Light - Reflection and Refraction
This chapter covers the reflection of light by spherical mirrors, refraction of light through lenses, and the laws governing these phenomena, along with the mirror and lens formulas used to solve numerical problems.
1. The angle of incidence is always equal to the angle of reflection. 2. The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.
The mirror formula relates object distance (u), image distance (v), and focal length (f): 1/v + 1/u = 1/f. Magnification (m) = height of image / height of object = -v/u. By sign convention, distances are measured from the pole of the mirror.
Q. Why does a convex mirror always produce a virtual, erect, and diminished image, regardless of object distance?
A convex mirror is diverging in nature, so the reflected rays always appear to diverge from a point behind the mirror, which means the image formed is always virtual, erect, and smaller than the object, no matter where the object is placed.
The refractive index of a medium is a measure of how much it slows down light compared to vacuum. A higher refractive index means light bends more and travels slower in that medium.
Q. Why do we use a convex mirror as a rear-view mirror in vehicles?
A convex mirror always gives an erect, though diminished, image and has a much wider field of view than a plane mirror of the same size, allowing the driver to see a larger area behind the vehicle.
A concave mirror always forms a virtual image when the object is placed:
Correct answer: (A) Between the pole and the focus
The bending of light as it passes from one medium to another is called _______.
Refraction
A concave lens can form a real image under certain conditions.
False — a concave (diverging) lens always forms a virtual, erect and diminished image of a real object, regardless of the object's position, since it always diverges light rays.
An object 4 cm in height is placed 25 cm in front of a concave mirror of focal length 15 cm. Find the distance of the image from the mirror.
Using the mirror formula 1/v + 1/u = 1/f with u = -25 cm and f = -15 cm (sign convention): 1/v = 1/f - 1/u = -1/15 - (-1/25) = -1/15 + 1/25 = (-5 + 3)/75 = -2/75, so v = -37.5 cm. The image forms 37.5 cm in front of the mirror.