Light Energy

20 cm

The relationship between focal length (f) and radius of curvature (R) is: R = 2 × f. Step 1: Write the formula — R = 2 × f. Step 2: Substitute the value — R = 2 × 10 cm. Step 3: Calculate — R = 20 cm. The radius of curvature is always TWICE the focal length. A common error is to halve the focal length instead of doubling it.

Virtual, erect and diminished

A convex mirror ALWAYS forms a virtual, erect (upright) and diminished (smaller) image, no matter where the object is placed. This is because a convex mirror is a diverging mirror — it spreads reflected rays apart. The image is always formed between the pole and focus, behind the mirror. This property makes it useful as a rear-view mirror in vehicles.

15 cm

The formula connecting focal length and radius of curvature is: f = R/2. Step 1: Write the formula — f = R ÷ 2. Step 2: Substitute — f = 30 cm ÷ 2. Step 3: Calculate — f = 15 cm. The focal length is HALF the radius of curvature. A common mistake is to multiply R by 2 instead of dividing.

It forms an erect and magnified image when the object is close to it

A concave mirror is used as a shaving mirror because when the face (object) is placed between the pole and focus of the mirror, it forms a VIRTUAL, ERECT and MAGNIFIED image. This enlarged image helps in seeing facial features clearly for shaving. If the face were placed beyond the focus, the image would be real and inverted — not useful for shaving!