Light Reflection And Refraction Class 10 Notes

Key Principles & Formulas to Remember

• Laws of Reflection: 1) Angle of incidence (∠i) equals angle of reflection (∠r). 2) Incident ray, reflected ray, and normal all lie in the same plane.
• Laws of Refraction (Snell's Law): 1) Incident ray, refracted ray, and normal all lie in the same plane. 2) The ratio of sine of angle of incidence to sine of angle of refraction is constant for a given pair of media (n = sin i / sin r).
• Mirror Formula: 1/f = 1/v + 1/u (where f = focal length, v = image distance, u = object distance).
• Lens Formula: 1/f = 1/v - 1/u (where f = focal length, v = image distance, u = object distance).
• Magnification (Mirrors): m = h'/h = -v/u (h' = image height, h = object height).
• Magnification (Lenses): m = h'/h = v/u.
• Power of a Lens (P): P = 1/f (in meters). Unit is Dioptre (D).
• Refractive Index (n): n = speed of light in vacuum / speed of light in medium (c/v). Also, n₂₁ = n₂/n₁.
• Sign Convention (New Cartesian): All distances measured from pole/optical centre. Distances in direction of incident light are positive, opposite are negative. Heights above principal axis are positive, below are negative.

Essential Terms & Definitions

Reflection

The phenomenon of bouncing back of light when it strikes a surface. Occurs from polished surfaces like mirrors.

Refraction

The bending of light as it passes from one transparent medium to another due to a change in its speed.

Focal Length (f)

The distance between the pole/optical centre and the principal focus (F) of a spherical mirror or lens.

Centre of Curvature (C)

The centre of the sphere of which the spherical mirror or lens forms a part.

Principal Axis

The straight line passing through the pole/optical centre and the centre of curvature.

Real Image

An image formed when reflected or refracted light rays actually meet at a point. Can be obtained on a screen.

Virtual Image

An image formed when reflected or refracted light rays appear to meet at a point. Cannot be obtained on a screen.

Optical Centre

The central point of a lens through which a ray of light passes undeviated.

Power of a Lens (P)

The degree of convergence or divergence of light rays achieved by a lens. It is the reciprocal of its focal length in metres.

Understanding Image Formation by Spherical Mirrors and Lenses

Image formation in spherical mirrors (concave and convex) and lenses (convex and concave) relies on understanding the path of specific principal rays. For concave mirrors, if an object is placed beyond C, the image is real, inverted, and diminished (between F and C). If at C, image is real, inverted, same size, at C. If between F and C, image is real, inverted, enlarged, beyond C. If at F, image is formed at infinity. If between P and F, the image is virtual, erect, and enlarged, behind the mirror. This last case is crucial for understanding shaving mirrors and dentist mirrors.

For convex mirrors, they always form a virtual, erect, and diminished image between the pole (P) and the focus (F), regardless of the object's position. This property makes them useful as rear-view mirrors in vehicles, providing a wider field of view.

Similarly, for convex lenses, if an object is placed beyond 2F₁, the image is real, inverted, diminished (between F₂ and 2F₂). If at 2F₁, image is real, inverted, same size, at 2F₂. If between F₁ and 2F₁, image is real, inverted, enlarged, beyond 2F₂. If at F₁, image is formed at infinity. If between F₁ and the optical centre, the image is virtual, erect, and enlarged, on the same side as the object. This is the principle behind a magnifying glass.

Concave lenses always produce a virtual, erect, and diminished image between the optical centre and F₁ on the same side as the object. They are used to correct myopia (short-sightedness). Mastering the ray diagrams for these cases is essential; remember to draw at least two principal rays (e.g., ray parallel to principal axis passes through F after reflection/refraction, or ray passing through C goes undeviated) to locate the image.

Real vs. Virtual Images

Worked Mini-Examples

• {"title":"Mirror Formula Application","bodyMarkdown":"Q: An object is placed 15 cm in front of a concave mirror of focal length 10 cm. Find the position of the image.\n\nA: Given: u = -15 cm (object always to left), f = -10 cm (concave mirror).\nUsing mirror formula: 1/f = 1/v + 1/u\n1/(-10) = 1/v + 1/(-15)\n-1/10 = 1/v - 1/15\n1/v = 1/15 - 1/10 = (2 - 3)/30 = -1/30\nv = -30 cm. The image is formed 30 cm in front of the mirror, meaning it is real and inverted."}
• {"title":"Power of a Lens","bodyMarkdown":"Q: A convex lens has a focal length of 20 cm. Calculate its power.\n\nA: Given: f = +20 cm = +0.20 m (convex lens has positive focal length).\nPower P = 1/f (in meters)\nP = 1 / 0.20 = 5 Dioptres (D)."}

Exam Tip: Mastering Sign Conventions & Ray Diagrams

A common mistake in numerical problems is incorrect application of New Cartesian Sign Conventions. Always remember: object distance (u) is almost always negative. Focal length (f) is negative for concave mirrors/lenses and positive for convex mirrors/lenses. Image distance (v) sign tells you if the image is real (negative for mirrors, positive for lenses) or virtual (positive for mirrors, negative for lenses). For ray diagrams, use a sharp pencil and ruler. Draw clear arrows on the rays to indicate direction. Extend virtual rays as dotted lines. Label F, C (or 2F₁/2F₂) and the pole/optical centre clearly. Practice drawing diagrams for all positions of the object for both mirrors and lenses.

Quick Revision Check

• Q: State the two laws of reflection. A: 1) The angle of incidence equals the angle of reflection. 2) The incident ray, the reflected ray, and the normal to the surface all lie in the same plane.
• Q: What is the power of a lens? What is its S.I. unit? A: The power of a lens is a measure of its convergence or divergence ability. It is the reciprocal of its focal length (in meters). Its S.I. unit is Dioptre (D).
• Q: Under what conditions does a concave mirror form a virtual image? A: A concave mirror forms a virtual, erect, and enlarged image when the object is placed between its pole (P) and principal focus (F).
• Q: Define absolute refractive index. A: 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 that medium (n = c/v).

Frequently Asked Questions