Class 8 Science · Chapter 10 NotesLight: Mirrors and Lenses

A spherical mirror is a curved mirror whose reflecting surface is part of a sphere. If the reflecting surface curves inwards, it is a concave mirror; if it curves outwards, it is a convex mirror. You can identify them by looking at their side view: concave curves in, convex bulges out. A shiny spoon acts like a spherical mirror — its inner curve gives an inverted image, while the outer curve gives an erect, smaller image. Concave mirrors are used in torches, headlights, and dental mirrors because they can converge light and enlarge images when the object is close. Convex mirrors are used as side-view mirrors and safety mirrors because they provide a wider field of view and always form erect, diminished images.

• Concave mirrorreflecting surface curves inwards.
• Convex mirrorreflecting surface curves outwards.
• Concave mirrors converge light; convex mirrors diverge light.

The image formed by a concave mirror depends on the distance of the object from the mirror. When the object is close, the image is erect and enlarged. As the object moves farther away, the image becomes inverted; initially enlarged, then it shrinks. In contrast, a convex mirror always forms an erect, diminished image, and the image size decreases slightly as the object moves away. Unlike plane mirrors, which always give an erect image of the same size, spherical mirrors can produce images of different sizes and orientations. Lateral inversion is observed in all three types of mirrors.

• Concave mirrorimage can be enlarged, diminished, erect, or inverted depending on object distance.
• Convex mirrorimage is always erect and diminished.
• Plane mirrorimage is always erect and same size as object.

The laws of reflection describe how light behaves when it bounces off a surface. The first law states that the angle of incidence is equal to the angle of reflection. The second law states that the incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane. These laws hold for all types of mirrors — plane, concave, and convex. When light falls along the normal, the angle of incidence is 0° and the angle of reflection is also 0°, meaning the ray reflects back along the same path. These laws help us understand image formation and predict the path of reflected light.

• Angle of incidence (i) = angle of reflection (r).
• Incident ray, normal, and reflected ray lie in the same plane.
• Laws apply to plane, concave, and convex mirrors.

When parallel rays of light strike a plane mirror, they remain parallel after reflection. However, a concave mirror causes parallel rays to converge to a point, while a convex mirror causes them to diverge. This happens because each ray still obeys the laws of reflection, but the curved surface changes the overall direction of the rays. The converging property of a concave mirror is used in solar concentrators to focus sunlight and generate heat, which can even burn paper. Convex mirrors spread light out, which is why they are useful for wide-angle viewing.

• Plane mirrorparallel rays stay parallel after reflection.
• Concave mirrorconverges parallel rays.
• Convex mirrordiverges parallel rays.

A lens is a transparent material with curved surfaces that refracts light. A convex lens is thicker in the middle and thinner at the edges; it converges light and is called a converging lens. A concave lens is thinner in the middle and thicker at the edges; it diverges light and is called a diverging lens. When you look through a convex lens at an object close by, the object appears erect and enlarged. As you move the object farther away, the image becomes inverted, first enlarged and then diminished. A concave lens always produces an erect, diminished image, regardless of the object's distance.

• Convex lensthicker in middle, converges light.
• Concave lensthinner in middle, diverges light.
• Convex lens can form enlarged or diminished, erect or inverted images.
• Concave lens always forms erect, diminished images.

Lenses are everywhere: eyeglasses correct vision, cameras and telescopes use lenses to focus light, and magnifying glasses make small print readable. The human eye contains a convex lens that changes shape to focus on near and far objects. Concave mirrors are used in torches, headlights, and dental mirrors, while convex mirrors serve as side-view mirrors, road safety mirrors, and surveillance mirrors in stores. Solar cookers use mirrors to concentrate sunlight for heating and cooking. Understanding these applications helps us appreciate the role of light in technology and daily life.

• Eyeglasses, cameras, telescopes, microscopes use lenses.
• Concave mirrorstorches, headlights, dental mirrors.
• Convex mirrorsvehicle side mirrors, road safety, surveillance.