Light rays that are parallel to the principal axis of a concave mirror converge at a specific point on its principal axis after reflecting from the mirror. This point is known as the principal focus of the concave mirror.

Radius of curvature, R= 20 cm
Radius of curvature of a spherical mirror = 2 × Focal length (f)R= 2f
f= R/2 = 20 / 2 = 10
Hence, the focal length of the given spherical mirror is 10 cm.

Concave Mirror.

We prefer a convex mirror as a rear-view mirror in vehicles because it gives a wider field of view, which allows the driver to see most of the traffic behind him. Convex mirrors always form a virtual, erect, and diminished image of the objects placed in front of it.

Radius of curvature, R= 32 cm
Radius of curvature = 2 × Focal length (f)
R= 2ff= R/2 = 32/2 = 16
Hence, the focal length of the given convex mirror is 16 cm.

Magnification produced by a spherical mirror is given by the relation,



Let the height of the object, ho= h

Then, height of the image, hI= - 3h (Image formed is real)



Object distance, u= - 10 cm

v= 3 � ( - 10) = - 30 cm

Here, the negative sign indicates that an inverted image is formed at a distance of
30 cm in front of the given concave mirror.

The ray of light bends towards the normal. When a ray of light enters from an optically rarer medium (having low refractive index) to an optically denser medium (having high refractive index), its speed slows down and it bends towards the normal. Since water is optically denser than air, a ray of light entering from air into water will bend towards the normal.

Refractive index of a medium, nm = Speed of light in vacuum/Speed of light in the medium
Speed of light in vacuum, c = 3 × 10⁸ ms^-1
Refractive index of glass, ng = 1.50
Speed of light in the glass, v = Speed of light in vacuum/ Refractive index of glass
= c/ng
=3 × 10⁸/1.50 = 2 x 10⁸ ms^-1.