Class 10 Science · Chapter 9 NotesLight – Reflection and Refraction

When light falls on a polished surface like a mirror, it bounces back. This phenomenon is called reflection. The laws of reflection state that the angle of incidence is equal to the angle of reflection, and 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 reflecting surfaces, including curved ones. A plane mirror forms a virtual, erect image of the same size as the object, located as far behind the mirror as the object is in front. The image is also laterally inverted, meaning left and right are swapped.

• Angle of incidence (i) = Angle of reflection (r)
• Incident ray, normal, and reflected ray are coplanar
• Plane mirror imagevirtual, erect, same size, laterally inverted

A spherical mirror is a part of a sphere whose reflecting surface is curved. If the reflecting surface curves inwards, it is a concave mirror; if it curves outwards, it is a convex mirror. Important terms include the pole (P), the centre of curvature (C), the radius of curvature (R), the principal axis, the principal focus (F), and the focal length (f). For mirrors with small apertures, the radius of curvature is twice the focal length: R = 2f. Concave mirrors can form real or virtual images depending on the object's position, while convex mirrors always form virtual, erect, diminished images.

• Concave mirrorconverging, reflecting surface curved inwards
• Convex mirrordiverging, reflecting surface curved outwards
• R = 2f for spherical mirrors of small aperture
• Convex mirror always gives virtual, erect, diminished image

The nature, position, and size of the image formed by a spherical mirror depend on the object's position relative to the pole, focus, and centre of curvature. For a concave mirror, when the object is beyond C, the image is real, inverted, and diminished; at C, it is real, inverted, and of the same size; between C and F, it is real, inverted, and enlarged; at F, no image is formed; and between P and F, the image is virtual, erect, and enlarged. For a convex mirror, the image is always virtual, erect, and diminished, regardless of the object's position. Ray diagrams use two rays: one parallel to the principal axis and one through the focus or centre of curvature.

• Concave mirrorobject beyond C → image between F and C, real, inverted, diminished
• Concave mirrorobject between P and F → image behind mirror, virtual, erect, enlarged
• Convex mirrorimage always between P and F, virtual, erect, diminished

The mirror formula relates the object distance (u), image distance (v), and focal length (f): 1/v + 1/u = 1/f. It is valid for all spherical mirrors and all object positions. The New Cartesian Sign Convention must be used: distances measured to the right of the pole are positive, to the left are negative; heights above the principal axis are positive, below are negative. Magnification (m) is the ratio of the image height (h′) to the object height (h): m = h′/h = –v/u. A negative magnification indicates a real image, while a positive magnification indicates a virtual image.

• Mirror formula1/v + 1/u = 1/f
• Magnificationm = h′/h = –v/u
• Sign conventiondistances to the left of pole are negative, to the right positive

Refraction is the change in direction of light when it travels obliquely from one transparent medium to another. This happens because the speed of light differs in different media. When light goes from a rarer to a denser medium, it bends towards the normal; from denser to rarer, it bends away from the normal. The laws of refraction state that the incident ray, refracted ray, and normal all lie in the same plane, and the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media and a given colour of light. This is Snell's law: sin i / sin r = constant.

• Refractionbending of light due to change in speed
• Snell's lawsin i / sin r = constant
• Rarer to denserbends towards normal; denser to rarer: bends away from normal

The constant in Snell's law is called the refractive index of the second medium with respect to the first. It is also defined as the ratio of the speed of light in the first medium to that in the second medium. The absolute refractive index of a medium is the ratio of the speed of light in vacuum (or air) to the speed of light in the medium: n = c/v. A higher refractive index means the medium is optically denser and light travels slower in it. The refractive index of water is 1.33, crown glass is 1.52, and diamond is 2.42. Optical density is not the same as mass density.

• Refractive index n = c/v (absolute)
• n21 = v1/v2 (relative)
• Higher n means optically denser medium

A lens is a transparent material bound by two surfaces, at least one of which is spherical. A convex lens is thicker in the middle and converges light rays; it is also called a converging lens. A concave lens is thicker at the edges and diverges light rays; it is called a diverging lens. Key terms include the optical centre (O), centres of curvature (C1 and C2), principal axis, principal focus (F1 and F2), and focal length (f). A ray passing through the optical centre goes undeviated. For a convex lens, the image can be real or virtual depending on object position; for a concave lens, the image is always virtual, erect, and diminished.

• Convex lensconverging, thicker at middle
• Concave lensdiverging, thicker at edges
• Ray through optical centre passes without deviation

The lens formula relates object distance (u), image distance (v), and focal length (f): 1/v – 1/u = 1/f. It is valid for all spherical lenses. Magnification by a lens is m = h′/h = v/u. The power of a lens is the reciprocal of its focal length: P = 1/f, where f is in metres. The SI unit of power is the dioptre (D). A convex lens has positive power, and a concave lens has negative power. When multiple thin lenses are in contact, their powers add algebraically: P = P1 + P2 + P3 + …

• Lens formula1/v – 1/u = 1/f
• Magnificationm = h′/h = v/u
• PowerP = 1/f, unit dioptre (D)
• Convex lenspositive power; concave lens: negative power