Light: Mirrors and Lenses

Light and how we see

Light travels approximately in straight lines through a uniform transparent medium. We see an object when light emitted or reflected from it enters our eyes. Opaque objects block transmission, translucent materials scatter or transmit some light, and transparent materials allow clearer transmission. Never look directly at the Sun or intense laser beams.

Reflection of light

Reflection occurs when light returns into the original medium after striking a surface. The incident ray approaches, the reflected ray leaves, and the normal is perpendicular to the surface at the point of incidence. The angle of incidence equals the angle of reflection, measured from the normal—not the mirror surface. Incident ray, reflected ray and normal lie in one plane.

Regular and diffused reflection

A smooth mirror produces regular reflection, maintaining ordered ray directions and clear images. Rough paper scatters reflected rays in different directions and usually does not form a clear image. Each individual ray still obeys the laws of reflection. A surface can look smooth to the eye yet be microscopically rough.

Plane mirrors and images

A plane mirror forms an upright, virtual image of the same size as the object, appearing the same perpendicular distance behind the mirror as the object is in front. The image cannot be projected on a screen. Lateral inversion is the apparent reversal of front–back direction that makes left/right labels appear swapped when we compare a person with their mirror image.

Spherical mirrors

A concave mirror curves inward on the reflecting side and can converge parallel incoming rays. It may produce a magnified upright virtual image for a nearby object, or a real inverted image for an object beyond its focal point. A convex mirror bulges outward and generally produces upright, diminished virtual images, useful for wide-angle rear-view and security mirrors. Image properties depend on object position.

Focus and focal length of mirrors

The principal axis passes through the pole and centre of curvature of a spherical mirror. The principal focus is the point where paraxial parallel rays meet after concave reflection or appear to diverge from after convex reflection. Focal length is distance between pole and focus. These are geometric optics models; curved mirrors show aberrations for rays far from the axis.

Refraction of light

Refraction is a change in direction when light crosses between media with different refractive indices at an angle. A pencil partly immersed in water may appear bent. At normal incidence, direction need not change, although speed does. Refraction does not mean the pencil itself bends.

Convex and concave lenses

A convex lens is thicker near the centre and generally converges parallel rays; a concave lens is thinner centrally and generally diverges them. A convex lens can form real inverted images when the object is outside the focal length and an upright magnified virtual image when inside it. A concave lens generally forms an upright diminished virtual image for real objects.

Uses of mirrors and lenses

Plane mirrors are used for dressing and periscopes; concave mirrors in shaving or dental examination and some reflectors; convex mirrors for wider fields of view; convex lenses in magnifiers and cameras; concave lenses in some spectacles. The appropriate optical design depends on intended field of view, image orientation, focus and safety.

Image formation comparison

Terminology and formulas

Safe observations and ray diagrams

Use a plane mirror and low-intensity torch to mark incident and reflected rays on paper. Draw the normal with a ruler and protractor. Place an object in front of a plane mirror and compare its image distance using a grid. Compare convex and concave mirrors with a spoon, noting that spoon surfaces are imperfect models. Observe an immersed pencil from the side. Under teacher supervision, use an LED ray box and approved convex/concave lenses; never focus sunlight onto skin, paper or eyes. Record real observations rather than inventing results.

Common errors and revision

Angles in reflection are measured from the normal. A convex mirror does not form an enlarged real image of an ordinary object. A convex lens does not always produce an inverted image: a nearby object within its focal length gives an upright virtual image. Real images can generally be projected onto a screen; virtual images cannot be caught directly on a screen. The laws of reflection apply to rough surfaces locally too.

Original practice questions with full explanations and answers

These are original competency-building questions. They are not presented as a verbatim or verified NCERT exercise set.

MCQs, HOTS and exam revision

MCQ: Which mirror offers a wide field of view: convex or concave? Convex. MCQ: If incidence angle is 35°, what is reflection angle? 35°. HOTS: Why can a convex lens act as a magnifier but also project a real image? Image type changes with object position relative to focal length. Exam trap: Measure angles from the normal, never the mirror surface.

Figure and curriculum audit note

Official textbook reference: NCERT Curiosity Chapter 10 PDF. The official source could not be fetched reliably during this session. Original textbook diagrams, numbered activities and exercise-by-exercise solutions need separate verification. The ray descriptions here are schematic and do not substitute for precise labelled textbook figures.