Question

A point on the principal axis of a spherical mirror where rays parallel to the principal axis after reflection meet or appear to meet is known as:

• A. centre of curvature
• B. principal focus
• C. pole
• D. aperture

Hint:

Concave mirror \(\to\) Real focus (rays physically meet).
Convex mirror \(\to\) Virtual focus (rays appear to meet when traced back).
Both points are called the principal focus (\(F\)).

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the standard optical term for the specific point on the principal axis of a spherical mirror where parallel incident rays converge or from which they seem to diverge after reflecting off the mirror.

Step 2: Key Formula or Approach:
Identify the definition of each term listed in the options:
- Centre of Curvature: The center of the hollow glass sphere of which the mirror is a part.
- Pole: The geometric center of the reflecting surface of the spherical mirror.
- Aperture: The effective diameter of the reflecting surface.
- Principal Focus: The point where parallel rays actually meet (concave) or appear to diverge from (convex).

Step 3: Detailed Explanation:

• When rays of light coming from infinity (which are parallel to the principal axis) strike a spherical mirror, they undergo reflection according to the laws of reflection.

• In a concave mirror, these reflected rays actually intersect at a specific point on the principal axis.

• In a convex mirror, the reflected rays diverge, but when projected backward, they appear to originate from a specific point on the principal axis behind the mirror.

• This common point (actual intersection or virtual origin) is defined as the principal focus of the mirror.


Step 4: Final Answer:
The point is known as the principal focus.