If the focal length of a lens is $1$ meter, its power will be $1 \text{ D}$.
Key Points:
- Convex Lens: Its power is always positive (+).
- Concave Lens: Its power is always negative (-).
- Other Options:
- Meter: The unit of distance.
- Watt: The unit of power (electrical or mechanical).
- Joule: The unit of energy or work.
Q. 2 How Ciliary Muscles Change Focal Length
The ability of the eye to adjust its focal length to see objects at different distances is called Accommodation. This process is managed entirely by the Ciliary Muscles.